Diseases of watermelon (
Pythium root rot and seedling damping-off
Pythium root rot of watermelon in Arizona is usually caused by Pythium aphanidermatum, but other Pythium species also may be involved. In mature plants, infected roots are brown then become soft and dark. Plants are wilted, stunted and/or chlorotic, and at high temperatures in mid summer may collapse quickly (photo 1). In seedlings, the roots rot quickly, and young seedlings fall over or "damp-off" when infected.
Pythium species are soil borne organisms in the oomycete group. They are common in agricultural soils in Arizona. They produce motile asexual spores called zoospores in wet soils, and these spores can infect roots within a few hours. A thick-walled sexual spore produced in infected tissue remains inactive in the soil for months or years, germinating when a susceptible host is present under suitable environmental conditions. Pythium aphanidermatum is active in wet soils when soil temperatures are warm, above about 85 F.
In Arizona, Pythium root rot and damping off are more common in furrow irrigation as opposed to drip irrigation since free water in the furrows incites zoospore production and spread, and chemicals are more difficult to apply effectively. Damping-off of seedlings can be reduced by planting into pre-irrigated
soils that are still moist. Under these circumstances, the soil
is cooler and seedlings grow faster, reducing chances of infection. Mefanoxam fungicide (Ridomil Gold) is effective for controlling Pythium aphanidermatum and can be applied as a soil drench or
through drip lines. Early application is critical, and preventive fungicide treatments may be warranted in fields with a known history of disease. Pythium aphanidermatum is not a pathogen of common Arizona crops such as cotton, alfalfa, small grains or sorghum, and infested fields should be rotated out of watermelon.
1 
July 1 , 2008