Use of sporulation and other leaf and vine traits for evaluation of resistance to downy mildew in cucumber

A higher level of downy mildew resistance is needed in cucumber (Cucumis sativus L.). One method for doing that would be to combine different mechanisms of resistance into a single cultivar. Disease symptoms are expressed as necrotic spots, chlorotic areas, amount of sporulation on the leaf underside, and Stunting of plant growth. Methods for evaluation of necrosis, chlorosis, and sporulation are available, but sporulation requires a lot of effort to evaluate, requiring spore counts oil a hemacytometer under a compound microscope. We evaluated two faster methods: a subjective rating under a dissecting scope, and a subjective rating with the unaided (naked) eye. The naked eye was the fastest method, and it also had the lowest error variation in an experiment run using susceptible 'Coolgreen' grown in the field. Leaves sampled from the base of the plants were sporulating the most compared with those taken from the middle or tip of the plant. A set of 65 diverse cultivars was evaluated in the field for necrosis, chlorosis, sporulation and stunting. Necrosis and chlorosis rating were highly correlated with each other. However, resistance to sporulation and stunting by downy mildew appeared to be relatively independent of necrosis and chlorosis. It may be possible to develop higher resistance by combining genes for low necrosis with those for low sporulation or with low stunting.