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The relationship between various abiotic and biotic soil factors and the capacity of the soil to allow soil-borne pathogens to produce disease (`soil receptivity', SR) was investigated for three major pea root rot pathogenic fungi. Several relationships were established. SR to Thielaviopsis basicola was shown to be positively associated with the abundance of pseudomonads, soil pH, organic matter content and C/N ratio. The number of saprophytic Fusarium solani in the rhizoplane was higher in soil less conducive to F. solani f. sp. pisi, whereas the abundance of indigenous F. solani in the rhizosphere was unrelated to receptivity. The amounts of soluble K, P, Mg and total C and N in soil, individually, were higher with increasing conduciveness to F. solani f. sp. pisi. The SR to Aphanomyces euteiches was not well associated with any soil aspect investigated, except Gliocladium in the rhizoplane and Acremonium in the rhizosphere, of which populations were higher in soil less receptive to this pathogen. Experiments with soil sterilization showed that soil biota were the main factor responsible for the inhibition of investigated pathogens. Therefore, despite its correlation with chemical and physical factors, SR to the investigated pathogens can be considered essentially of microbiological nature.

The virulence of Meloidogyne hapla, M. chitwoodi, and M. fallax was studied on genotypes of Solanum spp. in a greenhouse. Juveniles of 11 M. hapla race A isolates, 3 M. hapla race B isolates, and 5 mono-female lines of a M. hapla race A isolate were inoculated on S. chacoense, S. hougasii, and S. sparsipilum. Juveniles of eight M. chitwoodi isolates, five M. fallax isolates, and six mono-female lines of a M. chitwoodi isolate were inoculated on S. bulbocastanum, S. chacoense, S. hougasii, S. stoloniferum, and S. tuberosum. Virulence was expressed as nematode reproduction 8 weeks after inoculation. Nematode reproduction was estimated by the number of egg masses and, in one experiment, by the number of hatched second-stage juveniles per inoculated juvenile. Considerable variation in virulence and resistance was observed among M. hapla isolates and plant genotypes, respectively. The M. hapla isolate-plant species interaction was highly significant. The response to M. chitwoodi ranged from susceptible (S. tuberosum and S. chacoense) to highly resistant (S. bulbocastanum and S. hougasii). S. tuberosum was susceptible to M. fallax, whereas all four wild species were resistant. In contrast to M. hapla, no significant isolate-plant genotype interaction was obtained for M. chitwoodi or M. fallax, indicating no or little intraspecific variation in virulence. M. chitwoodi juveniles in species mixtures with M. fallax isolates appeared to be able to break the resistance of S. bulbocastanum and S. hougasii. Significant differences among mono-female lines of M. hapla and M. chitwoodi were observed, indicating heterogeneity of pathogenicity within meiotic parthenogenic Meloidogyne populations.

The insect pathogenic nematode Steinernema feltiae (Nematoda: Steinernematidae) was shown to offer an alternative to the use of diflubenzuron for the control of the mushroom fly Lycoriella auripila (Diptera: Sciaridae). The influence of diflubenzuron and S. feltiae treatments on the yield and numbers of the mushroom, Agaricus bisporus , was studied. Applications of diflubenzuron after casing significantly reduced total mushroom yields by 10-13%. The greatest yield reductions were observed in the first flush. In the second flush, high yields were found despite significantly lower numbers of mushrooms. These high yields, however, could not compensate for the yield loss of the first flush. Some of the treatments with S. feltiae significantly increased yields, but these were not associated with a particular time of application. As nematodes did not reduce mushroom yields they were able to compete with diflubenzuron. Although the purchase costs of nematodes were high, their use was more economical than that of...

