Greenhouse - Phytotoxicity in Greenhouses

The following is an article on pesticide phytotoxicity in greenhouses from the New England Greenhouse Update.

Pesticide phytotoxicity on plants can often be distinguished from pest problems by the pattern and timing of symptom development. Although the damage may take up to several days or more to occur, pesticide damage symptoms often occur all at once and have a regular distribution on the crop. Symptoms caused by pathogens usually develop over an extended period of time in random or grouped patterns. Pesticide phytotoxicity can be expressed by a number of different symptoms, including leaf speckling, cupping and twisting and other leaf distortions or plant death. Pesticides with hormone-type activity such as the insect growth regulator Distance and herbicides containing 2,4-D tend to produce leaf cupping and twisting. Other pesticides that have caused twisted growth include Judo applied to dracaena plugs and Botanigard ES applied to tomato plants.

Phytotoxicity can also be caused by the solvents in a formulation (EC formulation vs WP), impurities in spray water, using a higher rate of pesticide than is listed on the label, tank-mixing or inadequately mixing the spray solution. Environmental conditions such the temperature, humidity, and light can also influence phytotoxicity. High temperatures can speed up pesticide degradation and volatilization, but may also result in increased phytotoxicity for some products. Plants that are stressed are more susceptible to pesticide injury.

Growers can prevent pesticide damage to plants by applying pesticides during the cooler part of the day such as the early morning or evening. Treatments made in the early morning allow foliage to dry before temperatures reach 85-90°F. Take special precautions when using oils. Treat when conditions allow plants to dry quickly. Other suggestions to prevent burning plants with pesticides include:

1) Add surfactants only when recommended on the pesticide label.
2) Never use a sprayer for insecticides that was previously used to apply herbicides.
3) Do not apply pesticides to plants that are under moisture stress.
4) Avoid using more than one emulsifiable concentrate in a tank mix.
5) Do not apply pesticides with fertilizers unless the label states otherwise.
6) Never use broad-leaved weed killers and brush killers around the greenhouse.

To prevent injury due to pesticides, be sure to follow label directions exactly. See the company’s website to read any technical bulletins about the product or call the company’s technical representative before using a product for the first time.

If minor phytotoxicity is suspected from foliar applications of an insecticide, miticide or fungicide, watch the new growth as it emerges. Plants will often grow out of one-time spray damage. As plants grow, the damage will remain on the oldest leaves and the new growth will appear healthy.

Reprinted from the current edition of the New England Greenhouse Update.