Development and persistence of resistance to fungicides in Sphaerotheca fuliginea in cucumbers in the Netherlands

Ergosterol biosynthesis inhibitors (EBIs) have a remarkably broad spectrum of antifungal activity. They belong to the commercial fungicides which exhibit the highest activity known to date. Resistance to EBIs was found in vitro, but the level of resistance and the decreased fitness of resistant strains led to the hypothesis that resistance to EBIs was rather unlikely to develop in practice. Despite the fact that EBIs were introduced as early as 1972, up to 1980 only data on EBI-resistant laboratory strains were available. However, gradually there appeared reports on decreased sensitivity to EBIs in practice. This made it of interest to investigate the potential development of resistance to EBIs in vivo.In this study the pathogen used to gather the desired information was Sphaerotheca fuliginea, as this fungus is widespread in cucumber glasshouses in the Netherlands, intensively controlled with fungicides and resistance-prone. This thesis contains five papers describing several aspects of resistance to fungicides in S.fuliginea.In the first paper it has been shown that S.fuliginea is present on cucumber plants all year round. A gradual increase in the number of infected crops, in the form of S-shaped curves, was observed from planting until May. In the districts with the highest crop density (Pijnacker), mildew was generally observed early in the growing season, while the apparent rate of increase of infected crops was higher than in other districts. In the district with the lowest crop density (Northern Netherlands), mildew was observed late in the growing season and the apparent rate of increase of infected crops was low. Early in the growing season the pathogen is probably dispersed by transportation of infected planting stock and by man. When the disease pressure increases, inoculum is probably dispersed by wind. This implies that inoculum is highly mobile and that, in the case of development of resistance to fungicides, the resistant strains will disperse to crops where the spray regime did not cause resistance to develop.As S. fuliginea developed resistance to dimethirimol, benzimidazoles and pyrazophos in 1971, 1972 and 1979, respectively, it was thought that presence of strains resistant to these fungicides might provide information on the long-term fitness of these strains. The results of an investigation are presented in the second paper. Strains resistant to these fungicides still persisted in the pathogen population. Although this finding may indicate that the long-term fitness of these strains is equal to that of wild-type strains, several other factors might be involved in the persistence of resistance.From 1981 to 1984, S.fuliginea in the Netherlands was primarily controlled with EBIs. The short-term fitness of isolates of S. fuliginea resistant to EBIs and their behaviour under practical conditions are described in the third, fourth and fifth paper.Several fitness parameters of EBI-resistant isolates collected from commercial glasshouses were compared to those of isolates with a wild-type sensitivity to EBIs. Fitness parameters studied were germination of conidia, growth of germ tubes and mycelium, penetration, sporulation and competitive ability. The results are described in the third paper. One or more values of fitness parameters for EBI-resistant isolates were significantly lower than those for the wild-type isolates. However, in general it was concluded that the fitness of EBI-resistant S. fuliginea isolates, collected in commercial glasshouses, was hardly reduced. Within the group of EBI-resistant isolates tested no significant relation was observed between the degree of resistance to EBIs and the degree of fitness.In the fourth and fifth paper the resistance to EBIs of S.fuliginea over a three-year period has been described. In 1981 the sensitivity of glasshouse isolates to EBIs was lower than that of the wild-type isolates. In 1982 and 1983 the sensitivity decreased further. In 1984, no significant differences in sensitivity with the 1983 level were apparent. Per district, the degree of resistance appeared to be positively correlated to the frequency of application of EBIs. Isolates collected in the district of Limburg, where EBIs were applied less frequently than in the district of Pijnacker, showed a significantly higher sensitivity to EBIs than isolates collected in Pijnacker.Triforine showed a very low efficacy in controlling glasshouse isolates. This is in accordance with the experience of growers who, therefore, only use triforine when the disease pressure is low. It is concluded that the decreased sensitivity to triforine can be regarded as resistance. In the case of bitertanol, fenarimol and imazalil, a change to shorter spray intervals has up till now been sufficient to achieve proper control.Contrary to earlier presumptions that development of resistance to EBIs seemed rather unlikely, it gradually developed in normal commercial practice. The slowly decreasing sensitivity to EBIs of cereal powdery mildew is in line with the results described in this thesis. Strategies to delay resistance to EBIs are necessary. Alternation with other site-specific and multi-site fungicides must be advised and can be applied immediately. Non-EBI fungicides are registered for control of cucumber powdery mildew in the Netherlands. Appropriate information and guidance by extension officers and fungicide manufacturers may lower the reluctance of growers to use these strategies.In the long run, control of S. fuliginea might only be maintained by a form of integrated control, which accepts less than perfect disease control. Biological and chemical control, induced resistance of cucumber plants, epidermal coating and mildew-tolerant cucumber cultivars are elements that may fit into a strategy for integrated control of cucumber powdery mildew. Bundeling van artikelen betreffende: het optreden van meeldauw bij komkommers in Nederland en de verspreiding in kasteelten; persistentie van de resistentie tegen dimethirimol, benzimidazolen en pyrazophos; vitaliteit van Sphaerotheca fuliginea-isolaten welke resistent of gevoelig zijn voor ergosterol-biosynthese-remmende fungiciden; de gevoeligheid van S. fuliginea voor fungiciden, in Nederlandse glastuinbouw tussen 1981-1984