The effect of treatments with conidial suspensions of Ulocladium atrum and Gliocladium roseum on leaf rot of cyclamen caused by Botrytis cinerea was investigated under commercial greenhouse conditions. Spraying U. atrum (1 × 106 conidia per ml) or G. roseum (2 × 106 conidia per ml and 1 × 107 conidia per ml) at intervals of 2 to 3 weeks during the production period and spraying U. atrum (1 × 106 conidia per ml) at intervals of 4 to 6 weeks resulted in a significant reduction of natural infections of petioles by B. cinerea. U. atrum or G. roseum (1 × 107conidia per ml) was as effective as the standard fungicide program. B. cinerea colonized senesced leaves within the plant canopy and infected adjacent petioles and leaves later. The antagonists colonized senesced leaves and reduced B. cinerea development on these leaves. Thus, the inoculum potential on petioles adjacent to necrotic leaf tissues was reduced. The fate of U. atrum conidia on surfaces of green cyclamen leaves during a 70-day period after application was studied. The number of conidia per square centimeter of leaf surface remained relatively constant during the entire experiment. Sixty percent of the conidia sampled during the experiments retained the ability to germinate. When green leaves were removed from the plants to induce senescence and subsequently were incubated in a moist chamber, U. atrum colonized the dead leaves. Senesced leaves also were colonized by other naturally occurring fungi including B. cinerea. On leaves treated with U. atrum from all sampling dates, sporulation of B. cinerea was significantly less as compared with the untreated control. Our results indicate that early applications of U. atrum before canopy closure may be sufficient to achieve commercially satisfactory control of Botrytis leaf rot in cyclamen.

As the use of biotechnology products, such as genetically modified microorganisms (GMMs), in the environment might bring about undesirable ecological effects, it is important that the environmental fate of inoculant organisms, as well as any effects of their release, are assessed. Ideally, pilot studies in microcosms or small mesocosms are thus performed prior to a larger-scale (commercial) application, and the results of these studies serve to guide further environmental use of the GMM. In these pilot studies, the methods employed to assess the environmental fate and effects of the GMM will have to be fine-tuned and optimized so as to assess these phenomena in an optimal way. This review examines the methods that are currently available for the assessment of the environmental fate of genetically modified and unmodified microorganisms, as well as the impact following their release. The emphasis will be on monitoring of these phenomena in soil, as a paradigm of assessments in complex environmental matrices. Detection methods based on cultivation, which generally rely on the use of added or intrinsic markers, serve to assess the fate of the culturable fractions of released microorganisms. The specificity of this detection can be enhanced by using a combination of cultivation-based and immunology- and/or DNA-based assessments. Furthermore, specific immunofluorescence or in situ hybridization techniques are suitable to quantify populations of GMMs at the level of microscopy-detectable cells, even though fluorescent in situ hybridization is still plagued by sensitivity problems in oligotrophic environments. Detection methods based on nucleic acids (DNA or RNA) extracted from the environment offer the possibility to monitor the fate of the heterologous genes released, including that following a horizontal gene transfer. Moreover, these methods can also provide a picture of the dynamics of the total numbers of microbial cells released. For a sound assessment of the biosafety of environmental releases, a polyphasic approach to environmental monitoring is recommended, as in most analyses information will be needed on the fate of culturable and nonculturable cells, as well as on that of the specific gene sequences released. Finally, the use of impact analysis via an array of different methods is briefly reviewed, and the merits of novel approaches to assessments of community structure via molecular means (PCR/DGGE, ARDRA or T-RFLP) and metabolic profiling via Biolog are discussed as methods specifically aimed at the detection of shifts in community structure and function (metabolic complement analysis).

