Downy mildew of Cucurbits (Pseudoperonospora Cubensis): the Fungus and its hosts, distribution, epidemiology and control

Taxonomy of the genus Pseudoperonospora, morphology of Pseudoperonospora cubensis (Berk, et Curt.) Rostow. and occurrence of its oospores, are described briefly. A list is presented of over 40 cucurbitaceous host species, representing about 20 genera, on which P. cubensis has been recorded. Two or more races exist in Japan and the United States, but not in Europe or the Middle East. The distribution of P. cubensis is widest on all continents on cucumbers (70 countries) and muskmelon (50 countries); on Cucurbita and watermelons it extends to about 40 and 25 countries, respectively. P. cubensis may overwinter as oospores, though this seems rare, and on wild hosts or crops grown in the open or under cover. Airborne sporangia may also reach cooler countries from regions with mild winters. Apart from the leaf wetness essential for infection, the factors determining disease progress are: rate of foliage growth and physiological age of the host; amount of primary inoculum available, light, and the rate at which lesions necrotize. The interaction of these factors is described for early, mid-season, and late crops. Losses caused by P. cubensis depend on the growth stage at which the crop is attacked, and on the rate of foliage and pathogen development. Breeding has produced downy mildew resistant lines of cucumbers, used chiefly in the United States, and some resistant lines of melons and watermelons. The most important agricultural practices used to restrict downy mildew development are proper irrigation management and avoidance of sowing in proximity to infected crops. Success of control by protectant chemicals depends largely on proper timing of applications. Proximity of inoculum sources, hours of leaf wetness, age of crop, and irrigation practices are the principal factors that determine when to begin treatments. These factors and rate of leaf formation determine the frequency of applications. Application of systemic fungicides is much easier to time correctly.