Tomatoes Fail to Set Fruit – Blossoms Dry & Fall Off

Q.  We have great looking tomato plants 4′-high, but they are not setting any fruit.  The blossoms just dry up and fall off.  What’s wrong, and what can we do to fix it?
A.  In a greenhouse environment with no air movement, vibration or hand -brushing of the blossoms may be necessary for good pollination.  This is not necessary in the garden, however.
Many “authorities” say that low temperatures below 55 at night, and high temperatures above 90 during the day inhibit fruit set.  Our experience has been that these can have an influence, but that other factors may be more important.  We have grown tomatoes successfully, with blossoms continuing to set fruit, in temperatures beyond those extremes. 
Ample water is essential, and especially in hot weather the tomato plant must “sweat” or transpire, in order to stay cool.  Dr. Mittleider says that as much as 95% of the water passing through a tomato plant will be lost to transpiration on a hot day.  Therefore, make sure your plants have moisture at all times – even if it means watering more than once each day in hot weather.  Remember that your palnts also need oxygen, however, so don’t leave them in standing water for any length of time.
There are two other things to look for when your tomatoes look healthy, but fail to set fruit.  First, look and see if there are thrips around your blossoms.  “They are very tiny, slender, narrow insects. Thrips fly, and they inhabit nearly all farm and garden crops. Their special dessert is pollen!  If they are allowed to multiply, they will eat the pollen as it ripens and thus interfere with normal pollination. When there is no pollen, the female flowers die and fall off. Getting tomatoes to set fruit may be as simple as controlling the thrips.”   (Let’s Grow Tomatoes – Ch 22).
Second, inspect for Nematodes.  Here is some information on Nematodes from Chapter 20 of Let’s Grow Tomatoes:  “Nematodes are tiny eel-like worms. So tiny, in fact, that they are seldom seen with the naked eye.  These eel-like worms eat their way into the roots of plants and thereafter live off the essential liquids in the roots.
“They are especially fond of tomato plants. Infested roots have irregular brown-colored swellings which appear like rough knots.  Nematodes multiply rapidly and, as they increase, the knots become larger. Tomato roots can be heavily infested with nematodes without affecting the green color of the leaves or noticeably retarding plant growth.
Pollination and the ensuing conception are almost automatic for tomatoes.  “However, conception places a heavy load on the plant. If the roots are functioning properly, and the essential nutrients, water, air and temperature are satisfactory, the roots can easily support the added responsibility, which is to develop the tomatoes. But if nematodes are living in the roots, plant performance is reduced according to the number of nematodes present.
“By the time the plant is flowering, the nematodes have multiplied so much that the plant has a full load just to support the nematodes.  The plant cannot expel the nematodes; therefore, when the flowers mature and are pollinated, they are aborted along with the small fruit in order for the plant to survive and stay alive. But the struggling plant does not give up. It tries to produce seed. And it puts out new leaves, buds, and flowers.  And again, at the crucial moment, because of the nematodes, the plant aborts the fruit just to stay alive. This process is repeated again and again in nematode-infested plants. The plants cannot do otherwise.
Therefore, plants which appear to be healthy but which fail to set fruit should be carefully inspected for thrip and nematode infestation.