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Limited fruit set and fruit abortion in winter squash and pumpkins

Author: Natalie Hoidal, UMN Extension horticulture educator

We're seeing some cases of low fruit set and / or fruit abortion in pumpkins and winter squash. If this is happening on your farm, there are a couple of potential explanations:

1. Pollination

Pumpkins and squash are dependent upon insect pollination. Male flowers bloom about a week before female flowers, and flowers only bloom for a few hours in the morning, so it's crucial that conditions are right for pollination. Penn State Extension has an excellent, in-depth article on pollinators in pumpkins and squash. Some of the highlights include: 
  • Bumblebees and squash bees (Peponapis pruinosa) are the best pollinators of pumpkins since they forage in the morning, and because squash bees have a lifecyle that's perfectly timed with the lifecycles of cucurbits. Cucurbits include pumpkin, squash, melons, cucumbers, and zucchini. 
  • Creating habitat on your farm for nesting sites may help boost pollination on your farm. For bumblebees, this means consistent floral blooms, ideally with native plants throughout the growing season. For squash bees, which nest in the soil at a depth of 5-10 inches, this means minimizing tillage in pumpkin fields. 
Keep in mind that pesticide applications can also negatively impact pollinators. If you're spraying for cucumber beetle or squash bugs, try to time applications so as to avoid flowering times. 

Photo: Natalie Hoidal
2. Planting density

If your plants are too close together, they'll experience competition for both light and nutrients. In some of the fields where we've seen low fruit set, plants were too close together. When plants have to compete for sunlight, photosynthesis is reduced, which in turn reduces growth and development. Ideal spacing for pumpkins is: 

Compact / bush varieties: plants 18-24 inches apart with 4-6 foot rows (4-6 pounds seed / acre)
Miniature pumpkins: plants 2 feet apart with 6-8 foot rows
Vining varieties: plants 2-5 feet apart with 6-8 feet per row (2-3 pounds seed / acre)

At this point in the season, thinning pumpkins (especially vining varieties) is impractical and may not make a difference. Spacing recommendations are from the Midwest Vegetable Production Guide for Commercial Growers and this publication from University of Illinois Extension.

3. Too much nitrogen / N applied at the wrong time

Too much nitrogen can delay fruiting, and cause plants to put energy towards vegetative growth rather than reproductive growth (growing vines and leaves rather than fruit). While a split application is recommended for pumpkins - 1/2 N applied at planting, 1/2 applied when plants start to vine, applying too late once plants have begun to set fruit could prove detrimental to fruit production. Make sure to start your season with a soil test, and make fertilization decisions accordingly. For more complete fertilizer recommendations based on your soil test, consult the Nutrient Management Guide for Commercial Fruit and Vegetable Growers in Minnesota. 

4. Water stress

Water stress, either too much or too little water, can also cause these symptoms. In particular, drought stress can result in a skewed ratio of male to female flowers, and flooding prevents the roots from uptaking oxygen and other nutrients.

Additionally, since flowers only bloom for a few hours, conditions have to be just right for pollination. if flowers bloom during heavy rainfall (which we've had a lot of), pollinators are unlikely to be out pollinating. 

5. High temperatures
For all cucurbits, high temperatures during fruiting can cause fruiting problems. Daytime temperatures in the 90s or nighttime temperatures in the 70s can cause flower and small fruit abortion. For a more in-depth discussion of temperature and water dynamics on cucurbits, consult this article from University of Delaware. 

6. Diseases
Make sure to check whether aborted fruit is diseased - diseases like gummy stem blight and phytophthora can cause fruit to rot on the vine, which may be mistaken for fruit abortion. The "What's wrong with my plant" tool provides a nice overview of many of the diseases affecting squash and pumpkin fruit. However, disease symptoms are not always easily identified using pictures alone - consider sending plants to the UMN Plant Disease Clinic for diagnosis. 
Squash with discoloration from black rot
Image: Gummy stem blight in squash, S. Jensen, Cornell University,

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