Powdery mildew (caused by the fungus named Podosphaera xanthii) was consistently among the most prevalent diseases of watermelon in a survey of foliar diseases done in 2015 and 2016, second only to gummy stem blight.1 As no resistant cultivars of watermelon are available, specific fungicides are necessary to manage the disease.
Symptoms and Signs
The first symptoms of powdery mildew on watermelon are yellow spots on the leaves (figure 1) that reduce photosynthesis. Next, white colonies of fungus growth and spores form (figure 2) on both the top and the bottom of leaves. Symptoms are found on leaves and petioles and, rarely, on fruits. In severe cases, most of the leaf surface will be covered with powdery mildew, leading to leaf death and smaller or sunburned fruit. In greenhouses, powdery mildew can be a serious problem on seedlings of watermelon, as well as hybrid squash and bottle gourd rootstocks used in grafting.
How Powdery Mildew Spreads
Powdery mildew overwinters on cucurbits in southern Florida and other places without frost. Spores are produced in chains on the leaf surface (figure 3). Mature spores break off and are spread by wind. When they land on a leaf of a suitable host like watermelon, the spores germinate and penetrate the leaf surface. After extracting nutrients and growing across the leaf surface, powdery mildew forms new spores. This cycle repeats itself many times during the growing season. At 68 to 80°F, powdery mildew can develop quickly, and symptoms can appear within three days of infection. Spores remain alive up to eight days. Unlike many other foliar diseases of watermelons, such as gummy stem blight, anthracnose and downy mildew that thrive in wet conditions, dry conditions promote spread of powdery mildew (figure 3).
Cucurbit powdery mildew also grows on Verbena bonariensis (tall verbena), a tender perennial naturalized in the southeastern United States (figure 4). As powdery mildew often shows up on verbena before watermelon, verbena can be used as an indicator plant to time the first fungicide application.
Fungicides Recommended to Manage Powdery Mildew
Fungicides may be necessary to protect the yield and quality of watermelon crops affected with powdery mildew.2 The yield increase, up to 75%, on sprayed crops compensates for the fungicide cost and generates extra profits from the crop.
The powdery mildew fungus has a relatively high risk of becoming resistant to fungicides. Resistance to Quintec and Torino has occurred in New York.3,4 Always use at least two different fungicides in rotation when spraying watermelon and other cucurbits for powdery mildew (table 1).
Table 1. Recommended fungicides to manage powdery mildew.
|FRAC Group||Risk of Resistance|
|7 + 3||
Medium to High
|Switch||9 + 12||
Low to Medium
*Note: Maximum of two applications per crop.
Organic producers can choose from the following biofungicides allowed in organic production that are effective against powdery mildew: sulfur, potassium bicarbonate, and Organocide 3-in-1 Garden Spray.5
- Rennberger G, Gerard P, Keinath AP. Occurrence of foliar pathogens of watermelon on commercial farms in South Carolina estimated with stratified cluster sampling. Plant Dis. 2018 Sep; 102(11): 2285-2295. doi:10.1094/PDIS-03-18-0468-RE.
- Keinath AP. Efficacy of fungicides against powdery mildew on watermelon caused by Podosphaera xanthii. Crop Prot. 2015 Sep; 75: 70-76. doi:10.1016/j.cropro.2015.05.013.
- McGrath MT. First report of resistance to quinoxyfen in Podosphaera xanthii, causal agent of cucurbit powdery mildew in the United States. Plant Health Prog. 2017 May; 18(2): 94. doi:10.1094/PHP-03-17-0018-BR.
- McGrath MT, Sexton ZF. Poor Control of cucurbit powdery mildew associated with first detection of resistance to cyflufenamid in the causal agent, Podosphaera xanthii, in the United States. Plant Health Prog. 2018 Aug; 19(3): 222-223. doi:10.1094/PHP-06-18-0029-BR.
- Keinath AP, DuBose VB. Controlling powdery mildew on cucurbit rootstock seedlings in the greenhouse with fungicides and biofungicides. Crop Prot. 2012 Dec; 42: 338-344. doi:10.1016/j.cropro.2012.06.009.
Keinath AP, Miller G. Watermelon fungicide guide for 2019. Land-Grant Press by Clemson Extension. 2019; LGP 1001. http://lgpress.clemson.edu/publication/watermelon-fungicide-guide-for-2019.