Powdery mildew caused by Podosphaera xanthii has affected watermelon in the southeastern United States since the late 1990's. Watermelon is the largest acreage vegetable crop grown in South Carolina, where, currently, over 70% of the watermelons grown are triploid ("seedless") cultivars. Study have shown that triploid watermelon cultivars, on average, were less susceptible to powdery mildew than diploid cultivars.
Because P. xanthii isolates from multiple locations it is insensitive to fungicides in FRAC (Fungicide Resistance Action Committee) Groups 1 (methyl benzimidazole carbamates) and 11 (quinone outside inhibitors), Dr. Anthony Keinath tested fungicides in FRAC Groups 3, 7, 9, 12, 13, and U6 on seedless watermelon Tri-X 313 for their ability to reduce powdery mildew on foliage and prevent yield loss.
Nine powdery mildew-specific fungicides were applied three times in alternation with chlorothalonil, which was applied three times in 2013 and four times in 2014. Control treatments included water and chlorothalonil applied every other week.
In both years, four fungicide treatments, chlorothalonil alternated with triflumizole, fluopyram + tebuconazole, quinoxyfen, or both cyflufenamid and quinoxyfen reduced leaf area covered with powdery mildew compared to both control treatments.
Chlorothalonil alternated with cyflufenamid and quinoxyfen was significantly more effective on lower leaf surfaces than all other treatments. Chlorothalonil alternated with tebuconazole did not differ from chlorothalonil applied every other week.
All 10 fungicide treatments increased the number and weight of marketable-sized fruit in both years compared to the water control treatment. Fungicide active ingredients in FRAC Groups 3, 7, 13, and U6 consistently reduced the severity of powdery mildew on seedless watermelon.
Keinath writes: "In conclusion, of the fungicides applied regularly to watermelon to manage gummy stem blight in the southern and eastern United States, tebuconazole was ineffective against powdery mildew, and cyprodinil, difenoconazole, and fludioxonil resulted moderately effective. If powdery mildew is a concern, cyprodinil + fludioxonil should be applied instead of cyprodinil + difenoconazole. Fluopyram was highly effective against powdery mildew. Although fluopyram + tebuconazole cannot be used during the harvest period, due to the seven-day pre-harvest interval for tebuconazole, the combination would be very useful prior to harvest to manage both gummy stem blight and powdery mildew on watermelon and other cucurbits. In addition to fluopyram, cyflufenamid, quinoxyfen, and triflumizole are recommended to manage powdery mildew on watermelon."
Results are available online since May 23 at:
http://www.sciencedirect.com/science/article/pii/S0261219415300351
Source: Anthony P. Keinath, ‘Efficacy of fungicides against powdery mildew on watermelon caused by Podosphaera xanthii’, September 2015, Crop Protection, Vol. 75: 70-76.
Contacts:
Anthony P. Keinath
Coastal Research and Education Center, Clemson University
Charleston, SC 29414-5329, USA
Email: tknth@clemson.edu