Disease severity keys are described and illustrated for powdery mildew (Erysiphe pisiDC.) and downy mildew (Peronospora viciae(Berk.) Caspary) of pea (Pisum sativumL.), and powdery scab(Spongospora subterranea(Wallr.) Lagerh. f. sp.subterraneaTomlinson) of potato (Solanum tuberosumL.). The keys consist of illustrations of pea leaves or potato tubers in 11‐point (0–10) severity scales of arbitrary proportions of surface areas diseased. The keys were constructed from hand drawings 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 using computer‐assisted image analysis. The drawings were then scanned into computer storage for manipulation into illustrated keys for publication. The complete keys, or abbreviated versions, have been used for assessing severity of the diseases in recent research on crop loss assessment, disease control, susceptibility assessment, and disease epidemiology. They also provide bases for accurate disease severity assessment in commercial pea and potato crops. Copies of the disease keys are available from the senior author.