Symptoms and Signs

White, powdery fungal growth develops on both leaf surfaces, petioles, and stems (fig. 3).  This growth is primarily asexual spores called conidia.  It usually develops first on crown leaves, on shaded lower leaves and on leaf undersurfaces.  Yellow spots may form on upper leaf surfaces opposite powdery mildew colonies.  Older plants are affected first.  Infected leaves usually wither and die.  Plants may senesce prematurely.  Fruit infection occurs rarely on watermelon and cucumber.  Cleistothecia are dark brown, small (diameter of about 0.003 inches) structures that are barely discernable without a hand lens.  They develop late in the growing season.  The sexual spores within these structures are protected from adverse conditions.

Disease Cycle

Sources of initial inoculum for powdery mildew developing in New York have not been determined.  The primary initial inoculum is believed to be airborne conidia dispersed over long distances from southern states, where cucurbit crops are grown earlier in the year.  Conidia remain viable for 7-8 days based on results from laboratory studies.  The causal fungi are obligate parasites and therefore cannot survive in the absence of living host plants, except as cleistothecia.  Possible local sources of initial inoculum include conidia from greenhouse-grown cucurbits, cleistothecia, and alternate hosts.