Testing New Pesticides on Your Plants

Quick Control Tips Want to set up your own pesticide trials in your greenhouses? First, choose representative plants to test. At least eight to 10 plants might be used for each type, preferably in a sensitive or typical stage such as rapid shoot and leaf growth or during flowering. Flag or tag pots to be treated if it might be difficult to identify them later on, and note the application date. Mix up a sample of the product at the highest label rate and make the application according to label directions. Be sure plants are not under drought stress while running the trial and maintain them normally like other similar plants nearby. Drenches should be made to pots that are slightly on the dry side so material does not leach out. Sprays can be applied thoroughly to wet both sides of leaves and (if present) flowers (or according to label). If an adjuvant (such as a wetting agent or acidifier) is normally used, add it to the water prior to adding the pesticide. You may also want to treat a set of plants without the adjuvant. Ideally untreated control plants for comparison should be sprayed or drenched with water only (in case there is a chance that water alone may be causing symptoms (i.e., cold water on African violets). After trial applications are made After observing label re-entry intervals, check leaves and flowers the following day or as soon as possible. Symptoms can show up fairly quickly. On plants that quickly rebloom, like impatiens, it is important to check before the newest blooms drop or fade if sprays were applied. Continue to inspect plants periodically over the next 10 to 14 days. Compare the treated plants to untreated (or water-sprayed/drenched) plants nearby to see if there are any differences. Examine the flowers and youngest leaves most closely and check the underside of the foliage as well. Look for symptoms such as yellowing, distortion, brown spotting or bronzing, stunting or edge burn. It is a good idea to keep a simple record of the crop(s) and cultivars, crop stage, products, application rate and method, along with notes of observations and date(s) the observations were made. If a problem is found If you're convinced there may be a problem with a particular product on a new or older crop, it would be good to notify the manufacturer directly or through your local Extension staff or supplier. The manufacturer may follow up with a series of its own tests or be able to explain the situation. If the problem can be reproduced it will likely result in helpful information added to the label and conveyed to others. This can help growers avoid costly mistakes while keeping the manufacturers interested in developing new and better products for the horticulture industry.

Growers should test new control products or new crops in their own greenhouses with their application equipment.

When a new insect or mite control product comes onto the market there, are three questions most growers will ask. Is the product effective? What does it cost? Is it safe for plants?

I can usually answer the first two questions, but the third can be more challenging due to novel ornamental plant varieties and increasing diversity of plants being grown.

Every year we receive plant samples at the Cornell University diagnostic lab showing symptoms that could be due to some kind of chemical injury. While it is important to sort through all possible causes that can be mistaken for pesticide damage, such as broad mite damage, nutrient deficiency or cold temperature effects, we sometimes conclude that a chemical spray application (or a combination of chemical sprays) is the likely culprit. Sometimes this results from not heeding product label warnings, but once in a while there are no label warnings and growers are surprised by the results.

I am aware of some products that have little or no plant safety testing behind them. However, most manufacturers arrange trials to evaluate not only efficacy but also sensitivity of plants to new pesticide products, particularly when they are to be used on ornamentals. Obviously every plant cultivar and every production situation can't be tested, so here is where some due diligence is necessary during production.
 

1. What is phytotoxicity?

Phytotoxicity is the injury to plant tissue (leaves, stems, flowers, etc.) from exposure to a chemical. Symptoms include brown spotting on leaves or leaf edge burn, yellow patches on foliage, and stunting or distortion of new growth.


2. How does phytotoxicity occur?

Phytotoxicity from pesticide sprays or drenches can occur in many ways, such as chemicals entering and killing plant cells or damaging cellular components like chloroplasts, inhibiting leaf function like respiration, removal of the protective waxy cuticle layer of leaves, direct effects on the plant hormone system, or causing an osmotic imbalance in the root zone. Sometimes over-application is responsible for phytotoxicty. This can occur because of a miscalculation in rates or the products are used in tank mixes that can increase penetration into the leaves or cells. Occasionally products are inadvertently applied to known sensitive crops. Fortunately it is less common, but on occasion growers discover that certain crops show sensitivity to a new insecticide or miticide for which there was no prior knowledge.

Growers may want to treat a set of plants without an adjuvant to see if there are any negative effects. Damage to this New Guinea impatiens was caused by an adjuvant.

3. What steps should growers take to prevent phytotoxicity?

Before using a new product or even an older material on a new crop, be sure to check the label for any warnings concerning plant safety. Updated advisories may appear in manufacturers' product information bulletins posted on their websites. Extension specialists, experienced growers and local suppliers can also be helpful sources of information.


4. What else can growers do, particularly for unusual or new crops?

Growers can run their own in-house trials. Test a new control product or new crop in your own greenhouse range with your own equipment. It's relatively easy, though it may take several days to two weeks to be confident of the results. The advantage, however, is that the results reflect the conditions, application methods and plants particular to that individual greenhouse situation and crop stage and mix, and it helps minimize potentially costly mistakes.


5. Why up to two weeks?

In the diagnostic lab we look at the crop application history and focus on applications made within the prior two weeks. It is almost a rule that some kinds of spray injury to tender unexpanded leaves may be apparent only after the new foliage has grown out fully, which takes around 10 to 14 days. There is also a lag time for drenches to be taken up into the plants. In some cases, however, more acute symptoms are noticed within a day or two especially for foliar sprays.


Have a Question? You can contact Dan at dog1@cornell.edu.