Pathotypic diversity of Hyaloperonospora brassicae collected from Brassica oleracea

Downy mildew caused by Hyaloperonospora brassicae is an economically destructive disease of brassica crops in many growing regions throughout the world. Specialised pathogenicity of downy mildews from different Brassica species and closely related ornamental or wild relatives has been described from host range studies. Pathotypic variation amongst Hyaloperonospora brassicae isolates from Brassica oleracea has also been described; however, a standard set of B. oleracea lines that could enable reproducible classification of H. brassicae pathotypes was poorly developed. For this purpose, we examined the use of eight genetically refined host lines derived from our previous collaborative work on downy mildew resistance as a differential set to characterise pathotypes in the European population of H. brassicae. Interaction phenotypes for each combination of isolate and host line were assessed following drop inoculation of cotyledons and a spectrum of seven phenotypes was observed based on the level of sporulation on cotyledons and visible host responses. Two host lines were resistant or moderately resistant to the entire collection of isolates, and another was universally susceptible. Five lines showed differential responses to the H. brassicae isolates. A minimum of six pathotypes and five major effect resistance genes are proposed to explain all of the observed interaction phenotypes. The B. oleracea lines from this study can be useful for monitoring pathotype frequencies in H. brassicae populations in the same or other vegetable growing regions, and to assess the potential durability of disease control from different combinations of the predicted downy mildew resistance genes

Histological responses of host and non-host plants to Hyaloperonospora parasitica

Differences in Hyaloperonospora parasitica development and plant tissue responses were compared for 10 cruciferous hosts (including both resistant and susceptible genotypes), 3 leguminous and 1 graminaceous non-host species. Cotyledons, or true leaves in the case of Triticum aestivum and Pisum sativum, were studied at 2, 8, 24 h and 3, 5, 7 days post inoculation (dpi). The high levels of zoosporangial germination observed on all species tested, as well as on glass slides, suggested that inhibition of germination did not play a significant role in distinguishing host versus non-host resistance. During the early stages of infection, at spore germination and host penetration, there was no evidence of a clear-cut difference between Brassica host species which displayed a hypersensitive, partially resistant or susceptible reaction compared with non-host species. Haustoria formation was the key infection phase for the establishment of biotrophy. Across all tested species, haustoria were initiated inside the epidermal cells. However, there were significant differences in the frequency and timing of haustorial formation and the final size of haustoria among the tested species at early infection stage. Fully developed haustoria were never observed in Raphanus raphanistrum, Triticum aestivum, Lupinus angustifolius nor Trifolium subterraneum. Instead, the haustorium development appears to abort in the penetrated epidermal cells of these species. Although haustoria were formed in the epidermal and mesophyll cells of Sinapsis alba and Pisum sativum, subsequent hyphal growth and/or continued haustoria formation were rare or few, respectively. Hypersensitive reaction was the key resistance response observed among the host and non-host resistant species tested. It is noteworthy that, in the initial stages of pathogenesis, there was no differentiating point that separated the non-host species from those that were hosts