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Be prepared for cucurbit powdery mildew

Posted: July 18, 2012

In-season cucurbit powdery mildew management is most effective when initiated early (one powdery mildew colony per 50 leaves).
Signs of powdery mildew on summer squash (Photo: Beth K. Gugino)

Signs of powdery mildew on summer squash (Photo: Beth K. Gugino)

The first sign of the pathogen is white powdery fungal growth on both the upper and/or lower leaf surface of the older leaves. If left unmanaged, severely infected leaves can die leading to reduced fruit size, quality and sunburn. Although powdery mildew does not infect the fruit, it can infect the handles thus reducing the overall marketability of the pumpkin. I have seen cases when powdery mildew presents its self as a chlorotic/yellow spot on the upper surface of the leaf and it is not until you look at the underside of the leaf that you see the white sporulation.

There is considerable concern over the development of fungicide resistance with powdery mildew. For resistance management, it is best to start applying the most effective products when you first start seeing symptoms (1 lesion on 50 leaves) and then later in the season when switch to a protectant spray program rather than the reverse. In the long-run this will reduce the selection pressure for powdery mildew spores that are resistant to the fungicide because fewer spores are exposed to the active ingredient when disease severity is low.

For powdery mildew, the suggested alternation for pumpkins and winter squash is Quintec (FRAC code 13), Pristine (FRAC codes 11+7), Quintec, Procure (FRAC code 3), etc. Each product should be applied at the highest labeled rate and tank mixed with a protectant fungicide. There is increasing concern about the development of powdery mildew resistance to FRAC code 3 fungicides which includes Procure. Management failures have been documented in several Mid-Atlantic states and in the 2011 Fungicide Resistance Management Guidelines for Curcurbit Downy and Powdery Mildew Control in the Northeast United States, FRAC code 3 fungicides are now color coded red.

Gowan Company has just received EPA registration for its new fungicide Torino. The active ingredient is cyflufenamid and is in the FRAC group U6. It has performed well in University trials and offers another mode of action that can be included in powdery mildew programs to help manage for resistance. State registrations are currently pending.