Search

Liquid organic fertilisers allow growers to abandon the use of conventional de novo (mined or synthesised) fertilisers without the need for major technological adaptions in the greenhouse. It was decided to run a cultivation experiment to find practical information for producers and growers when using liquid organic fertilisers in cultivation. The objective was to compare crop production between plants grown with liquid organic fertilisers and plants grown with conventional de novo fertilisers. Cucumbers were propagated in coir and peat and planted on slabs of respectively coir and peat. The plants on the two rooting media received either liquid organic or chemical fertilisers, resulting in four treatments in total. Plants fed with liquid chemical fertilisers were shown to initially produce 10-20% more fresh weight per unit time than plants fed with liquid organic fertilisers. When organic fertiliser was used during cultivation, ammonium accumulated to levels of 10-20 mmol L-1, while nitrate levels dropped to below 1 mmol L-1. To explain the results, the conversion of nitrogen was studied. Conversion of organic nitrogen into nitrate requires enzymatic hydrolysis to ammonium, followed by bacterial oxidation to nitrate. This microbiological process depends on two physiological groups of naturally occurring aerobic soil bacteria; ammonia oxidising bacteria (AOB) and nitrite oxidizing bacteria (NOB). In conclusion nitrate concentration rather than the concentrations of organic nitrogen or ammonium, were growth rate determining. The low conversion rate of ammonium into nitrate by bacteria present in substrate limited the leaf area growth of cucumbers. Suggestions to overcome the problem are made.

Liquid organic fertilisers allow growers to abandon the use of conventional de novo (mined or synthesised) fertilisers without major technological adaptions to the cultivation system. In prior experiments the conversion by aerobic substrate born bacteria of ammonium into nitrate was plant growth limiting. To improve the formulation of a commercial liquid organic fertiliser by increasing the nitrate concentration, a simple bench scale moving bed bioreactor of 20 L volume was constructed and used in continuous mode. The objective was to convert 80% or more of the organic N present in the original liquid organic fertiliser into nitrate. In the bioreactor we used naturally occurring aerobic nitrifying soil bacteria. Bacterial material from a soil sample was multiplied in vitro in selective media. The bioreactor was inoculated with ammonia and nitrite oxidising bacteria. Bacteria were maintained on a bio-carrier with a high specific surface area and a low specific mass. The bioreactor was aerated from below which allowed the bio-carrier to float and fertiliser solution could freely reach all bacterial mass. After initial inoculation, bacteria established themselves on the bio-carrier over a three week period after which accumulation of nitrite ended and stable production of nitrate as an only product of nitrification started. The nitrate production was monitored for several months during which it produced continuously. The bioreactor was supplied with fertiliser solution containing 6 mmol L-1 (total N, no nitrate). Over 90% of nitrogen supplied was converted to nitrate. Influent hydraulic retention time in the bioreactor was around 12.5 h. In conclusion, in line continuous conversion of liquid organic-N fertiliser into nitrate can be achieved for over 90% by a simple and reliable technique. The technique may be used by growers supported by the fertiliser industry or may be scaled to pre-treat commercial liquid organic fertilisers before marketing.

The management of pests is one of the major challenges in organic greenhouse cropping systems. In this paper, I summarize the currently most problematic and persistent, as well as the newly emerging pest species in organic tomato, sweet pepper, cucumber and aubergine crops in Europe. Furthermore, I discuss 3 new developments in biological pest control with arthropod natural enemies: 1) the integration of augmentative and conservation biological control, 2) the increased use of omnivorous predators as biological control agents and 3) the presence of other factors that affect directly or indirectly the performance of pests and natural enemies, such as endophytes, plant nutrition, disease control and greenhouse climate management. Finally, I suggest some directions for future research.

Limiting the use of phosphorous (P) in intensive agriculture is necessary to decrease losses to surface waters. Balanced fertilizer application (P supply equals P offtake by the crop) is a first step to limit the use of P. However, it is questioned whether this balance approach is sufficient to maintain soil fertility. A long-term field experiment (17 yr), on grazed grassland, has been conducted on sandy soil, marine clay soil and peat soil to obtain insight into the effects of balanced P fertilizer application on soil test P values and to explain the results by changes in P pools in the soil. The balance approach led to a gradual decline in plant available P, measured as P-AL, in the topsoil (

Biological pest control with mass-produced arthropod natural enemies is well developed in greenhouse crops and has often resulted in the evolution of complex ecosystems with persistent populations of multiple arthropod natural enemy species. However, there are cases where arthropod natural enemies are either not effective enough, not available, or their use is rather costly. For these reasons, biological control based on microorganisms, also referred to as ‘microbials’, represents a complementary strategy for further development. Although commercially available microbials have been around for quite some time, research on and the applied use of combinations of arthropod natural enemies and microbials have remained relatively under explored. Here, we review current uses of entomopathogenic fungi, bacteria and viruses, and their possible direct and indirect effects on arthropod natural enemies in European greenhouses. We discuss how microbials might be combined with arthropod natural enemies in the light of new methodologies and technologies such as conservation biological control, greenhouse climate management, and formulation and delivery. Furthermore, we explore the possibilities of using other microorganisms for biological control, such as endophytes, and the need to understand the effect of insect-associated microorganisms, or symbionts, on the success of biological control. Finally, we suggest future research directions to optimize the combined use of microbials and arthropod natural enemies in greenhouse production. peerReviewed

PurposeThe Aristotelian notion of phronèsis inspired innovative work in the realm of project management as well as in literature on sustainability and societal transformations. We argue that both literatures may benefit from a dialogue between the two, especially in view of outlining project managers' skills and competencies.MethodologyThe paper contributes to such a dialogue by reflecting on two practical experiences with managing sustainability-oriented, highly ambitious projects from the perspective of phronètic project management. The argument develops by reasoning to and fro between theory and practical experience.FindingsA key skill is a manager's capacity to distinguish between adopting a responsive attitude (a 'learning attitude') to attune a high-ambition project to the particularities of the participants' circumstances, and a 'leading attitude' to help the project move away from participants' current ideas on what is feasible and desirable in order do justice to its sustainability-oriented ambitions.ImplicationsThe competency to carefully alternate between these two attitudes in the light of a project's specific circumstances is quintessential in reflexive project management.ContributionThe paper explores the concept of reflexivity on micro-level, in project management, and on macro-level, in view of societal transformations, and explores the link between the two, both theoretically and in practical terms.