An AluI satellite DNA family has been isolated in the genome of the root-knot nematode Meloidogyne chitwoodi. This repeated sequence was shown to be present at approximately 11,400 copies per haploid genome, and represents about 3.5% of the total genomic DNA. Nineteen monomers were cloned and sequenced. Their length ranged from 142 to 180 bp, and their A + T content was high (from 65.7 to 79.1%), with frequent runs of As and Ts. An unexpected heterogeneity in primary structure was observed between monomers, and multiple alignment analysis showed that the 19 repeats could be unambiguously clustered in six subfamilies. A consensus sequence has been deduced for each subfamily, within which the number of positions conserved is very high, ranging from 86.7% to 98.6%. Even though blocks of conserved regions could be observed, multiple alignment of the six consensus sequences did not enable the establishment of a general unambiguous consensus sequence. Screening of the six consensus sequences for evidence of internal repeated subunits revealed a 6-bp motif (AAATTT), present in both direct and inverted orientation. This motif was found up to nine times in the consensus sequences, also with the occurrence of degenerated subrepeats. Along with the meiotic parthenogenetic mode of reproduction of this nematode, such structural features may argue for the evolution of this satellite DNA family either (1) from a common ancestral sequence by amplification followed by mechanisms of sequence divergence, or (2) through independent mutations of the ancestral sequence in isolated amphimictic nematode populations and subsequent hybridization events. Overall, our results suggest the ancient origin of this satellite DNA family, and may reflect for M. chitwoodi a phylogenetic position close to the ancestral amphimictic forms of root-knot nematodes.

We propose the 'Seinhorst Research Program', derived from Seinhorst's empirical philosophy. All theories of the 'Seinhorst Research Program' are developed by searching for recurring regularities (patterns) in a collection of observations, named 'the empirical base'. To prevent 'ghost theories from sloppy data', all assumptions underlying the empirical base are carefully described in theories with respect to methodology and technology, including statistics. The patterns to be recognised are summarised by mathematical equations, which must be connected with biological processes to bridge the gap between 'normal' language and mathematical language for the description of biological theories. Often, the patterns result from more than one biological process. If so, the basic patterns are disentangled from one another using a method of pattern analysis. The procedure is best carried out when only a limited number of more or less congruent patterns are involved. Therefore, attention must be given to the choice of the hierarchic level and the complexity of the investigated system. Investigations proceed from simple experimental systems to complex natural systems at a hierarchic level that is neither so high that manifesting processes are very dissimilar nor so low that one runs the risk of describing processes irrelevant for the purpose of the investigation. In the 'Seinhorst Research Program', this purpose is finding methods for improvement of financial returns of host crops attacked by plant-parasitic nematodes through calculating risks of nematode population development and subsequent yield reduction. Pattern analysis yields theories about causes of phenomena observed at the investigated hierarchic level and about properties of processes at the nearest lower hierarchic level. Predictions at the next higher hierarchic level are made by synthesising several patterns in (stochastic) simulation models. Synthesis is also applied to compound patterns of processes in simple experimental systems, with the objective of explaining complicated patterns in complex systems.

The diversity of dinitrogenase reductase gene (nifH) fragments in Paenibacillus azotofixans strains was investigated by using molecular methods. The partial nifH gene sequences of eight P. azotofixans strains, as well as one strain each of the close relatives Paenibacillus durum, Paenibacillus polymyxa, and Paenibacillus macerans, were amplified by PCR by using degenerate primers and were characterized by DNA sequencing. We found that there are two nifH sequence clusters, designated clusters I and II, in P. azotofixans. The data further indicated that there was sequence divergence among the nifH genes of P. azotofixans strains at the DNA level. However, the gene products were more conserved at the protein level. Phylogenetic analysis showed that all nifH cluster II sequences were similar to the alternative (anf) nitrogenase sequence. A nested PCR assay for the detection of nifH (cluster I) of P. azotofixans was developed by using the degenerate primers as outer primers and two specific primers, designed on the basis of the sequence information obtained, as inner primers. The specificity of the inner primers was tested with several diazotrophic bacteria, and PCR revealed that these primers are specific for the P. azotofixans nifH gene. A GC clamp was attached to one inner primer, and a denaturing gradient gel electrophoresis (DGGE) protocol was developed to study the genetic diversity of this region of nifH in P. azotofixans strains, as well as in soil and rhizosphere samples. The results revealed sequence heterogeneity among different nifH genes. Moreover, nifH is probably a multicopy gene in P. azotofixans. Both similarities and differences were detected in the P. azotofixans nifH DGGE profiles generated with soil and rhizosphere DNAs. The DGGE assay developed here is reproducible and provides a rapid way to assess the intraspecific genetic diversity of an important functional gene in pure cultures, as well as in environmental samples.

