Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
The heat wave we have had and will continue having into next week will have repercussions over the next month as the flowers of several crops including cucurbits, peppers and especially tomato will not pollinate or fertilize properly to develop into fruit. Daytime highs of >92o F and nighttime lows only getting down to 71o F in much of the mid-Atlantic will cause blossom drop and fruit abortion in tomatoes. Normally tomato pollination is achieved by the action of the wind. Pollen is released from the tomato flower and falls downward onto the stigma. Without pollination flowers die and drop. In tomatoes, blossom drop is usually preceded by the yellowing of the pedicel (Fig. 1). Tomato flowers must be pollinated within 50 hours of formation or they will die. This is about the time it takes for the pollen to germinate and move up the style to fertilize the ovary.
Tomato plants can tolerate extreme temperatures for short periods, but several days or nights with too high of temperatures will cause the plant to abort flowers and fruit (Fig. 2). At these high temperatures the pollen can become sticky and nonviable, preventing pollination from occurring and causing the blossom to dry and drop. Relative humidity also plays a role in pollination with high levels (>80% RH) during pollen shed causing the pollen not to be released properly resulting in poor or incomplete pollination.
Most tomato cultivars can make up for this set back of low fruit production with increased flower formation once it cools a bit. It would help to have some cultivars that are better able to take the high temperatures and humidity without as much flower/fruit abortion. Variety trials are being conducted by Maryland County Educators to see what cultivars of tomato do well in high temperatures and humidity.
Figure 1. Pedicels of tomato (arrows) turning yellow prior to aborting
Figure 2. All of the flowers on these 2 fruit clusters have aborted because of high temperature/humidity