Hairy roots disease is an important problem in cultivation of greenhouse vegetables (tomato, aubergine and cucumber). Infection is caused by rhizogenic bacteria from Agrobacterium/Rhizobium group. It has been shown that infection process is regulated by environmental factors and quorum sensing mechanism. Rhizogenic bacteria produce acyl homoserine lactones (AHL) as signal molecules, which enable cell-to-cell communication. Disruption of this communication process will most likely lead to lower infection and symptoms. There are several possible routes of quorum quenching. One of them is degradation of pathogenic bacteria’s signal molecules by other microorganisms present in the root environment. Bacteria capable of AHL degradation were previously isolated from natural environment such as soil and water. In this research AHL degrading bacteria were successfully isolated from greenhouse growing substrate rockwool. Application of these micro-organisms in greenhouse practice is also discussed.

PurposeThe Aristotelian notion of phronèsis inspired innovative work in the realm of project management as well as in literature on sustainability and societal transformations. We argue that both literatures may benefit from a dialogue between the two, especially in view of outlining project managers' skills and competencies.MethodologyThe paper contributes to such a dialogue by reflecting on two practical experiences with managing sustainability-oriented, highly ambitious projects from the perspective of phronètic project management. The argument develops by reasoning to and fro between theory and practical experience.FindingsA key skill is a manager's capacity to distinguish between adopting a responsive attitude (a 'learning attitude') to attune a high-ambition project to the particularities of the participants' circumstances, and a 'leading attitude' to help the project move away from participants' current ideas on what is feasible and desirable in order do justice to its sustainability-oriented ambitions.ImplicationsThe competency to carefully alternate between these two attitudes in the light of a project's specific circumstances is quintessential in reflexive project management.ContributionThe paper explores the concept of reflexivity on micro-level, in project management, and on macro-level, in view of societal transformations, and explores the link between the two, both theoretically and in practical terms.

In ‘KAS Winter’ werd verteld over lopend onderzoek waarbij meer kennis moet worden ontwikkeld om een teeltstrategie op te zetten tegen kasluchtschimmels. Daarmee kan niet alleen meeldauw, maar ook Botrytis worden beheerst. Dit artikel gaat dieper in op onderzoek dat in 2013 werd uitgevoerd en waarbij werd geprobeerd een verdiepingsslag te maken naar metingen van inhoudsstoffen en genen in pottomaat en potgerbera, die een rol spelen in de afweer tegen Botrytis.

The Ethiopian Horticulture Producer Exporters Association (EHPEA) is responsible for the Ethio-Dutch Program for Horticulture Development (EDPHD). One of the components of this program is the Integrated Pest Management (IPM) which has been executed by Wageningen University and Research, Business Unit Greenhouse Horticulture in collaboration with Ethiopian research organizations and EHPEA. This final report of the WUR team reports on the following project activities: 1) Technical advice on formulation of the best microbial bca’s obtained from the conducted research, 2) Survey, collection and commercialization of indigenous natural enemies for the management of major horticultural pests, 3) Support and advice on the study of the biology of identified potential indigenous natural enemies, 4) Support and advice on small-scale mass production for evaluation trials and methods to establish persistent populations in crops, 5) Technical advice on the efficacy trials under laboratory and field conditions on the identified predators and parasitoids, 6) Review of IPM course delivered at Jimma University, 7) Sharing information with growers and companies, 8) Train the trainer, and 9) All collection and other missions De Ethiopian Horticulture Producer Exporters Association (EHPEA) is verantwoordelijk voor het Ethio-Dutch Program for Horticulture Development (EDPHD). Een van de componenten is het Integrated Pest Management (IPM) programma dat is uitgevoerd door Wageningen University and Research, Business Unit Glastuinbouw in samenwerking met Ethiopische onderzoeksorganisaties en EHPEA. Dit rapport van het WUR team geeft verslag van de volgende activiteiten: 1) Technisch advies op het gebied van formulering van de beste microbiële bca’s die in het onderzoek zijn gevonden, 2) Zoeken, verzamelen en vercommercialiseren van inheemse natuurlijke vijanden voor het beheer van belangrijke plagen in de tuinbouw, 3) Ondersteuning en advies op het gebied van biologische studies van geïdentificeerde potentiële inheemse natuurlijke vijanden, 4) Ondersteuning en advies op het gebied van kleinschalige massaproductie voor evaluaties en methoden om persistente populaties in gewassen te vestigen, 5) Technisch advies op het gebied van efficacy (werkzaamheid) evaluaties in laboratorium- en veldomstandigheden voor de geïdentificeerde predatoren en parasitoïden, 6) Evaluatie van de IPM cursus gegeven aan de Jimma universiteit, 7) Het delen van informatie met tuinders en bedrijven, 8) Training van trainers, en 9) Alle verzamelmissies en andere missies

In this project, solutions are developed to minimise leaching of nutrients and pesticides from greenhouses to the environment (esp. surface water), in order to comply with legislation and societal demands. In 2017 the following questions have been addressed: To prevent emission, drain solutions are reused or purified. Other water flows may deviate in compositionand possibilities for reuse or purification. The option for reuse or purification for these water flows has been investigated, and a working methodology for the end of a cultivation (e.g. cleaning) has been developed. Applications of Forward Osmosis in horticulture have been investigated. Water extracted from the discharge flow with Forward Osmosis using the concentrated nutrient solution holds prospects, but extracting irrigation water from brackish groundwater seems less feasible. In a long-term experiment, sodium (Na) standards for sweet pepper have been reinterpreted. It was shown that an increase in the Na standard up to 8-10 mmol/l causes no damage or loss in pepper production. Inaddition, it was shown that the split-root system can be used for uptake of extra Na without growth hampering. Furthermore, applying humate can prevent negative sodium effects at high sodium levels (Chinese cabbage). Finally, insight was gained into the risks associated with the use of chlorinated cleaning products in zero-discharge cultivations. In dit tweejarige project zijn openliggende kennisvragen opgepakt om de laatste stappen naar de gewenste nulemissie bij substraatteelten mogelijk te maken. In 2017 zijn de volgende vragen onderzocht: Voor waterstromen die sterk afwijken van gangbaar lozingswater, is de samenstelling geanalyseerd en zijn mogelijkheden tot hergebruik dan wel zuivering verkend. Tevens is onderzocht hoe bedrijven in de praktijk omgaan met deze waterstromen. Voor waterstromen die bij de teeltwisseling vrijkomen is een werkwijze ontwikkeld; in2018 komen best practices beschikbaar voor de andere waterstromen. De toepassing van Forward Osmosis in de tuinbouw is onderzocht. Voor het indikken van de spuiwaterstroom biedt Forward Osmosis met als trekvloeistof een geconcentreerde nutriëntenoplossing perspectief, voor het onttrekken van gietwater uit brak grondwater minder. In 2018 volgt een pilot proef en optimalisatie van de scenario’s. De Na-normen voor paprika zijn geherinterpreteerd met een duurproef, waarbij bleek dat een verhoging van Na-norm voor paprika tot 8-10 mmol/l veilig kan. Daarnaast is aangetoond dat de restgoot-methode (gescheiden wortelsysteem) gebruikt kan worden om extra Na op te nemen zonder negatieve gevolgen voor groei. In 2018wordt deze methode op semi-praktijkschaal getest. Ook het toedienen van humaat kan de negatieve gevolgen vaneen hoog natrium teniet doen (Chinese kool). Tenslotte is er inzicht verkregen in de risico’s bij het gebruik van chloorhoudende reinigingsmiddelen in emissieloze teelten.

