Diseases of melon (
Pythium root rot and seedling damping-off
Root rot of melons in Arizona is usually caused by Pythium aphanidermatum, but other Pythium species may also be involved. Pythium species are soil borne organisms that are common in agricultural soils. Pythium aphanidermatum is active when soil temperatures are warm, above about 85 F, and very moist. Motile asexual spores called zoospores are produced in wet soils, and these spores can infect roots within a few hours. A thick-walled sexual spore, produced in infected tissue, can remain inactive in the soil for months or years, germinating only when a susceptible host is present under suitable environmental conditions. Infected roots are brown then become soft and very dark. In seedlings, the roots are rotted quickly and young seedlings fall over or "damp-off" when infected.
Damping-off of seedlings can be reduced by planting into pre-irrigated
soils that are still moist since under these circumstances the soil
is cooler and the seedling begins to grow faster, both of which reduce
chances of infection. Metalaxyl fungicide is effective for controlling Pythium aphanidermatum and can be applied as a soil drench or
through drip lines. It is important to get a confirmed diagnosis before
application of metalaxyl since there have been reports of resistance
with repeated applications. There are no tolerant melon varieties.
July 31, 2003