Powdery Mildew of Cucurbits
M. Babadoost
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 less
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.