Powdery mildew (PM) is a very serious disease affecting glasshouse-grown roses and tomatoes in the Netherlands. Control is limited because of resistance to existing fungicides. Anhydrous milk fat (AMF) and soybean oil (SBO) emulsions were evaluated for control of PM in roses and tomatoes. Both AMF (14 g/litre) and SBO (14 g/litre) provided powdery mildew control on rose leaves and blooms that was significantly better (P

Greenhouse vegetable production systems require high N and irrigation inputs. Commonly, these systems are associated with environmental problems caused by nitrate leaching. Given increasing societal pressure to reduce these problems, there is a requirement to optimally use N and water inputs. Optimal management practices will be influenced by the greenhouse technological level, the growing medium and the quality of the available water supply. Four broad categories of greenhouse vegetable production can be considered: (i) high technology (HT) in soil, (ii) HT in substrate, (iii) low to medium technology (LMT) in soil, and (iv) LMT in substrate. Examples of HT systems are glasshouses in The Netherlands and Belgium, and of LMT are plastic greenhouses in the Mediterranean Basin. Combined drip irrigation and fertigation are being increasingly used which provide a high technical capacity for precise N and irrigation management. Tools are required that enable growers to take advantage of this technical capacity. For optimal irrigation management, suitable tools are decision support systems (DSS) incorporating simulation models to estimate daily crop water requirements and soil/substrate moisture sensors. For optimal N management, suitable tools are DSS employing simulation models that estimate N fertilizer requirements based on an N balance approach combined with soil or soil solution testing. Crop/plant monitoring is potentially useful, but as yet further work is required. For both irrigation and N management, the recommended approach is combined prescriptive-corrective management, which can also be described as combining modelling and monitoring approaches whereby models are used to prescribe management plans and monitoring is used to identify when adjustments are required.

Zoophytophagous predators of the family Miridae (Heteroptera), which feed both on plant and prey, often maintain a close relationship with certain host plants. In this study, we aimed to select a suitable mirid predatory bug for aphid control in sweet pepper. Four species were compared: Macrolophus pygmaeus (Rambur), Dicyphus errans (Wolff), Dicyphus tamaninii Wagner and Deraeocoris pallens (Reuter). They were assessed on their establishment on sweet pepper plants with and without supplemental food (eggs of the flour moth Ephestia kuehniella Zeller and decapsulated cysts of the brine shrimp Artemia franciscana Kellogg) and on their effects on aphids with releases before and after aphid infestations. None of the predator species was able to control an established population of aphids on sweet pepper plants; however, the predators M. pygmaeus and D. tamaninii could successfully reduce aphid populations when released prior to an artificially introduced aphid infestation. The best results were achieved with M. pygmaeus in combination with a weekly application of supplemental food. Hence, our results demonstrate that the order and level of plant colonization by mirid predators and aphids determines how successful biological control is. Further studies are needed to evaluate the performance of mirid predatory bugs in sweet pepper crops in commercial greenhouses with multiple pests and natural enemies, in particular to understand how increased variation in food sources affects their feeding behaviour and preferences.

Planten halen nutriënten normaal gesproken binnen via hun wortels. Het opname- en transportsysteem is daar volledig op ingericht. Bij bemesting via het blad is er niet zo’n gestroomlijnde route. Dat maakt het resultaat van bladbemesting erg wisselend. Immobiele elementen blijven snel steken bij de plek waar ze terecht zijn gekomen.

Supplemental food sources to support natural enemies in crops are increasingly being tested and used. This is particularly interesting for generalist predators that can reproduce on these food sources. However, a potential risk for pest control could occur when herbivores also benefit from supplemental food sources. In order to optimize biological control, it may be important to select food sources that support predator populations more than herbivore populations. In this study we evaluated the nutritional quality of four types of supplemental food for the generalist predatory mites Amblyseius swirskii Athias-Henriot and Amblydromalus (Typhlodromalus) limonicus (Garman and McGregor), both important thrips predators, and for the herbivore western flower thrips Frankliniella occidentalis Pergande, by assessing oviposition rates. These tests showed that application of corn pollen, cattail pollen or sterilized eggs of Ephestia kuehniella Zeller to chrysanthemum leaves resulted in three times higher oviposition rates of thrips compared to leaves without additional food. None of the tested food sources promoted predatory mites or western flower thrips exclusively. Decapsulated cysts of Artemia franciscana Kellogg were not suitable, whereas cattail pollen was very suitable for both predatory mites and western flower thrips. In addition, we found that the rate of thrips predation by A. swirskii can be reduced by 50 %, when pollen is present. Nevertheless, application of pollen or Ephestia eggs to a chrysanthemum crop still strongly enhanced the biological control of thrips with A. swirskii, both at low and high release densities of predatory mites through the strong numerical response of the predators. Despite these positive results, application in a crop should be approached with caution, as the results may strongly depend on the initial predator–prey ratio, the nutritional quality of the supplemental food source, the species of predatory mites, the distribution of the food in the crop and the type of crop.

