Diseases of melon (
Powdery mildew
Powdery mildew of melons is caused by Podosphaera xanthii (Sphaerotheca fuliginea). Disease is found anywhere cantaloupe, honeydew and other 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 (photo 1). The mycelium of the fungus grows over the leaf surface and produces many asexual spores that are easily carried in the wind (photo 2). 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. Photosynthetic area is reduced and fruit exposed to sun burning. Variability in pathogenicity has been demonstrated in the fungus, and several races have been described based on host range. Podosphaera xanthii ( Sphaerotheca fuliginea) has two mating types which is probably why the sexual spores of the fungus are rarely found.
Registered fungicides used for control include sulfur dust and micronized sulfur; strobulurin fungicides azoxystrobin (Quadris), pyraclostrobin (Cabrio) and trifloxystrobin (Flint); thiophanate-methyl (Topsin-M); chlorothalonil (Bravo); quinoxyfen (Quintec); triflumizole (Procure); and mancozeb. Disease incidence may be reduced when foliar applications of neem extract (Trilogy) or potassium bicarbonate products (Armicarb 100, Kaligreen) are applied. Fungicides with the same mode of action such as the strobilurins should not be used in rotation with each other. Results of effective rotations and fungicide efficacy are available from yearly field trials in Yuma by Matheron and Porchas (http://cals.arizona.edu/pubs/crops/az1419/9_WEB.PDF).
Early detection and treatments are important since control is very difficult once disease develops. Varieties with some tolerance to Podosphaera xanthii (Sphaerotheca fuliginea) are available.
For more information on powdery mildews see:
http://ag.arizona.edu/pubs/crops/az1033.pdf
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December 27, 2007