Abstract
Viticulture is continuously suffering significant economic loss from downy mildew epidemics. Although the infection biology of the pathogen Plasmopara viticola is well understood, little is known regarding the population diversity of this oomycete and on the mechanisms responsible for compatible or incompatible reactions with different host genotypes. The discrimination of strains with different virulence is a fundamental step for the assessment of pathotypes in local populations and could help to develop measures for preventing economic loss. We here describe an extended and slightly modified bioassay for the assessment of virulence by means of sporulation intensity. Simultaneously, the necrotic reaction of host genotypes is considered and allows identification of different resistance strategies. Comparison of infections symptoms of 12 host genotypes after inoculation with five different single sporangium strains of Plasmopara viticola showed that: 1. resistance diversity is high in some Vitis vinifera. Cultivars; 2. Vitis riparia showed the strongest resistance amongst the four tested North American species; 3. Asian Vitaceae harbor fully resistant as well as highly susceptible genotypes. In addition, microscopic investigation of inoculated leaf discs from Vitis amurensis and Ampelopsis japonica unraveled, that despite lack of sporulation, a mycelium may grow to a certain point in resistant hosts. The necrotic reaction in V. amurensis indicates a different mechanism of resistance when compared to A. japonica, where no necrosis was found.
Highlights
The downy mildew of grapevine is one of the most destructive diseases in viticulture resulting in severe epidemics and enormous economic costs
The leaf disc assays with an extended range of Vitis genotypes showed that in some cases (e.g. V. riparia, or V. aestivalis) the evaluation, when based predominantly on the degree of sporulation, would classify some phenotypes in the same category, the necrotic reaction of the host clearly indicated differences in the host-pathogen interaction (Figure 2)
Using the five previously categories established for pathogen aggressiveness ranging from unlimited (A) to no sporulation (E), the resistance reaction of the plant was categorized with four additional classes that ranked necrosis from strong (+++) to absent ( ) (Table 1)
Summary
The downy mildew of grapevine is one of the most destructive diseases in viticulture resulting in severe epidemics and enormous economic costs. Due to the lack of natural resistance in the European grapevine Vitis vinifera against the new pathogen, chemical measures for disease control soon became necessary. Increasing amounts of fungicides and multiple applications throughout the season are necessary for adequate disease control [2] and fungicide resistance is frequently found in pathogen populations of commercial vineyards [3]. Considering the economic impact of P. viticola, there is still very limited knowledge on mechanisms relevant for the pathogenesis of this oomycete, and the development of alternative control methods for an integrated pest management in grapevine downy mildew would be highly desirable [6]
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