1. Plant traits that enhance the attraction of the natural enemies of their herbivores have been postulated to function as an ‘indirect defence’. An important underlying assumption is that this enhanced attraction results in increased plant fitness due to reduced herbivory. This assumption has been rarely tested. 2. We investigated whether there are fitness consequences for the charlock mustard Sinapis arvensis, a short-lived outcrossing annual weedy plant, when exposed to groups of large cabbage white (Pieris brassicae) caterpillars parasitized by either one of two wasp species, Hyposoter ebeninus and Cotesia glomerata, that allow the host to grow during parasitism. Hyposoter ebeninus is solitary and greatly reduces host growth compared with healthy caterpillars, whereas C. glomerata is gregarious and allows the host to grow approximately as large as unparasitized caterpillars. Both healthy and parasitized P. brassicae caterpillars initially feed on the foliage, but later stages preferentially consume the flowers. 3. In a garden experiment, plants damaged by parasitized caterpillars produced more seeds than conspecific plants damaged by unparasitized caterpillars. Reproductive potential (germination success multiplied by total seed number) was similar for plants that were not exposed to herbivory and those that were damaged by parasitized caterpillars and lower for plants that were damaged by healthy unparasitized caterpillars. However, these quantitative seed traits negatively correlated with the qualitative seed traits, individual seed size and germination success, suggesting a trade-off between these two types of traits. 4. We show that parasitism of insect herbivores that feed on reproductive plant tissues may have positive fitness consequences for S. arvensis. The extent to which plant fitness may benefit depends on parasitoid lifestyle (solitary or gregarious), which is correlated with the amount of damage inflicted on these tissues by the parasitized host.

Een machine-vision-setup en bijbehorende analyse software zijn in ontwikkeling voor het meten van tripsschade in chrysantenbladeren.

Supplying predators with alternative food can have short-term positive effects on prey densities through predator satiation (functional response) and long-term negative effects through increases of predator populations (numerical response). In biological control, alternative food sources for predators are normally supplied on the crop plants; using the litter-inhabiting food web as a source of alternative food for plant-inhabiting predators has received less attention. We investigated the effect of supplying plant-inhabiting predatory mites with alternative prey (astigmatic mites) in the litter on pest control. Predator (Amblyseius swirskii) movement and population dynamics of the pest (western flower thrips) and predators were studied on rose plants in greenhouses. Predators commuted between the above-ground plant parts where they controlled thrips, and the litter, where they fed on alternative prey, although the latter were a superior diet. Predators controlled thrips better in the presence of the astigmatic mites than in their absence. We show that predatory mites can form a link between above-ground pests and the litter food web, and propose that adding alternative prey to the litter of ornamental greenhouses can result in higher predator densities and increased biological control.

Worldwide increase of water shortage and competition for fresh water is raising the awareness that an increase in water use efficiency in horticulture is a necessity. At the same time, environmental awareness and legislation (e.g. the EU Water Framework Directive 2000/60/EC) restricts the emissions of nutrients and plant protection products (PPPs). In the Netherlands in soilless growing systems much effort is put into measures to enable maximal recycling of nutrient solutions, thereby decreasing the required amount of fresh water needed and the emission of nutrients and PPPs. Recycling of water fluxes also implicates that the drain water has to be disinfected to prevent rapid dispersal of pathogens. Heat treatment, filtration, Ozone and UV treatment are commonly used disinfection technologies. Some of these technologies (Ozone and UV) are known for their ability to break down PPPs and therefore their joint application (disinfection and end-of-pipe purification of discharged drain water) have been studied. In this paper we present the results of the integration and optimisation of ozone technology in a cucumber and a sweet pepper soilless cultivation. An Agrozone ozone unit that alternately treated the recycled nutrient solution for the elimination of pathogens and removed PPPs from discharge water was investigated. The ozone unit eliminates pathogens and PPPs by direct oxidation of organic material batchwise. It was shown that both disinfection of the recirculating nutrient solution and removal of PPPs from discharge water are possible with the same ozone equipment, if installed correctly. Preliminary tests with Tomato Mosaic Virus have shown that this technology can completely disinfect (from 402 to 0 lesions per plant), and remove up to 98% of PPPs from standardised greenhouse discharge water. For disinfection purposes shorter treatment times and lower ozone doses can be applied.

Fertigation in combination with drip irrigation is being increasingly used in vegetable crop production. From a nutrient management perspective, this combination provides the technical capacity for precise nitrogen (N) nutrition, both spatially and temporally. With these systems, N and other nutrients can be spoon-fed to crops, through frequent applications of small amounts, to the immediate root zone as required by the crop. In commercial farming practice, management of combined drip/fertigation systems generally does not take advantage of this potential for precise N management. As is common in commercial farming, management of both irrigation and N with drip/fertigation systems is generally based on growers' experience, with the objective of avoiding deficiencies that may limit production. Because of frequent N application, the established improved nutrient management strategies, based on infrequent soil testing, pre-plant and one or two side-dress applications, are of limited value. With drip/fertigation, dynamic N management approaches are required so that the capacity for frequent small applications can be fully exploited to provide (a) site and season specific management and (b) dynamic responses to temporal N requirements. Dynamic irrigation management is required for the same reasons. Modelling and monitoring approaches and combinations of the two enable exploitation of the technical capacity for precise N and irrigation management. Decision support systems (DSS) based on simple simulation models with limited data inputs can provide crop specific plans of daily N and irrigation requirements. The use of soil moisture sensors is an effective and proven monitoring approach for irrigation management. For monitoring of soil/crop N status, soil monitoring through regular sampling of soil-water extracts and soil solution is being used, and crop/plant monitoring approaches such as with proximal optical sensors and petiole sap analysis are promising methods.

Nu het gewasbeschermingspakket permanent lijkt in te krimpen, is weerbaar telen van modeterm tot speerpunt in het onderzoek geworden. Het vergt een aanpak waarbij de teler voortdurend rekening houdt met de interactie tussen ras, ziekteverwekker, biostimulanten, substraat en klimaat. De inzichten vorderen, maar er valt nog veel te leren.

Soil fertility management systems in organic greenhouses are quite diversified within Europe. The challenge is to identify and implement strategies which are in agreement with the organic principles stated by (EC) Reg. 834/2007 and (EC) Reg. 889/2008 as well as environmentally, socially and economically sustainable. In this paper, written by a group of scientists of different geographical origin and with different background, the state of the art of the sector and the main characteristics of the European greenhouse cropping systems are described. The main bottlenecks and constraints are discussed with a particular reference to the regulatory framework in force. The more relevant issues that may influence the enforcement and future development of the sector have been identified as specific knowledge gaps. For each of them, the appropriate research needs were elaborated in a multidisciplinary perspective as forthcoming challenges for the whole sector. Far to be exhaustive, given the wide heterogeneity of the implemented systems, this paper is able, for the first time, to give a structured outlook, on soil fertility in protected organic condition at an European scale.

