Powdery Mildew on Cucurbits

Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu

Symptoms typically begin on older, lower leaves and can spread rapidly under dry, humid conditions. Control of powdery mildew begins with regular scouting for symptoms and weekly fungicide applications. Begin a fungicide program when PM has been found in the region and/or when 1 lesion is found on the underside of 45 leaves. Fungicide resistance management of the fungus which causes powdery mildew is critical in the Mid-Atlantic region! Fungicides with a high risk for resistance development, such as the strobilurin fungicides (Pristine, FRAC code 11) and Rally or Procure (FRAC code 3), should be tank mixed with a protectant fungicide such as chlorothalonil (M5) and rotated with fungicides of a different chemistry.

The following are some fungicide recommendations for control of powdery mildew in a variety of crops:

To control powdery mildew in pumpkin and winter squash:
Alternate:
Nova or Rally (myclobutanil, 3) at 5.0 oz 40WP/A plus chlorothalonil at 2.0-3.0 pt 6F/A
or
Procure (triflumizole, 3) at 4.0-8.0 oz 50WS/A plus chlorothalonil at 2.0-3.0 pt 6F/A

With:
Micronized Wettable Sulfur (M2) at 4.0 lb 80W/A; Sulfur may injure plants especially at high temperatures. Certain varieties can be more sensitive. Consult label for precautions.
or
chlorothalonil plus Pristine (pyraclostrobin + boscalid, 11 + 7) at 12.5-18.5 oz 38WG/A

If powdery mildew has become well established in the mid to late part of the season, only apply protectant fungicides such as chlorothalonil or sulfur or Quintec* (quinoxyfen, 13) at 6.0 oz 2.08F/A plus chlorothalonil at 2.0-3.0 pt 6F/A.

*Quintec (quinoxyfen, FRAC code 13) from Dow AgroSciences has a section 3 supplemental label for powdery mildew control on pumpkin, winter squash and gourd. The label is available at http://www.rec.udel.edu/update09/Quintec.pdf.

To control powdery mildew in summer squash and cucumbers:
Alternate:
Nova or Rally (myclobutanil, 3) at 5.0 oz 40WP/A plus chlorothalonil at 2.0-3.0 pt 6F/A,
or
Procure (triflumizole, 3) at 4.0-8.0 oz 50WS/A plus chlorothalonil at 2.0-3.0 pt 6F/A

With:
chlorothalonil plus Pristine (pyraclostrobin + boscalid, 11 + 7) at 12.5-18.5 oz 38WG/A

To control powdery mildew in muskmelon and watermelon:
Alternate:
Nova or Rally (myclobutanil, 3) at 5.0 oz 40WP/A plus chlorothalonil at 2.0-3.0 pt 6F/A
or
Procure (triflumizole, 3) at 4.0-8.0 oz 50WS/A plus chlorothalonil at 2.0-3.0 pt 6F/A

With:
Quintec (quinoxyfen, 13) at 6.0 oz 2.08F/A plus chlorothalonil at 2.0-3.0 pt 6F/A
or
Pristine (pyraclostrobin + boscalid, 11 + 7) at 12.5-18.5 oz 38WG/A plus chlorothalonil at 2.0-3.0 pt 6F/A

For more information on control of powdery mildew of cucurbits please see the Delaware Commercial Vegetable Production Recommendations.

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