DISEASE SEVERITY KEYS FOR POWDERY AND DOWNY MILDEWS OF PEA, AND POWDERY SCAB OF POTATO

Disease severity keys are described and illustrated for powdery mildew (Erysiphe pisi DC.) and downy mildew (Peronospora viciae (Berk.) Casp ary) of pea (Pisum sativum L.), and powdery scab (Spongospora subterra nea (Wallr.) Lagerh. f. sp. subterranea Tomlinson) of potato (Solanum tuberosum L.). The keys consist of illustrations of pea leaves or pota to tubers in 11-point (0-10) severity scales of arbitrary proportions of surface areas diseased. The keys were constructed from hand drawing s of pea leaflets or potato tubers, with colonies or lesions drawn to resemble typical infections of the respective diseases. Proportions of surface area of plant parts ''diseased'' on drawings were confirmed u sing computer-assisted image analysis. The drawings were then scanned into computer storage for manipulation into illustrated keys for publi cation. The complete keys, or abbreviated versions, have been used for assessing severity of the diseases in recent research on crop loss as sessment, disease control, susceptibility assessment, and disease epid emiology. They also provide bases for accurate disease severity assess ment in commercial pea and potato crops. Copies of the disease keys ar e available from the senior author.