New diagnostic and rapid ‘nextgeneration sequencing’ methods facilitate thediscovery of new plant viruses. However, virus detection or elucidation of thesequence of its dna/rna is no guarantee for understanding its epidemiology.In order to develop effective control measures in field crops or in greenhouses,knowledge of how viruses move from one plant to another is essential.

Application of a mobile unit for discharge water purification is one of four options to apply to the purification obligation per 1-1-2018. Depending on the amount of discharge, future water strategy and investment options, mobile purification can be an interesting option. The amount of discharge water varies with crop, irrigation strategy and quality of the irrigation water and is between 122 and 3.340 m3/ha/year for surveyed companies. About 65% of greenhouse companies discharges Mobiele zuivering is één van de vier opties om te voldoen aan de zuiveringsplicht per 1-1-2018. Afhankelijk van de hoeveelheid lozingswater, toekomststrategie en investeringsruimte kan mobiele zuivering een interessante optie zijn. De hoeveelheid lozingswater varieert afhankelijk van de teelt, waterstrategie en kwaliteit gietwater en ligt bij de onderzochte bedrijven tussen 122 en 3.340 m3/ha/jaar. Ongeveer 65% van de bedrijven loost minder dan 600 m3/ha/jaar. De nadruk van de lozingen ligt in de zomer, omdat de filters dan vaak gespoeld worden en de kwaliteit van het gietwater zorgt voor de meeste incidentele lozingen. Op basis van de onderzochte bedrijven kan de sector opgedeeld worden in vier groepen qua lozingsintensiteit, die allen een kwart van het areaal beslaan: 4, 7.5, 12.5 en 40 m3/ha/week. Met name voor bedrijven met weinig lozingswater per week en met een klein oppervlak (en weinig investeringsruimte) is mobiele zuivering een interessante oplossing. Als voor alle bedrijven het lozingsvolume gehalveerd wordt, wordt het voor veel meer bedrijven interessant om te kiezen voor de optie mobiele waterzuivering. Het potentieel totaal te zuiveren volume water wordt hierdoor kleiner, maar het potentieel aantal bedrijven neemt toe. De mobiele installatie moet een goedkeuring van de Beoordelingscommissie Zuiveringsinstallaties Glastuinbouw hebben voor 95% zuivering en moet de gezuiverde hoeveelheid registreren voor handhavingsdoeleinden.

The role of natural enemy diversity in biological pest control has been debated in many studies, and understanding how interactions amongst predators and parasitoids affect herbivore populations is crucial for pest management. In this study, we assessed the individual and combined use of two species of natural enemies, the parasitoid Aphidius ervi Haliday, and the predatory brown lacewing Micromus variegatus (Fabricius), on their shared prey, the foxglove aphid, Aulacorthum solani (Kaltenbach), on sweet pepper. We hypothesized that the presence of intraguild predation (IGP) and predator facilitation (through induced aphid dropping behaviour) might have both negative and positive effects on aphid control, respectively. Our greenhouse trial showed that overall, the greatest suppression of aphids occurred in the treatment with both the parasitoid and the lacewing. While the combination of lacewings and parasitoids significantly increased aphid control compared to the use of parasitoids alone, the effect was not significantly different to the treatment with only predators, although there was a clear trend of enhanced suppression. Thus, the combined effects of both species of natural enemies were between additive and non-additive, suggesting that the combination is neither positive nor negative for aphid control. High levels of IGP, as proven in the laboratory, were probably compensated for by the strong aphid suppression provided by the lacewings, whether or not supplemented with some level of predator facilitation. For aphid management over a longer time scale, it might still be useful to combine lacewings and parasitoids to ensure stable and resilient aphid control., Centro de Estudios Parasitológicos y de Vectores

In ‘KAS Winter’ werd verteld over lopend onderzoek waarbij meer kennis moet worden ontwikkeld om een teeltstrategie op te zetten tegen kasluchtschimmels. Daarmee kan niet alleen meeldauw, maar ook Botrytis worden beheerst. Dit artikel gaat dieper in op onderzoek dat in 2013 werd uitgevoerd en waarbij werd geprobeerd een verdiepingsslag te maken naar metingen van inhoudsstoffen en genen in pottomaat en potgerbera, die een rol spelen in de afweer tegen Botrytis.

Aphids are a huge problem in organic greenhouse production systems of sweet pepper, eggplant and cucumber in northern Europe. Biological control with arthropod natural enemies is often not effective, resulting in large crop losses yearly. Entomopathogenic fungi of the order Entomophthorales are known for their ability to cause epizootics and reduce host populations dramatically in a short time. These abilities make them potentially more effective biological control agents of pest species than commercially available entomopathogens of the order Hypocreales (e.g., fungi of the genus Beauveria, Metarhizium, Lecanicillium and Isaria). However, a major stumbling block to utilizing these fungi as biological control agents has been the difficulties encountered in growing them in vitro, which is one of the reasons why the biocontrol industry did not develop commercial products based on these fungi. Another approach in utilizing these fungi could be to try inducing epizootics by introducing inoculum of infected aphids into the crop. Here we present our attempts to induce such epizootics with banker plants of wheat with grain aphids infected by the entomophthoralean fungi Pandora neoaphidis. The system of introducing "Pandora bankers" has now been applied by 8 organic greenhouse growers in The Netherlands with various results. The requirements for causing a successful epizootic with this fungus will be discussed.

At the end of the cultivation period a considerable amount of nitrate and phosphate is still present in the water in slabs and in the irrigation system. To reduce the emission of this amount of nitrate and phosphate a strategy was developed to use the plant to use this nitrate and phosphate in the last few weeks of the cultivation period. This however should be done without affecting the production or quality of the lasts fruits to be harvested. The strategy consisted of a gradual decrease over the last five weeks of the cultivation of nitrate, EC and water quantity in the supply. The ammonium to nitrate level was increased to acidify the slabs and release eventual precipitated phosphate. Chloride was used to replace nitrate as anion in the supply to allow to maintain proper levels of cations, notably potassium, in the supply and slab. Aan het einde van een teelt is er nog een aanzienlijke hoeveelheid nitraat en fosfaat aanwezig in het water in de teeltmatten en in de leidingen en voorraadvaten van het irrigatiesysteem. Om de eindeteelt emissie van nitraat en fosfaat vanuit het teeltsysteem te verminderen, is er een strategie ontwikkeld om in de laatste weken van de teelt de plant de voorraden aan nitraat en fosfaat te laten opeten. Dit moet uiteraard gebeuren zonder dat de productie of de kwaliteit van de productie in de laatste weken daaronder leidt. De strategie bestond uit het, in de laatste weken, geleidelijk verlagen van de aanvoer van nitraat, EC en water. De verhouding ammonium/ nitraat werd juist verhoogd om de matten enigszins te verzuren zodat mogelijk neergeslagen fosfaat in oplossing komt. Chloride werd gebruikt om nitraat te vervangen als belangrijkste anion in de aanvoer zodat het niveau van kationen aanbod, met name kalium, zo lang mogelijk hoog kon blijven.