Isolates of Fusarium oxysporum from lily were screened for pathogenicity, vegetative compatibility and DNA restriction fragment length polymorphisms, and compared to reference isolates of F. oxysporum f. sp. gladioli and F. oxysporum f. sp. tulipae to justify the distinction of F. oxysporum f. sp. lilii. Twenty-four isolates from different locations in The Netherlands (18 isolates), Italy (4 isolates), Poland and the United States (1 isolate each) shared unique RFLP patterns with probes D4 and pFOM7, while hybridization did not occur with a third probe (F9). Except for a self-incompatible isolate, these 24 isolates all belonged to a single vegetative compatibility group (VCG 0190). Isolates belonging to VCG 0190 were highly pathogenic to lily, but not to gladiolus or tulip, except for a single nonpathogenic isolate. Six saprophytic isolates of F. oxysporum from lily were nonpathogenic or only slightly aggressive to lily, gladiolus and tulip, belonged to unique VCGs and had distinct RFLP patterns. Three pathogenic isolates previously considered to belong to E oxysporum f. sp. lilii were identified as F. proliferatum var. minus; all three belonged to the same VCG and shared unique RFLP patterns. These three isolates were moderately pathogenic to lily and nonpathogenic to gladiolus and tulip. The reference isolates of F. oxysporum f. sp. tulipae were pathogenic to tulip, but not to lily and gladiolus; they shared a distinct RFLP pattern, different from those encountered among pathogenic and saprophytic isolates from lily, and formed a separate new VCG (VCG 0230). Reference isolates of F. oxysporum f. sp. gladioli belonging to VCG 0340 proved pathogenic to both gladiolus and lily, but not to tulip. These isolates, as well as isolates belonging to VCGs 0341, 0342 and 0343 of F. oxysporum f. sp gladioli, had RFLP patterns different from those encountered among the isolates from lily or tulip. These findings identify F. oxysporum f. sp. lilii as a single clonal lineage, distinct from F. oxysporum f. sp. gladioli and f. sp. tulipae.

Three Orthosia species were studied to forecast larval emergence and development and to find out the optimal moments for monitoring and control. The duration of embryonic and larval development of Orthosia incerta and Orthosia stabiliz was determined at various constant temperatures, and validated in field experiments. Embryonic development of Orthosia gothica was followed in the field. No significant differences between embryonic development of the species under study were observed. The estimated heat sum for embryonic development is 72 degree days above a developmental threshold of 7.0°C. Differences in development of the first larval stage depending on the quality of the substrate were found for O. incerta. Stage one of this species developed faster on artificial food than on fresh apple leaves, but total larval development rates of the species observed showed hardly any difference. The regression equation Y = 0.0017 × T – 0.0045 gives a good description of the total larval development rate (L1-pupa) for both species. The larval development rates were implemented in an age-structured phenological computer model INSIM. Results of simulations were in close agreement with field observations. Opportunities for use of these data in practice are discussed.

Oogenesis was investigated using a fluorescent staining technique with Hoechst 33258 in several isolates of facultative meiotic parthenogenetic -Meloidogyne hapla race A, M. chitwoodi, M. fallax - and obligate ameiotic (mitotic) parthenogenetic - M. hapla race B, M. javanica - and isolate Xa of an unidentified species. Without fertilisation, the somatic chromosome number was restored by fusion of pronucleus and second polar body in M. hapla race A, M. chitwoodi, and M. Sallax isolates. In oocytes of isolate Xa, a second meiotic division occurred in a few cases while in all other cases the somatic chromosome number was restored by duplication of the chromosomes of the egg nucleus after the first division. In one mono-female line of Xa, embryonic development occurred within the body of the female and was genetically controlled. In M. hapla race B and M. javanica, maturation of the oocytes consisted of one mitotic division. This study confirmed that a large variation of the mode of reproduction exists in Meloidogyne spp., which may be a reason for their world-wide success. In one M. hapla race A isolate, only 12 % of the oocytes of inseminated females contained sperm, and prophase I instead of prometaphase I oocytes were found posterior to the spermatheca. (C) Orstom/Elsevier, Paris

