Management of Powdery Mildew of Cucurbits.

Powdery mildew of cucurbits, caused by the fungus Sphaerotheca fuliginea, is an important disease of cucurbit crops in Illinois and worldwide. This disease can result in serious losses on muskmelon, pumpkin, and squash. Most of the cucumber varieties are resistant, and watermelons are rarely affected. The fungus is an obligate parasite. Spores dispersed over long distance from alternate hosts are the primary source of inoculum.

Powdery mildew is diagnosed by white, powdery mold on plant tissues. The disease first appears on lower stems and petioles (Figure 1). As the disease continues to develop, the white, moldy spots occur on the underside of leaves (Figure 2). Symptoms on the upper leaf surfaces (Figure 3) usually signal an outbreak.

Powdery mildew can be managed effectively by planting resistant cultivars, crop rotation, and application of fungicides. Resistance in the plants is usually partial and may require additional complementary control practices. Rotations with non-cucurbit crops will help prevent serious early season epidemics. Fungicide application is a common control practice of powdery mildew of cucurbit crops.

Several fungicides are effective against powdery mildew. Timing of application of fungicides is very important; they should be applied before symptoms develop. Fungicides most commonly recommended for control of powdery mildew are strobilurins (for example, Quadris, Flint, and Cabrio) and DMI (strole-inhibiting) fungicides (such as Nova and Procure).

Development of resistance in powdery mildew fungi to fungicides is a common phenomenon, and resistance to both strobilurin and DMI fungicides in S. fuliginea has already been reported.

Resistance management to strobilurins and DMI could be improved by application of strobilurins and DMI fungicides mixed with contact fungicides such as Microthiol Disperss. Also, to prevent rapid development of resistance in powdery mildew fungi against DMI fungicides, these fungicides should be applied at the manufacturer’s higher label rates and shorter application intervals.

The most effective approach for managing powdery mildew of cucurbits is IPM strategies using plant resistance, cultural practices, and fungicide treatments.