New diagnostic and rapid ‘nextgeneration sequencing’ methods facilitate thediscovery of new plant viruses. However, virus detection or elucidation of thesequence of its dna/rna is no guarantee for understanding its epidemiology.In order to develop effective control measures in field crops or in greenhouses,knowledge of how viruses move from one plant to another is essential.

The Palaearctic predator Dicyphus errans (Hemiptera: Miridae) lives omnivorously on various host plants, preying on a wide range of small arthropods, including some new invasive alien species. These characteristics make it a promising biological control agent (BCA) in organic greenhouses. The capacity of a BCA to find, kill and consume prey plays a fundamental role in trophic interactions and population dynamics in a predator-prey system. The functional response of a predator, which describes how the individual rate of prey consumption changes in response to prey density, is a key component to assess its effectiveness in pest control and the stability of its own populations. Therefore, the functional response of D. errans on different prey was studied to improve our knowledge on the potential of this mirid, which is naturally widespread in European organic greenhouses. Laboratory experiments were carried out on three exotic pests: the poinsettia thrips Echinothrips americanus (Thysanoptera: Thripidae), the greenhouse whitefly Trialeurodes vaporariorum (Hemiptera: Aleyrodidae), and the tomato borer Tuta absoluta (Lepidoptera: Gelechiidae), to build functional response curves. Prey was offered at different densities to single females of D. errans for 24 h. The predation behaviour of D. errans on all the prey species was defined by Type II functional response curves. The female could daily prey about 62 adults of E. americanus, 114 pupae of T. vaporariorum, and 236 eggs of T. absoluta. The high voracity of this generalist predator on different prey confirmed its suitability as a BCA. For effective and stable pest control strategies, a prior to pest establishment of D. errans in organic greenhouses may prevent pest escaping in case of high infestation rates, even if the type II functional response reaches saturation at very high prey densities.

Hoe weet je hoe het staat met de weerbaarheid van het gewas? Kun je dat meten? Sinds enige jaren groeit het inzicht in activiteit van genen die daarbij zijn betrokken. Maar dat blijkt zo'n complexe zaak, dat vooralsnog meten van weerbaarheidseiwitten en andere verdedigingsstoffen het snelst inzichten geeft.

Within a project Sustainable control strategies Wageningen University & Research Greenhouse Horticulture literature screening to develop an integrated pest management strategy to control airborne fungi. This project was funded by the former Dutch Horticultural Production Board and accompanied by the Dutch growers association, LTO Glaskracht Nederland. In 2016 two greenhouse trials were performed with pot tomato, Petunia and pot gerbera. Additionally, a greenhouse test was performed at a commercial greenhouse with pot gerbera. Goal was to test the side effects of synthetic elicitors which stimulate systemic acquired resistance against biotrophic fungi on disease resistance against nectrotrophic fungi like Botrytis. A commercially available synthetic elicitor, Fado (Nufarm) was able to reduce the Botrytis infection in pot tomato and in flowers of pot gerbera. Effectivity was supported by a combined treatment with Jiffy mineral (soil addition) and Serenade. Two experimental products of BASF and Nufarm were effective in reducing Botrytis in pot gerbera. Botrytis infection was not only related to light and relative humidity, but was also correlated with nutrients and chlorophyll content. Based on this knowledge, prediction models for Botrytis can be improved. Wageningen University & Research Glastuinbouw heeft binnen het project Duurzame teeltstrategie een brede kennisinventarisatie uitgevoerd om een geïntegreerde systeemaanpak voor beheersing van kasluchtschimmels te ontwikkelen. Dit project is gefinancierd door het voormalige Productschap Tuinbouw en begeleid door LTO Glaskracht Nederland. In 2016 zijn twee kasproeven bij WUR Glastuinbouw uitgevoerd om nieuwe teeltstrategieën te testen in pottomaat, potgerbera en Petunia. Daarna is een praktijkproef in potgerbera uitgevoerd bij de JHL group in Pijnacker. Doel was om de nevenwerking te testen van synthetische elicitors die de salicylzuurroute tegen biotrofe schimmels activeren op het verminderen van de gevoeligheid voor necrotrofe schimmels, zoals Botrytis. Zowel in optimale als in suboptimale groeicondities met relatief lage lichtniveaus. In proeven met een commercieel beschikbare elicitor die de salicylzuurroute activeert, Fado (Nufarm), werd in pottomaat de gevoeligheid voor Botrytis verlaagd en in potgerbera werden bloemen minder gevoelig voor smet in de naoogstfase na een preventieve gewasbehandeling. De werking neemt toe in combinatie met Serenade en Jiffy mineral (toegediend in potgrond) en blijft goed zowel in lage als in hoge lichtcondities (stralingssom 4, 15 en 30 mol/dag). Twee gewasbeschermingsmiddelen die nog niet toegelaten zijn in gerbera (BASF exp.1 en Nufarm exp.1) werken effectief tegen Botrytis. De gevoeligheid voor Botrytis was niet alleen afhankelijk van de stralingssom en luchtvochtigheid, maar toonde ook een relatie met voedingsstoffen van plantsapanalyses en bladgroenwaardes. Deze kennis kan bijdragen aan verbetering van voorspellingsmodellen voor Botrytis.

