While cantaloupe and pumpkin growers are used to combating powdery mildew in Indiana, watermelon growers may not be familiar with the disease. Occasionally, I observe this disease on watermelon as well. If left uncontrolled, this disease can cause loss of foliage, loss of yield and lower quality fruit. This article will discuss the biology and management of powdery mildew of cucurbits with an emphasis on watermelon.
While powdery mildew often causes a white talc-like growth on either side of the leaf, on watermelon the symptom may show up as a chlorotic lesion on the upper side of the leaf (Figure 1). The talc-like growth on the lower side of the leaf may be more idiffuclut to observe than on other hosts. Occasionally, powdery mildew may be observed on watermelon fruit (Figure 2). This article https://vegcropshotline.org/powdery-mildew-symptoms-vs-variegated-leaves/ has additional information about powdery mildew symptoms. The fungus that causes powdery mildew, Podosphaera xanthii, does not require leaf wetness for infection of leaves, only high humidity. The optimum temperature for disease development is 68 to 81°F. P. xanthii may survive for a period in crop residue as a resilient fungal structure, but the disease is so easily windborne, that crop rotation is not always a practical control measure.
Figure 1. Powdery mildew may cause a round, chlorotic lesion on the top of the leaf, whereas, the white fungus may often be observed on the bottom of the leaf.
Figure 2. Although unusual in Indiana, powdery mildew can cause infections on watermelon fruit as seen here.
The fungus that causes powdery mildew of cucurbits does not cause powdery mildew on other plant families. In the same way, powdery mildew of other plant families does not affect cucurbits.
Fortunately, commercial varieties of pumpkin and cantaloupe exist with partial resistance to powdery mildew. Most growers, however, find it necessary to apply systemic fungicides to manage powdery mildew, even when using partially resistant varieties. As far as I know, there are no differences in the susceptibilities of watermelon varieties to powdery mildew.
To avoid additional sprays, watermelon growers who are worried about powdery mildew and wish to apply fungicides may want to choose products that are effective on more diseases than just powdery mildew. For example, Luna Experience® should be effective on powdery mildew and gummy stem blight. Merivon®should be effective on anthracnose and powdery mildew. Aprovia Top® has proven effective on powdery mildew of cucurbits plus it is labeled on anthracnose and gummy stem blight. However, I don’t have any information about how effective Aprovia Top® is on the latter two diseases.
Inspire Super®, may not be as effective as the products listed above on powdery mildew, however it should be effective against both anthracnose and gummy stem blight. Similarly, fungicides with the active ingredient tebuconazole (e.g., Monsoon®, Onset®, Toledo®, Vibe®) should have good efficacy against gummy stem blight and moderate efficacy against powdery mildew.
More information about powdery mildew management can be found in the Midwest Vegetable Production Guide 2017 mwveguide.org. A proposed fungicide schedule can be found at purdue.ag/pumpkinfs. More information about powdery mildew can be found at https://vegcropshotline.org/article/powdery-mildew-of-cucurbits/. Finally, always read the label.