A method for the indirect (cell extraction followed by nucleic acid extraction) isolation of bacterial ribosomal RNA (rRNA) and genomic DNA from soil was developed. The protocol allowed for the rapid parallel extraction of genomic DNA as well as small and large ribosomal subunit RNA from four soils of different texture. DNA and rRNA yields from these soils were 15–30 and 0.25–1.0 μg g −1 soil, respectively. Following different purification steps, the rRNA as well as genomic DNA extracts obtained were sufficiently pure for either reverse transcription and polymerase chain reaction (PCR) amplification, or direct PCR amplification. Using a set of universal bacterial primers based on conserved regions of the 16S rRNA sequence, both approaches yielded mixed target molecules for subsequent denaturing gradient gel electrophoresis fingerprinting of soil microbial diversity. The amplified rRNA-based bacterial diversity assessment was compared with diversity assessments based on amplified DNA in one selected soil. Results showed similarities as well as differences between the profiles generated on the basis of rRNA and those based on genomic DNA, which suggested that the bacterial communities defined on the basis of their genomic DNA contained variable amounts of rRNA.

Strawberry mottle virus was transmitted from Fragaria vesca to Chenopodium quinoa and Nicotiana occidentalis 37B. In all plants, SMoV infection-associated dsRNA was detected but dsRNA patterns showed some differences between the herbaceous hosts. The differences were more evident when the dsRNA preparations were electrophoresed employing a SDS-PAGE method followed by silver staining of polyacrylamide gels than when employing traditional agarose gel electrophoresis and EtBr staining. Double-stranded RNA, purified from N. occidentalis 37B, was used to synthesize cDNA and for the development a cDNA library in Escherichia coli. Recombinant plasmids were analyzed by restriction enzyme digestion, and specific cDNA clones were selected by means of a 32P-labelled probe prepared from SMoV dsRNA. After a miniprep step, the plasmids were digested and the length of the inserts was determined to be between 200 and 2000 bp. The largest inserts were selected and used for further research. Hybridization experiments confirmed the specificity of the cloned material for SMoV dsRNA.

Three similar isolates of Phytophthora (Phytophthora sp-h) were obtained from diseased Spathiphyllum and Primula plants. Cultural characteristics did not fit any known description of Phytophthora species. The Phytophthora sp-h isolates are papillate, are homothallic, possess 80 to 86% amphigynous antheridia, and have a maximum temperature for growth of 36.5°C. Isozyme analysis of the Phytophthora sp-h isolates revealed a three-banded pattern with malic enzyme and a three-banded pattern with malate dehydrogenase on the second putative locus. The fastest band at both enzyme loci comigrated with the single P. nicotianae band, the slowest band comigrated with the single P. cactorum (and also P. pseudotsugae) band, and one band in between was concluded to represent the heterodimeric isozyme. The random amplified polymorphic DNA patterns of the Phytophthora sp-h isolates almost exclusively consisted of bands that were also present in either P. nicotianae or P. cactorum. Southern hybridization showed that bands specific for P. nicotianae were present as comigrating bands in the Phytophthora sp-h isolates. The same was found for species-specific bands of P. cactorum. It is concluded that the three Phytophthora sp-h isolates represent interspecific hybrids, P. nicotianae being the one parent and P. cactorum the other. Analysis of mito-chondrial DNA with restriction enzymes revealed banding patterns in all the Phytophthora sp-h isolates identical with those of P. nicotianae, confirming that indeed P. nicotianae was one of the parents.

