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Karen Delahaut, Outreach Specialist, UW-Madison
Last month I discussed pesticide use. At the end of the article I described some of the problems you may encounter when using pesticides including pest resurgence, secondary pest outbreaks, and resistance. There is another undesirable effect related to pesticide use and that’s phytotoxicity. All pesticides whether it’s an herbicide, insecticide, or fungicide are capable of causing untoward effects on the plants you are growing in the greenhouse. Often it’s not the active ingredient that causes the most damage but the adjuvants and inert ingredients that are the culprits.
Phytotoxicity can result from using a pesticide when environmental conditions are inappropriate such as extreme heat or intense light. Pay attention not only to the current conditions but the predicted conditions for the few days following the application. Phytotoxicity can also occur if two or more pesticides are tank-mixed, particularly if these products aren’t regularly combined. You should also know the persistence of the pesticides you routinely apply. If you apply a product that has a long residual and then re-treat the same crop with a second pesticide during which time some of the original product is still effective, this combination may cause problems. Even applying the same product at regular intervals can have an additive effect that results in damage. Sometimes it’s the formulation of the product that causes the problem. Emulsifiable concentrates (EC) are far more toxic than wettable powders (WP) or dusts. Similarly, ECs are more likely to volatilize under warm conditions so refrain from using this formulation of herbicide near your greenhouse in mid-summer. Be sure to read the product label carefully and only apply the product according to directions. Increasing the concentration can increase the risk to the crop.
Pesticide phytotoxicity may damage the foliage or shoots. Stunting, distortion, discoloration, and even death may result. If a pesticide is applied to flowering plants, there may damage to the blossoms or bracts. The plant growth stage also determines whether a plant will become damaged. Seedlings and new cuttings are most susceptible to damage as are plants under stress from pests or adverse environmental conditions. Water-stressed plants are particularly susceptible to damage while vigorously growing, mature plants are most resistant.
Herbicides are not routinely used inside greenhouses but if you do apply an herbicide to control vegetation beneath the benches when the greenhouse is empty, make sure there is adequate ventilation and sufficient time before you move plants into the house. More commonly you will be using herbicides outside the greenhouse to control vegetation that may serve as an alternative host to pests. The herbicide used, depending upon type and formulation, could volatilize or drift through screens, vents, or doors and damage susceptible plants. Keeping accurate records of pesticide use will help you identify the cause when plant symptoms arise.
Insecticides are frequently used in greenhouses to manage the various insect pests that plague specific crops. Phytotoxic responses are more likely when insecticides or miticides are applied when the air temperature is above 90°F or if the plant is drought stressed. Reducing the frequency with which you are spraying your plants through the incorporation of other IPM practices will also reduce the likelihood for damage.
Other things that you can do to reduce your chances of damaging susceptible plants include testing a new pesticide on a new cultivar or plant species before spraying the entire crop to determine whether there will be an adverse effect. Always follow the label instructions explicitly and if a crop is not listed on the label, be sure that it is safe to treat that crop with the product you are considering. Avoid adjuvants such as spreaders and stickers unless specified on the product label. It is often these additives that cause the most damage to the crop.
Finally, keep extensive, detailed records of past pesticide applications and any adverse effects that may have resulted. Include environmental conditions at the time of the application as well as for the period directly following.