Irrigation in protected cultivation is essential due to the absence of natural precipitation. High evapotranspiration, due to higher temperature and prolonged cropping period, requires ample an adequate supply of water. The water supply in a greenhouse is solely carried out by irrigation and thus enables full control over the water management of the greenhouse soil. There are no specific prescriptions for organic greenhouse horticulture (OGH) regarding water sources or irrigation. However, according to the basic principles, growers are obliged to responsibly use natural resources such as water. Thus, efficient water use and/or water recycling are important issues for organic farming. Salinity should be avoided through proper fertilisation management and the use of water sources of adequate quality. Moreover, rainwater collection is strongly encouraged or requested for operation sites situated in climatic suitable areas. Optimal and tuned irrigation is quite challenging for OGH. Obviously, supplying the crop with sufficient water is an important precondition for optimum crop health and performance. In addition, sufficient soil moisture is required for optimal functioning of the soil biota to secure mineral delivery from the applied organic amendments. Also accumulation of undesired salts must be avoided. On the other hand, nutrient leaching should be avoided or limited to a minimum. The irrigation management should take heterogeneities due to irregular water delivery of the irrigation system as well as site variations in soil and plants into account Minimizing leaching should be one of the main drivers for optimization of irrigation in OGH. Since OGH in Europe are strictly soil bound cultures, leaching and nutrient emission to groundwater is a threat. For soils with groundwater within reach of the root zone, capillary rise may add to the water supply of crops. However, irrigation strategies aiming at deficit irrigation, in other words, relying on capillary rise, can be used for the short term only. Given the presence of a surplus of ions in groundwater not only salts like Na and Cl, but also Ca, Mg, SO4 and even K, salinity problems will occur on the long run. Irrigation management tuned to crop requirements is an important issue for sustainable organic greenhouse production. To achieve sustainable management of OGH production, irrigation should be tuned to the demand of the crop, taking into account also the variations in climatic zones, soil types, growing seasons, planting dates and cultivars. Consequently, tools are required for irrigation management with the flexibility to deal with such variation. This paper will review the important aspects of the crop water demand and tools for irrigation management in organic greenhouse vegetables.

The conclusion of a previous experiment showed that the compost quantity was the most determining parameter for the production volume of mushrooms, despite the addition of hemi cellulose as carbon source to the compost. The present experiment focuses on the mycelium action with regard to the carbon availability in the compost. Two factors were studied: (1) The effect of breaking the mycelium at a predetermined depth in the compost layer and before the first mushrooms flush. The hypothesis was that mycelium which is no longer in contact with the mycelium in the casing material would free the carbon nutrition in the lower compost layer. This increased amount of free carbon would be beneficial during the budding. It was expected that breaking the compost layer during the growth of the mycelium would increase the amount of buds. The observed effect of breaking the mycelium is that the formation of buds is delayed in comparison with the reference. (2) The effect of distributing the compost in a longer layer which remains partly uncovered by casing material. The compost is distributed into a thin layer with 2-3 times the surface area of the reference. The casing material just covers the same surface area of the compost as in the reference. The hypothesis is that the mycelium can exploit the total extended compost layer and increase the mushroom production in correlation with the volume of compost accessible, but independent of the surface of the casing material. The observed effect is that the longer the compost layer is, the lower the production gets. In summary, the depth of the compost layer is the most determining factor for yield. In een eerder project bleek al dat de compostlaag sterk bepaald hoeveel opbrengst behaald wordt, ondanks het toevoegen van hemicellulose als extra koolstofbron. Dit project beschrijft een proef die op twee andere wijzen laat zien op welke manier de compostlaag de belangrijkste opbrengstbepalende factor voor de champignonkweek is. De proef bestudeert het gebruik van organische voeding door het mycelium. Twee factoren zijn onderzocht: (1) Het effect van het verbreken van het mycelium op verschillende hoogten in de compostlaag. Door de compostlaag op een bepaalde hoogte te scheiden in een onder- en bovenlaag, wordt het mycelium verbroken. De veronderstelling was dat het mycelium dat in de onderlaag groeit, extra organische voeding zou vrijmaken. Het resultaat was echter dat de onderbreking de knopvorming vertraagt. (2) Het effect van extra compost bereikbaar voor mycelium in horizontale richting in plaats van verticale richting (dikkere laag). Het volume compost wordt aangeboden als een dunnere laag die doorloopt buiten het met dekaarde bedekte deel. De veronderstelling was dat het mycelium in het horizontaal bereikbare extra compost zou groeien, waardoor de opbrengst in het deel met dekaarde zou toenemen. Het resultaat was dat hoe langer de niet afgedekt compostlaag is, hoe lager de opbrengst is. Samenvattend blijkt dus dat de dikte van de compostlaag de opbrengst bepaald.

In de periode van 2009-2016 is er onderzoek gedaan naar de mogelijkheden van substraatloze teelt voor chrysanten. Er is vooral gewerkt met teeltvijvers - een diepe waterlaag, waarboven de chrysanten in een houder geplaatst waren. Op onderzoeksschaal werden zeer goede resultaten behaald, tot wel 25% meerproductie ten opzichte van de grondteelt. Bij het opschalen van dit teeltsysteem op een praktijkbedrijf bleek het systeem echter zeer kwetsbaar voor zieken. Gedurende meerdere jaren is geprobeerd te achterhalen welke factoren doorslaggevend waren voor de ziekteontwikkeling om zo een weerbaar systeem te kunnen opleveren. Een sluitend bewijs is hier echter niet gevonden. Het onderzoek heeft de basis gelegd voor studies aan het microbioom in watersystemen. Met een eerste uitgebreide screening van micro-organismen in goed gedocumenteerde waterbassins is het begin gemaakt voor verder onderzoek. De studies geven tevens aan dat sturing op het microbioom loont. Aanenten met verschillende populaties en sturing op populaties gedurende de teelt beinvloeden de uitkomst. Verder onderzoek naar specifieke populaties en sturingsmechanismen kunnen in de toekomst handvatten opleveren om deze kennis toepasbaar te maken in waterteelten en in grond of substraatteelten.Over a period of seven years (2009-2016) hydroponic chyrsanthemum cultivation has been developed and tested in practice. Where the system delivered up to 25% higher yields at smaller scale, the larger systems of 250-300 m2 turned out very sensitive to root infection. Research then focussed to understanding the key factors that caused the plants to become sensitive. However, where the larger system showed much disease incidence every summer for three years in a row, at smaller scale the symptoms could not be induced despite application of extreme cultivation measures. The studies were the first to apply next generation sequencing to microbial populations in cultivation systems. The results gave evidence for shifting population dynamics due to inoculation and water temperature. Also the inoculation with beneficial microbes was found to have a positive effect on recovery of the roots upon infection. This effect, however, was only found in the research-facility. At larger scale potential beneficial effect could not prevent significant yield loss.