Peanut (Arachis hypogaea) plants dually infected with tomato spotted wilt tospovirus (TSWV) and peanut mottle potyvirus (PMV) exhibited a wide variety of symptoms, ranging from PMV-like symptoms of transient mild leaf mottle to TSWV-like symptoms of severe leaf distortion and stunting of the plant. Dual infection did not cause greater symptom severity than infection with either virus alone. In the early stages of disease development, PMV symptoms were similar to the first leaf symptoms of TSWV infection, suggesting that identification of TSWV in field-grown peanuts should not depend on visual observation. The virus titer, determined by enzyme-linked immunosorbent assay, indicated a delay in TSWV disease progress in doubleinfected plants, compared to plants infected with TSWV alone. In the later phase of disease progress, the virus titer in dually infected plants was not significantly different from that of singly infected plants. Infection with TSWV and PMV alone and with both viruses in combination was consistent among commercially grown peanut cultivars. In plants inoculated with TSWV or PMV alone or with both viruses in combination, the length of the latent period and final disease incidence, as measured by the number of plants showing symptoms, did not depend on the cultivar.

Realistic values of population growth rates are needed when used in forecasting programmes, e.g., in a programme of integrated control. Therefore, comparisons were made in a chrysanthemum - aphid system between different methods of assessing population growth rates. The reproductive performances of the aphid species Aphis gossypii and Myzus persicae were measured on two chrysanthemum cultivars using three plant growth stages (young vegetative, budding and flowering). In the first set of experiments, development time and reproduction were used to estimate the population growth rate r m . The mean relative growth rates (MRGR) were also assessed. It was shown for the first time that the relationship between r m and MRGR was influenced by aphid species. In a second experiment, the aphid population increase on a whole plant was measured and r m was estimated by calculating the slope of the (In transformed) population increase. It is shown that population growth rate is affected by the growth stage of the plant, and that cultivar and aphid species interact with plant growth stage in influencing population growth rate. Thus, no single growth stage of chrysanthemum for maximal aphid population growth can be assigned, but the budding and flowering stage are the most suitable in three out of four aphid x cultivar combinations. Comparison between the results from both experiments demonstrates clearly that more realistic values for r m are obtained when measured on whole plants.

Little is known about the composition of the mosquito midgut which plays a central role in the development and subsequent transmission of malaria parasites. As a first step towards the characterization of mosquito midgut molecules involved in the transmission of malaria parasites, we analysed two-dimensional gel electrophoresis patterns of the midgut proteins of sugar-fed and blood-fed Anopheles stephensi lines of different susceptibility to P. falciparum infection. Two lines fully susceptible and one line (Pb3-9A) of reduced susceptibility were used. In the refractory line ookinetes do develop but are only inefficiently transformed into oocysts (Feldmann and Ponnudurai, 1989). The protein profiles of midguts from all sugar-fed mosquito lines were similar. However, after blood feeding, the midgut of the fully susceptible lines contained proteins not found in the midgut of line Pb3-9A. Twenty-nine such proteins were detected and are candidates for involvement in the interaction between the mosquito midgut and P. falciparum.

Summary The nematode Steinemema feltiae (Nematoda: Steinernematidae) was tested for its ability to control two main mushroom pests i.e. the sciarid Lycoriella auripila (Diptera: Sciaridae) and the phorid Megaselia halterata (Diptera: Phoridae) in growing-rooms filled with spawned compost. A clear difference between female and male sciarid control was observed. A nematode application 1 day after casing preceded by an application 1 day before casing on the compost caused an almost complete control (97%) of the F1-generation of female sciarids. The FZgeneration of females was similarly controlled (95%) by an application 7 days after casing. A dosage of 1 x lo6 nematodes mP2 was found to be equally effective as higher dosages. Diflubenzuron remained active throughout entire the cropping period with high sciarid mortality rates varying from 72% to 99%. Phorid control was variable and seemed to depend on the presence of sciarids. In one occasion the control rate of F2-generation phorid larvae was 75% and was possibly caused by the presence of new infective juvenile nematodes recycled in FZgeneration sciarid larvae. Diflubenzuron did not significantly reduce phorid numbers.

