Friday, August 15, 2008

Greenhouse and Nursery - Avoid Phytotoxicities

It is important to avoid phytotoxicities on greenhouse and nursery plants by pesticides. The following is an article on the subject.

Pesticide phytotoxicity 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 often 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 even 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.

Poinsettias become more sensitive to spray injury as bracts turn color. Pesticide applications to poinsettia bracts are risky since there are so many variables. Differences in cultivars, culture and environmental conditions may increase or decrease bract sensitivity to pesticides. If you are inexperienced with a particular pesticide, always test a small portion of your crop before treating your entire crop.

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.

Information from the New England Greenhouse Update Newsletter.

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