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The global distribution of citrus black spot (CBS) disease, caused by Phyllosticta citricarpa, is climatically constrained, which is evident from its occurrence in citrus growing areas with warm, summer rainfall and its absence from areas with cooler, Mediterranean-type winter rainfall. Various epidemiological and modelling studies have supported this observation, predominantly estimating unsuitability for P. citricarpa in Mediterranean type climates, with no more than marginal suitability estimated at a few localities within some regions with Mediterranean type climates. The study by Martinez-Minaya et al. (European Journal of Plant Pathology, 143, 69–83, 2015), describes an historic sequence of recorded CBS occurrence in parts of South Africa, conducts an autocorrelation analysis and a correlative analysis with Koppen-Geiger climate zones and makes observations about the occurrence of certain Koppen-Geiger climate zones in the European Union. The study suggests that significant portions of the European Union and the broader Mediterranean basin are climatically similar to warm, summer rainfall areas in South Africa where P. citricarpa persists and causes CBS disease and concludes that the potential distribution of P. citricarpa is less constrained by climatic factors than spatial contagion. However, in this critique we expose methodological shortcomings in the Martinez-Minaya et al. (European Journal of Plant Pathology, 143, 69–83, 2015) study and conclude that the study grossly overestimated the extent of the geographical area that could support P. citricarpa, thereby rendering the findings scientifically unreliable.

Botrytis cinerea causes severe losses of rooibos seedlings in the Clanwilliam region of the Western Cape of South Africa. A total of 207 isolates were collected from diseased seedlings representing five nurseries. The cryptic species status of these isolates was determined with restriction enzyme analysis of the Bc-hch gene with 206 of the isolates identified as B. cinerea and one isolate as B. pseudocinerea. Analysis of the B. cinerea populations using seven microsatellite loci revealed a high total gene diversity (H), with a mean of 0.67. Analysis of molecular variance indicated that 97 % of the total genetic variation is distributed within subpopulations. Genotype flow was evident between nurseries indicated by shared occurrence of clonal lineages, in agreement with the relatively low but significant population differentiation between nurseries (mean F ST = 0.030, P = 0.001). The distribution of the mating type alleles MAT1-1 and MAT1-2 differed significantly from the ratio of 1:1 for the total population and in two of the five nursery subpopulations. Together with significant linkage disequilibrium found, populations appear to mainly reproduce asexually. Fungicide resistance frequency against iprodione for 198 of the genotyped isolates, showed highly varying levels of resistance between the nurseries. The mean incidence of resistance towards iprodione was 44 % in total, ranging from 0 % to 81 % for the five nurseries. This study has shown an adaptive capacity to overcome current chemical means of control in the studied B. cinerea population.

Trunk disease pathogens of grapevines, viz. Phaeomoniella chlamydospora, Eutypa lata and several species in Botryosphaeriaceae, Phaeoacremonium and Phomopsis are known to infect fresh pruning wounds by means of air-borne inoculum released after rainfall or prolonged periods of high relative humidity. Recent surveys have demonstrated that most or all of these pathogens are present in climatically diverse grape growing regions of South Africa. However, the factors controlling spore dispersal of these pathogens in vineyards were largely unknown. To address this question, spore trapping was done in a Chenin Blanc vineyard in the Stellenbosch area, South Africa, for 14 weeks during the grapevine pruning period from June to mid-September of 2004 and 2005. Hourly recordings of weather data were done by a weather station in the row adjacent to the spore trap. Spores of E. lata and Phomopsis and species in Botryosphaeriaceae were trapped throughout the trapping periods of 2004 and 2005, with higher levels of trapped spores recorded in 2005. The spores of all three pathogens were trapped during or after periods of rainfall and/or high relative humidity. In neither of the 2 years were spores of Pa. chlamydospora or Phaeoacremonium spp. trapped. Results indicated that spore event incidence, as well as the amount of spores released during a spore event of above-mentioned pathogens, were governed by rainfall, relative humidity, temperature and wind speed prior to and during the spore events.

Wound protection during all stages of grapevine propagation is of utmost importance to prevent infection of propagation material by decline and dieback pathogens. In semi-commercial nursery trials, grapevine rootstock and scion cuttings were soaked in water (control), chemical or biological sanitation products prior to cold storage, prior to grafting (machine- or hand-grafting) and prior to planting in field nurseries. Natural infection levels in basal ends and graft unions of uprooted nursery grapevines were evaluated 8 months after planting. Total pathogen incidences in the water-treated control plants ranged from 30% in basal ends to 13.5% in graft unions. Phaeomoniella chlamydospora was the most commonly isolated pathogen, followed by Phaeoacremonium, Cylindrocarpon + Campylocarpon, Botryosphaeria and Phomopsis spp. Machine-grafted unions generally had lower pathogen incidences compared with hand-grafted graft unions. In general, repeated soak-treatments of propagation material in the tested products resulted in reduced pathogen incidences in nursery grapevines. However, products containing T. harzianum (Trichoflow-T), hydrogen peroxide (Bio-sterilizer) and 8-hydroxyquinoline sulphate (Chinosol) gave inconsistent results, whereas Bronocide (a blend of halogenated alcohols and water) proved to be a good sterilising agent, but reduced certifiable plant yield significantly. Benomyl (at 100 g/100 l), Sporekill (a patented didecyldimethylammonium chloride formulation at 150 ml/100 l) and captan (at 1000 ml/100 l) were consistently the best treatments as growth parameters were not negatively influenced and pathogen incidences in basal ends and graft unions of uprooted plants were reduced.

Petri disease of grapevine is primarily caused by Phaeomoniella chlamydospora. This pathogen affects mostly young grapevines, but is also implicated in esca disease of older grapevines. Little is known about the disease cycle of this fungus. Infected propagation material was identified as a major means of dissemination of the pathogen. Recently, the pathogen was also detected from soil in South Africa and airborne conidia have been found in vineyards. The aim of this study was to use a molecular detection technique to test different samples collected from nurseries in South Africa at different nursery stages for the presence of P. chlamydospora. A one-tube nested-PCR technique was optimised for detecting P. chlamydospora in DNA extracted from soil, water, callusing medium and grapevine wood. The one-tube nested-PCR was sensitive enough to detect as little as 1 fg of P. chlamydospora genomic DNA from water and 10 fg from wood, callusing medium and soil. PCR analyses of the different nursery samples revealed the presence of several putative 360 bp P. chlamydospora specific bands. Subsequent sequence analyses and/or restriction enzyme digestions of all 360 bp PCR bands confirmed that all bands were P. chlamydospora specific, except for five bands obtained from callusing media and one from water. Phaeomoniella chlamydospora was positively detected in 25% of rootstock cane sections collected from mother blocks, 42% of rootstock cuttings and 16% of scion cuttings collected during grafting, 40% of water samples collected after pre-storage hydration, 67% of water samples collected during grafting, 8% of callusing medium samples and 17% of soil samples collected from mother blocks. These media can therefore be considered as possible inoculum sources of the pathogen during the nursery stages.