Diseases of watermelon (
Powdery mildew
Powdery mildew of watermelon is caused by Podosphaera xanthii (Sphaerotheca fuliginea). Disease is found wherever watermelon and melons are grown in Arizona; it is favored by moderate to high humidity and low light intensity. A white powdery growth on the leaf surface starts out as small spots, often on the lower leaf surface, and gradually covers the entire leaf. The mycelium of the fungus grows over the leaf surface and produces many asexual spores that are easily carried in the wind (photo 1). These spores germinate on the leaf surface where new infections develop within a few days. Advanced infections cause necrotic areas in leaves that can become extensive and result in a severe decline in foliage.
Registered fungicides used for control include sulfur dust, micronized sulfur, thiophanate methyl, chlorothalonil, mancozeb, myclobutanil and azoxystrobin. Disease incidence may be reduced when foliar feeds of monopotassium phosphate, potassium nitrate or potassium bicarbonate are applied. Early detection and treatments are important since control is very difficult once disease develops.
For more information on powdery mildews see:
http://ag.arizona.edu/pubs/crops/az1033.pdf
Reference to new pathogen name:
http://cals.arizona.edu/pubs/crops/az1419/9_WEB.PDF
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January 29, 2007