High Temperature Affects Fruit Set of Vegetable Crops

Recent high temperatures have been above optimum for vegetable crops. Around Indiana, from June 12 to 25 the daily high was more than 90°F on at least 5 days and the low overnight was above 70°F on at least 3 dates (Figure 1). High night temperature is a particular concern because it causes plants to use up more stored carbohydrates. For fruiting crops, a common heat-related problem is abortion of flower buds, flowers, or young fruit (Figures 2 and 3). Depending on crop and timing, this can result in a delay of harvest, or a gap in the harvest period when fruit are not available. The loss of buds, flowers, or young fruit may not be obvious without close inspection to see where they have fallen off. Because the plants aren’t spending energy on developing that fruit, vegetative growth may be more vigorous. Based on a windshield survey the plants might look very good.

It will take time for new buds to develop into flowers and fruit. There is not a lot one can do to speed up the process, but here are a few things that might help. Reduce potential stress on the crop. This could include providing timely irrigation and removing weeds that shade the crop, as well as smaller weeds that could compete for water and nutrients. Avoid pesticide and other foliar applications that you know may cause phytotoxicity. For crops that rely on pollinators, avoid pesticide applications that could harm them.

Because high levels of nitrogen can promote vegetative growth at the expense of fruiting, reassess plans for sidedressing or fertigating. I am not aware of research that pins down the best way to do this. Consider the crop condition and your estimate of nitrogen available in the soil. Take into account likely nitrogen losses from denitrification or leaching. Leaf tissue and soil nitrate tests could be helpful. If there is sufficient nitrogen at this time, additional application could be delayed to allow the next fruit set, without stimulating excess vegetative growth.

For hand-harvested tomatoes, peppers, snap beans, and other crops with smaller fruit, the overall yield is less likely to be affected by losing one set. In machine-harvested crops a split set might reduce the amount ready at the time of a once-over harvest. For a crop like jack-o-lantern pumpkins, with larger fruit and just a few per plant, yield could be reduced if there is not enough time remaining in the season for the next fruit to develop. This heat has occurred early in the season so any affected pumpkins should have time to develop more flowers.

Crop varieties differ in their tolerance to high temperatures. In some cases that information is included in seed catalogs, or can be found by close reading of variety trial reports. But a grower’s own experience is invaluable. This would be a good time to make notes about how different varieties have responded to the high temperatures. Have some varieties continued to bloom and set fruit while others have aborted buds, flowers, or young fruit? If they aborted, how long until the next successful fruit set? Factors like planting date, field or tunnel location, nutrient applications, etc., that are different for varieties could affect the response, so include notes on those too.