Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu
Cucurbit Powdery Mildew
Powdery mildew has been found on zucchini in New Jersey. Usually powdery mildew occurs from mid-July until the end of the season. 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 strobilurins (Pristine, FRAC code 11) and Nova 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.
For control of cucurbit powdery mildew in:
Pumpkin and winter squash fields:
Nova or Rally (myclobutanil, 3) at 5 oz 40WP/A plus chlorothalonil at 2-3 pt 6F/A,
or
Procure (triflumizole, 3) at 4-8 oz 50WS/A plus chlorothalonil at 2-3 pt 6F/A
Alternated with:
Micronized Wettable Sulfur (M2) at 4 lb 80W/A Sulfur may injure plants, especially at high temperatures. Certain varieties can be more sensitive. Consult label for precautions.
or
A tank mix containing 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.
Summer squash and cucumber fields:
Nova or Rally (myclobutanil, 3) at 5 oz 40WP/A plus chlorothalonil at 2-3 pt 6F/A,
or
Procure (triflumizole, 3) at 4-8 oz 50WS/A plus chlorothalonil at 2-3 pt 6F/A
Alternated with a tank mix containing:
Chlorothalonil plus Pristine (pyraclostrobin + boscalid, 11 + 7) at 12.5-18.5 oz 38WG/A
Muskmelon and watermelon fields:
Nova or Rally (myclobutanil, 3) at 5 oz 40WP/A plus chlorothalonil at 2-3 pt 6F/A,
or
Procure (triflumizole, 3) at 4-8 oz 50WS/A plus chlorothalonil at 2-3 pt 6F/A
Alternated with a tank mix containing:
Quintec (quinoxyfen, 13) at 6 oz 2.08F/A plus chlorothalonil at 2-3 pt 6F/A,
or
Chlorothalonil plus Pristine (pyraclostrobin + boscalid, 11 + 7) at 12.5-18.5 oz 38WG/A
For more information on control of powdery mildew of cucurbits please see the 2008 Delaware Commercial Vegetable Production Recommendations Guide.
Cucurbit Downy Mildew
Continue to check the website (http://www.ces.ncsu.edu/depts/pp/cucurbit/forecasts/c080625.php?month=06&year=08) for movement of downy mildew. The latest find in the South has been Reidsville, GA on butternut squash and slicing cucumber in a regional sentinel plot. There are reports of downy mildew from GA, SC and FL. The threat of movement north is very low as of June 26.