Due to the hot and humid weather, lately many gardeners are noticing "blossom drop" on their tomato plants. Blossoms are drying up and dropping off before the tomato fruit sets. As explained by J.M. Kemble, Extension Vegetable Specialist and associate Professor, Alabama Cooperative Extension System:

"This condition is NOT related to any nutritional disorder, or any disease or insect damage. It is related to temperature. Despite the fact that tomatoes evolved in the tropics, flowering in tomato is sensitive to temperature. When day temperatures exceed 85°F and night temperatures exceed 72°F, tomato flowers will abort. An important factor involved with temperature is time of exposure. The longer the plants are exposed to these high temperatures, the longer the condition will last and the more serious the effect on flowering. Short exposures such as a week or less should not cause much of a problem. It is interesting to note that although the combination of high day and night temperature causes blossom drop, high night temperatures alone can be detrimental to flowering even if day temperatures are not over 85°F.

Older or heirloom, home garden varieties are more sensitive to high temperatures than many of the newer hybrids that are presently available. When fruit do not set and all other conditions are otherwise favorable (sufficient water and fertilizer, good pest control, appropriate pH) plants generally become vigorous and dark green. Even new hybrids, however, are susceptible to blossom drop.

Commercial growers in the southeastern US have suffered with this problem for many years until the recent advent of “heat set” tomato varieties. These varieties have been bred for tolerance to high day and night temperatures common in the summer and early fall. In fact, many of these varieties set fruit under poor growing conditions – extended cool, rainy periods as well as during extended periods of hot weather.

What to do: For this year or for an existing planting, keep the plants healthy. Keep plants well watered. Maintain fertility levels and control any pest problems as any additional stress will make the condition worse. The plants will produce flowers and set fruit when temperatures become more favorable."

High levels of humidity can "gum" up the pollen in the flowers, preventing the pollen from moving from the anther to the stigma, which would normally result in pollination. Some gardeners have suggested shaking the plants to help the pollen move around. Blossom drop is a frustrating, but (if the weather co-operates!) temporary problem.  You will likely still be enjoying a beautiful crop of tomatoes by the end of the summer!