Foreign substances, or pollutants, represent a greater threat to tomato production than to any other vegetable crop. Damage from pollutants is said to be â€œabioticâ€ in that it is not caused by a living organism. Instead, it is caused by something in the environment in which the affected plant is growing. Abiotic disorders cannot be spread from one plant to another but they can have devastating consequences, nevertheless.
The most frequently encountered pollutant damage to tomato undoubtedly is caused by the gas ethylene. Symptoms include 'epinastic' growth--cupping of the new leaves and downward twisting of the leaf petioles. The leading cause for ethylene damage to greenhouse tomatoes is malfunctioning unit heaters. When functioning properly, greenhouse heaters combust (oxidize) propane or natural gas. This process releases carbon dioxide, water vapor and heat energy. Carbon dioxide and water vapor are harmless; the heat energy is needed to keep internal greenhouse temperatures at desirable levels.
Greenhouse heaters that are not supplied with sufficient amounts of fresh air to completely combust their fuel source tend to form products of incomplete combustion. Ethylene is one of these products and the results can be devastating because of tomato's extreme sensitivity to the gas. Normally, ethylene produced by unit heaters is evacuated from the greenhouse via the exhaust pipe (flue) of the heater. The exit route includes passage through heat exchange tubes, which if cracked or warped, can allow the gas to be introduced into the greenhouse atmosphere by the air circulating fan located behind the heat exchange tubes.
Preventing ethylene damage to greenhouse tomatoes starts with proper heater installation and maintenance. To provide adequate oxygen, unit heaters should be provided with one square inch of free area per 2000 Btu's of heater rating. Free area is defined as an unobstructed opening to the outside of the greenhouse. This is often accomplished by ducting air from the outside and releasing it just below the unit heater. Some of the better greenhouse heaters come with this feature builtin. Additionally, heaters should be adequately vented to the outside. The exhaust pipe should extend up and out of the greenhouse to a height of two feet above the ridge of the greenhouse. The exhaust pipe should be equipped with a back draft preventer. This will prevent outside air currents (e.g. gusty conditions) from impeding combustion air from properly evacuating the inside environment. Finally, growers should check the status of unit heaters on an annual basis. This includes checking to make sure fuel combustion is complete, exchange tubes are in good repair and the exhaust pipes contain no impediment to air flow.
Herbicides represent a second major pollutant threat to a tomato crop in a greenhouse. First, it is important to state that there are no herbicides labelled for greenhouse use while plants are growing in the structure. Sadly, several growers I have visited applied a preemergence herbicide in their greenhouse and lost their crop as a result. Most growers are aware of the sensitivity of tomatoes to herbicides but their neighbours are not. 2,4-D remains a common herbicide for the control of broadleaf weeds in lawns and is a leading cause of herbicide damage to tomatoes. Tomatoes are sensitive to the point if one can detect the odor of 2,4-D in the greenhouse, damage is likely no matter how far away it was applied. For this reason growers should contact neighbors and ask them either not to apply herbicides at all if they or located nearby or to use a compound other than 2,4-D.
A final caution about herbicides involves the use of animal manure for compost applied to greenhouse soil. There are several herbicides used to control weeds in pastures that can remain active even after passing through the digestive system of an animal. When the manure from animals grazing on treated pastures is composted, residual herbicide contained by the compost has been sufficient in concentration to cause damage to tomato plants. While the concentration of herbicide is not great enough to kill tomatoes, it can affect their growth and render them relatively worthless.
Diagnosing pollutant damage often is difficult. If symptoms appear uniformly over an entire crop or large portion of the greenhouse, pollutant damage is a possibility. Unfortunately, tests to determine which pollutant might be the culprit are limited and expensive.
REVISED: December 1, 2015