Update on Downy and Powdery Mildews in Fall Cucurbits

In the past few weeks, downy mildew (DM) has been reported on several cucurbit crops in Kentucky. In central KY, DM has been reported on cucumber, cantaloupe, butternut squash, cushaw, and pumpkin. Western and eastern Kentucky have confirmed reports on cucumber and pumpkin, and are at risk from disease pressure in watermelon and cantaloupe from neighboring states.

Cucurbit growers raising fall crops, particularly pumpkin, should act now to protect their plants for the remainder of the season. DM can destroy foliage very quickly; it thrives in rainy, humid weather, particularly from the South.

At the same time, in areas that have had intermittent dry weather, powdery mildew (PM) is also being reported on cucurbits, especially pumpkin. While PM may move a bit more slowly, it can still damage yields, and result in sunburn and in breakage of the pumpkin “handle,” all of which hurt jack o-lantern profits.

Figure 1. Butternut squash affected by both downy mildew (DM) and powdery mildew (PM). DM causes yellow, “window-pane” lesions on the upper surface of leaves, while PM shows white-gray growth on the top of leaves. (Photo: Emily Pfeufer, UK)

Figure 1. Butternut squash affected by both downy mildew (DM) and powdery mildew (PM). DM causes yellow, “window-pane” lesions on the upper surface of leaves, while PM shows white-gray growth on the top of leaves. (Photo: Emily Pfeufer, UK)

Figure 2. Dark gray-purple downy mildew sporulation occurs only on the underside of leaves and does not spread over leaf veins. White powdery mildew sporulation may occur on the tops or bottoms of leaves and easily spreads over leaf veins.(Photo: Emily Pfeufer, UK)

Figure 2. Dark gray-purple downy mildew sporulation occurs only on the underside of leaves and does not spread over leaf veins. White powdery mildew sporulation may occur on the tops or bottoms of leaves and easily spreads over leaf veins.(Photo: Emily Pfeufer, UK)


Quick Facts About Downy and Powdery Mildews on Cucurbits

Downy Mildew

Powdery Mildew

Thrives in wet, rainy weather Thrives in dry but humid weather
Dark sporulation only on bottoms of leaves and edged by leaf veins White-gray sporulation on tops and bottoms of leaves, grows over veins
Defoliates plants in a matter of days Prevalent on leaves, does not infect fruit
Fungicides: Ranman, Previcur Flex, Tanos, or Zampro. Tank-mix with mancozeb or chlorothalonil Fungicides: Quintec, Torino, Vivando, Procure, Pristine, or Fontelis. Tank-mix with mancozeb or chlorothalonil

Management With Fungicides

Protectant fungicides (chlorothalonil, mancozeb) used up to this point are minimally effective on both downy and powdery mildews, yet should be used as tank-mix partners with fungicides that have specific activity for the diseases of concern. As shown in Figures 1 and 2, downy and powdery mildews can be co-occurring, so fields with both diseases require active management. Sample spray programs are suggested below:

Downy Mildew Present

Powdery Mildew Present

Tanos tank-mixed with chlorothalonil Quintec tank-mixed with mancozeb
7 days later, Previcur Flex tank-mixed with mancozeb 7 days later, Torino tank-mixed with chlorothalonil
Repeat above Repeat above

Any of the DM and PM specific fungicides listed in the “Quick facts” (above) can replace those defined in the programs above, but it is important to alternate two different modes of action for each managed disease. Use the highest labeled rate of the specific fungicides.

In fields with both diseases, apply the first DM spray, then follow 3 to 4 days later with the first PM spray. Follow the PM spray with the second DM spray 3 to 4 days later, then apply the second PM spray 3 to 4 days after that. Repeat this program as long as disease pressure remains high or until 10 days preharvest. In resource-limited situations, downy mildew management should take precedence over powdery mildew management.

Follow all label instructions, including warnings about tank-mix partners, requirements for safe application, and preharvest intervals.


For Additional Information

  • Vegetable Production Guide for Commercial Growers (ID-36)(Vining Crops section, pg 116)
  • IPM Scouting Guide for Cucurbit Crops (ID-91)
  • Cucurbit Downy Mildew Forcasting (IPM pipe website) (useful for monitoring the spread of cucurbit downy mildew and estimating risk on individual farms)

 

By Emily Pfeufer, Extension Plant Pathologist and Shubin K. Saha, Extension Horticulturist

 

Posted in Vegetables