Fertigation in combination with drip irrigation is being increasingly used in vegetable crop production. From a nutrient management perspective, this combination provides the technical capacity for precise nitrogen (N) nutrition, both spatially and temporally. With these systems, N and other nutrients can be spoon-fed to crops, through frequent applications of small amounts, to the immediate root zone as required by the crop. In commercial farming practice, management of combined drip/fertigation systems generally does not take advantage of this potential for precise N management. As is common in commercial farming, management of both irrigation and N with drip/fertigation systems is generally based on growers' experience, with the objective of avoiding deficiencies that may limit production. Because of frequent N application, the established improved nutrient management strategies, based on infrequent soil testing, pre-plant and one or two side-dress applications, are of limited value. With drip/fertigation, dynamic N management approaches are required so that the capacity for frequent small applications can be fully exploited to provide (a) site and season specific management and (b) dynamic responses to temporal N requirements. Dynamic irrigation management is required for the same reasons. Modelling and monitoring approaches and combinations of the two enable exploitation of the technical capacity for precise N and irrigation management. Decision support systems (DSS) based on simple simulation models with limited data inputs can provide crop specific plans of daily N and irrigation requirements. The use of soil moisture sensors is an effective and proven monitoring approach for irrigation management. For monitoring of soil/crop N status, soil monitoring through regular sampling of soil-water extracts and soil solution is being used, and crop/plant monitoring approaches such as with proximal optical sensors and petiole sap analysis are promising methods.

The Dutch greenhouse horticultural sector has committed itself to reach a (practically) zero emission fornutrients and plant protection products at latest in 2027. A zero emission production strategy is demonstratedfor the production of sweet pepper (Maranello) on either Rockwool and coco peat. Readily available technologiesand strategies, like ozonation for disinfection of recirculation water and Uptake Analysis for control of nutrients, were combined to reach this goal, together with a newly developed strategy to minimise discharge of water, nutrients and plant protection products at the end-of-season crop change. Productivity on Rockwool substrate (28.1 kg/m2) was slightly higher than on coco peat (26.5 kg/m2), due to less generative steering on the coco peat substrate from the start of the cropping season. Quality of the fruits was good on both substrates. Technical failure of equipment caused overflow of drain tanks (3.8% on Rockwool, 0.7% on coco peat), but this did not significantly reduce the amount of sodium in the water (main reason for discharge). No other water quality parameters were observed that could have negatively influenced productivity. In het Platform Duurzame Glastuinbouw heeft de glastuinbouwsector met de landelijke overheid en andere partijen afgesproken toe te werken naar een (nagenoeg) nulemissie voor nutriënten en gewasbeschermingsmiddelen (GBM) in 2027. Voor een paprikateelt (Maranello) is een emissieloze teeltstrategiegedemonstreerd voor zowel een teelt op steenwol- als op kokossubstraat. Hiervoor is gebruik gemaakt van beschikbare technieken en strategieën, zoals ozonisatie voor ontsmetting van recirculatiewater en OpnameAnalyse voor sturing bemesting, en een nieuw ontwikkelde einde-teeltstrategie voor minimalisatie van afvoer van water, meststoffen en gewasbeschermingsmiddelen. Op steenwol (28,1 kg/m2) werd een iets hogereproductie gehaald dan op kokos (26,5 kg/m2), doordat vanaf de start de teelt op kokos niet generatief genoeggestuurd is. De kwaliteit van de vruchten was op orde. Door een storing aan de apparatuur is tijdens de teeltwater verloren gegaan (3,8% bij steenwol, 0,7% bij kokos), maar dit heeft niet significant bijgedragen aan hetverlagen van de concentratie natrium in het recirculatiewater (belangrijkste reden in de praktijk om te lozen).Er zijn geen overige parameters in het water waargenomen die een negatieve invloed hebben gehad op deproductiviteit.

Planten sterker maken waardoor ze beter bestand zijn tegen ziekten en plagen. Met dat doel worden biostimulanten of plantversterkers aan het wortelmilieu toegediend. Ze zijn sterk in opkomst, want ze vormen een alternatief voor chemische gewasbeschermingsmiddelen, waarvan het gebruik steeds meer onder druk komt te staan.

Understanding how arthropod pests and their natural enemies interact in complex agroecosystems is essential for pest management programmes. Theory predicts that prey sharing a predator, such as a biological control agent, can indirectly reduce each other's density at equilibrium (apparent competition). From this premise, we (i) discuss the complexity of indirect interactions among pests in agroecosystems and highlight the importance of natural enemy-mediated indirect interactions other than apparent competition, (ii) outline factors that affect the nature of enemy-mediated indirect interactions in the field and (iii) identify the way to manipulate enemy-mediated interactions for biological control. We argue that there is a need to increase the link between community ecology theory and biological control to develop better agroecological methods of crop protection via conservation biological control. In conclusion, we identify (i) interventions to be chosen depending on agroecosystem characteristics and (ii) several lines of research that will improve the potential for enemy-mediated indirect interactions to be applied to biological control. © 2014 Society of Chemical Industry

De topsectoren zijn van groot belang omdat hierbij actief wordt ingezet op de negen sectoren die voor Nederland BV erg belangrijk zijn. Het midden- en kleinbedrijf (MKB) neemt hierin een sleutelpositie in en kan binnen de topsectoren een rol vervullen die cruciaal is voor onze economie. De sector van de Tuinbouw en Uitgangsmaterialen bedraagt wel 10% van het bruto nationaal product (BNP). Ook andere Topsectoren, met name Agri&Food, zijn belangrijk voor specifiek de plantenziektekunde en voor de plantaardige productie als geheel. In dit artikel wordt ingegaan op het KNPV najaarssymposium ‘Plantenziektekunde in de Topsector op 19 november 2014.

Zijn potwormen in orchidee misschien minder schadelijk dan altijd werd gedacht? Adviseur René ‘t Hoen denkt van wel. Hij claimt dat ondanks de aanwezigheid van potwormen, een gezonde plant zonder problemen kan uitgroeien tot een prima verkoopbaar product. Dit zou betekenen dat de teeltstrategie belangrijker is dan bestrijding van de schadelijke muggenlarven. Tegelijkertijd vindt hij dat zijn waarnemingen moeten worden gestaafd door onderzoek. Er is namelijk nog zeer weinig bekend over de biologie van potwormen.

De auteurs Jantineke Hofland (WageningenvUR Glastuinbouw) & Luc Stevens (PRI) hebben dit artikel geschreven m.m.v. Els ten Dam en Rozemarijn de Vries.