A new virus was isolated from the pea aphid, Acyrthosiphon pisum, and tentatively named Acyrthosiphon pisum virus (APV). The isometric virus particles were approximately 31 nm in diameter and contained a single-stranded RNA molecule of approximately 10 kb. Four structural proteins were observed with molecular masses of approximately 23.3, 24.2, 34.5, and 66.2 kDa. The 34.5-kDa capsid protein was the most abundant product in purified virions. Computer-assisted analysis revealed no significant homology between an internal sequence of 37 amino acids of the 34.5-kDa protein of APV and other polypeptides of viral origin. APV was not immunologically related to other ssRNA viruses from hemipteroid insects, such as aphid lethal paralysis virus, Rhopalosiphum padi virus, and Nezara viridula virus type 1. Immunolocalization on ultrathin sections of 3-day-old nymphs of A. pisum showed that APV antigen was predominantly present in the epithelial cells of the digestive tract. Virus particles were also observed associated with the microvilli of the intestine. Occasionally, muscle cells and mycetocyte cells were found infected. Purified APV, fed to 1-day-old A. pisum nymphs, significantly reduced the growth of the aphid and increased the time needed to reach maturity.

Luteoviruses and the luteovirus-like pea enation mosaic virus (PEMV; genus Enamovirus) are transmitted by aphids in a circulative, nonreplicative manner. Acquired virus particles persist for several weeks in the aphid hemolymph, in which a GroEL homolog, produced by the primary endosymbiont of the aphid, is abundantly present. Six subgroup II luteoviruses and PEMV displayed a specific but differential affinity for Escherichia coli GroEL and GroEL homologs isolated from the endosymbiotic bacteria of both vector and nonvector aphid species. These observations suggest that the basic virus-binding capacity resides in a conserved region of the GroEL molecule, although other GroEL domains may influence the efficiency of binding. Purified luteovirus and enamovirus particles contain a major 22-kDa coat protein (CP) and lesser amounts of an approximately 54-kDa readthrough protein, expressed by translational readthrough of the CP into the adjacent open reading frame. Beet western yellows luteovirus (BWYV) mutants devoid of the readthrough domain (RTD) did not bind to Buchnera GroEL, demonstrating that the RTD (and not the highly conserved CP) contains the determinants for GroEL binding. In vivo studies showed that virions of these BWYV mutants were significantly less persistent in the aphid hemolymph than were virions containing the readthrough protein. These data suggest that the Buchnera GroEL-RTD interaction protects the virus from rapid degradation in the aphid. Sequence comparison analysis of the RTDs of different luteoviruses and PEMV identified conserved residues potentially important in the interaction with Buchnera GroEL.

The use of intercropping as an integrated pest management (IPM) tool to reduce population densities of pests of field grown vegetables, and the growth and yields of cabbage and clover cover crops grown in the Netherlands, were investigated. In the first 2 experiments, the clover (Trifolium repens cv. Pertina or T. subterraneum cv. Geraldton) crop significantly reduced pest damage of fresh market cabbage cv. Minicole. The associated yield losses due to competition with the cover crop were 15-24%. In other experiments designed to investigate these yield losses in cabbage for storage (cv. Slawdena), the losses were 18-43%. The decrease in weight/head was reduced by decreasing the row distance from 0.75 to 0.50 m, although yield losses were still high. Delaying the sowing of the clover crop, or transplanting the cabbage into a rotary cultivated strip in the clover stand instead of transplanting in the clover stand itself, did not reduce yield losses. It is suggested that intercropping with clover as an IPM tool is not suitable at present for cabbage crops in the Netherlands.