Powdery Mildew Problems

June 20, 2001

Within the past few days I have been notified of several watermelon fields experiencing problems with powdery mildew. The symptoms first appear as large chlorotic areas in the field with defoliation in the very center of the affected areas. Infected leaves often are turned upside down with the bottom of the leaf facing upward. This causes the vines to appear as if the have been blown around or moved. The powdery sporulation is very difficult to observe in infected leaves. On older, chlorotic leaves, look on the underside of the leaf and hold the leaf at
an angle such that the leaf edge is facing you. You should see a sparse, grayish-white, velvety growth covering the underside of the leaf. On younger leaves that are still upright and green, look for faint, chlorotic
patches on the top of the leaf. Turn the leaf over and the velvety sporulation can be seen beneath the chlorotic spots that were seen on the top side of the leaf. Powdery mildew on watermelon generally occurs
sporadically in fields that are within 2-weeks of harvest and can be very damaging. Fungicides that work best on powdery mildew are Nova, Flint and Quadris and their efficacy is in that same order (see 2001 Pest Control
Handbook page 229). Applications should be made at the very onset of disease (especially with Flint and Quadris). Fields that are already severely infested will not benefit much from curative treatments from any
of these fungicides, however, Nova will have the best curative activity of the three. Look at the May issue of the vegetable team newsletter available on the web at www.ugaveg.org for more information and images of
the disease.

David B. Langston, Jr. University of Georgia Extension Plant Pathologist-Vegetables