Understanding Tomato Fruit Set

Published: April 1, 2013

Few things are as frustrating to tomato producers as to pro-duce vigorous, health plants that fail to set fruit. Unfortu-nately, failure of tomato plants to set fruit (or blossom drop) seemingly is becoming a more common problem. Undoubted-ly, we can attribute fruit set problems experienced last sum-mer to the brutal, hot summer. High temperature, however, is not the only cause for failure of tomato fruit to set. This article explores tomato fruit set and causes for the failure of fruit to set properly.

Tomato bears “perfect” flowers meaning they have both male (stamens) and female (pistil) parts in the same flower. The pistil of a tomato flower is shaped somewhat like a bowl-ing pin. The top of the pistil is known as the “stigma”, the middle part (neck of the bowl-ing pin) is called the “style”, and the bottom part is the termed the “ovary”. The yel-low stamens are fused together to form a cone around the pis-til which is in the center of the flower. Pollination is accom-plished when mature pollen produced by the stamens lands on the mature stigma of the pistil. Simultaneous maturity of both male and female parts is important since tomatoes are about 98 percent self-pollinated.

Most often the transfer of pollen from stamen to pistil oc-curs because of air movement in the field or mechanically shaking (vibrating) the flowers in green-house/high tunnel settings. The latter causes pollen to shed from the stamens of a flower and land on its mature stigma. Bumble bees represent a biotic form of pollination assistance often em-ployed by larger greenhouse operations in an attempt to save labor costs.

Research has shown that blossom drop in tomato can be influ-enced both by the environment (and related stress) as well as by plant nutrition. When daytime temperatures exceed 85 degrees F or nighttime temperatures exceed 70 degrees F, pollination suffers due to pollen becoming “tacky” and non-viable. Conversely, nighttime temperatures below 55 degrees F also promotes blossom drop. Low temperatures affect pollen viability as well as the growth rate of the pollen tube that forms shortly after a pollen grain lands on a ma-ture sigma.

Relative humidity plays a major role in the transfer of pollen from the stamen to the sigma. The ideal relative humidity range for polli-nation of tomato is 40 to 70 percent. At lower humidity levels the pollen tends to dry and has trouble sticking to the stigma; at higher levels the pollen does not shed from the stamen properly.

Nitrogen nutrition also has been demonstrated by research to influence blossom drop. At high levels of nitrogen the plant is en-couraged to produce excessive vegetative growth at the expense of fruit set. There is a fine line between having adequate amounts of nitrogen for good growth and excessive amounts.

Water stress is yet another reason for fruit set failure of tomato. Lack of water or excessive wind represents two types of stress that are detrimental to good fruit set. Water stress coupled with high temperatures is a very unfortunate combination of environmental circumstances when it comes to tomato fruit set.

Nutritional stress also has deleterious effects to fruit set. After a tomato plant has set a large number of fruit on its bottom clusters, there is competition between developing fruit and developing flowers for nutrients. Commonly, the fruit wins this competition resulting in poor fruit set of the middle flower clusters. Once the bottom fruit is harvested and the nutritional stress reduced, upper flower clusters tend to set normally, temperature permitting.

Finally, insects and disease also can adversely affect fruit set. Botrytis (gray mold) and heavy infestations of bacterial spot or speck have been shown to reduce fruit set. Although pests and diseases might amplify the problem of failure of fruit to set, they rarely are its primary cause

The question arises, then, “What can be done?” Clearly we have little control over temperature. Greenhouse and high tunnels can be cooled somewhat by covering them with shade cloth after temperatures have warmed in late spring/early summer. Growers producing tomatoes on a small scale outdoors might consider the use of floating row cover or a similar material to shade their plants. Of course shading will have no affect on nighttime temper-atures which we know (when above 70 degrees F) will reduce fruit set.

Additionally, keeping plants vigorous through proper watering and fertilizing is helpful. Be mindful of the fact that both the water and nutritional demands of a tomato increase as it grows and sets fruit. MU Extension publication G6462 (Watering and Fertilizing Tomatoes in a High Tunnel) is a useful reference for tomato grow-ers on the subject of water and fertilizer requirements for tomato.

If tomato plants do an extraordinary job of setting fruit on their bottom clusters, fruit pruning should be considered. If prob-ably is unrealistic to expect a tomato plant to “size” more than four (or five) fruits per cluster. Allowing all fruits that set to ma-ture adds to the nutritional stress the lower fruits put on the plant and add to blossom drop problems.

Finally, good insect and disease control will help to assure that these factors do not add to blossom-drop problems but also will help assure a high-yielding crop with good quality.