Testing For Phytotoxicity

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Testing For Phytotoxicity

In these times of belt tightening, it is a good idea to know how to get the most out of your fungicide dollar. Simply replacing a product with one you think is less expensive may not be the answer. In this series, I will examine some of the places you should focus your efforts to maximize fungicide benefits, including this first article on phytotoxicity.

The aim of this series is to remind you of the ways we lose money by skipping steps. Other discussions in this series will be how cultural controls impact fungicide efficacy, choosing the right fungicide, the importance of rate, interval and timing in controlling disease losses, and rotating or tank mixing to avoid fungicide resistance.

Phytotoxicity Costs You Money

Pesticides are routinely employed by many ornamental plant producers. They are generally effective and safe for the labeled crops, but whenever a new product hits the market questions of safety must be addressed.

Remember, phytotoxicity directly impacts your bottom line by reducing crop quality and increasing production times. Many of the newer products are labeled in a way to allow use of ornamental crops that are not specifically listed on their labels. Wording like: “For use on ornamentals such as …”

Figure 1. Contact burn on undersides of aster leaves.

This is very helpful in our industry because no fungicide manufacturer can test their new products on all the ornamentals we grow and under all the conditions in which we grow them. So it is critical to evaluate fungicides for safety to your crops under your very special conditions.

If you do not test new products (or older products on new crops), you are taking a chance with phytotoxicity. Use of fungicides on sensitive plants, such as asters, impatiens, poinsettias, begonias, gerberas, snapdragons, scheffleras and geraniums can sometimes result in plant injury. If any of these are in your product mix, they would make good test subjects for a phytotoxicity trial.

The test description below is best used for foliar products. If you have a drench product that will be used in propagation, you will have to test the possibility that it can delay rooting. Be sure to compare untreated cuttings of exactly the same cultivar and age when evaluating pesticide safety.

Performing A Phytotoxicity Text

Below, I have listed the basic steps I would suggest to conduct a fungicide (or any other pesticide) phytotoxicity test.

1. Read the label of the product and determine how you will most likely use it: drench or spray. Choose the highest labeled rate for that use pattern.

2. Choose healthy, well-established test plants. Do not use stressed or overgrown plants.

3. Place one block of plants (at least 10 containers) in a group and another block of the same size in a separate group. Do the test under your normal growing conditions. Be sure to label the area clearly so plants won’t be moved, and you can tell which ones were treated one or two months later.

4. Mark one group of plants to treat with water only. This is the control (such as one label color or the letter “A”). Mark the other group differently to denote the product being tested (such as a different label color or “B”).

Figure 2. Leaf distortion and curl on gerbera daisy leaves.

5. Treat the control plants with plain water. This will help you make sure any damage is from the test fungicide and not water alone.

6. Use your normal sprayer if possible. If not, use a pump action hand sprayer and spray to drip (or follow the label).

7. Take pictures of the two groups of plants right before each treatment. This will help you compare results later.

8. Re-treat the plants on the closest labeled interval such as weekly and apply a total of three times to make sure multiple applications are safe.

9. At the end of the trial you can compare overall appearance. You can measure the plant height if you suspect stunting and compare the averages of the water treated to the test fungicide.

10. Look for signs of phytotoxicity including: yellowing, leaf distortion, burning (speckles or burned tips or edges particularly), darkening green color, smaller leaves and stunting.

11. Take notes and add to the pictures. Relying on memory is usually not effective.

What Does Phytotoxicity Look Like?

This is not always easy to answer, and that is the real purpose of including a group of water-treated plants in any phytotoxicity test. Some of the symptoms I have seen are: tip or edge burns, overall yellowing, stunting, small leaf size, leaf curling, cupping and other distortions, dark green color (typical of triazole fungicides), speckling, delays in flowering and delays in rooting.

Once you have completed the phytotoxicity test, you can start using the product in your general production. Because many practices of plant production can increase the chances of phytotoxicity, you should remember a single test may not tell you how safe the product is on all your plants, all year-round. The best ways to avoid phytotoxicity on products that are typically safe include the following rules:

1. Do not allow sprays of herbicides outside a greenhouse to drift and do not use herbicides that are known to volatilize to be used in enclosed structures.

2. Do not drench with any product that is not specifically labeled for drench applications.

3. Try not to apply a copper product when the spray takes a long time to dry.

4. Do not spray when it is too hot or too cold. Whenever possible, pesticide applications should be made between 60 and 85°F using water at or near the air temperature.

5. Do not apply a copper product with an acidic one because this can make too much copper available.

6. Never combine products against label directions. This includes adding adjuvant when they are prohibited or using the wrong type of adjuvant.

7. If all else fails, read the product label. It is usually a great source of information on how to avoid crop damage.GG

A.R. Chase is president and plant pathologist at Chase Horticultural Research. Chase can be reached at archase@chaseresearch.com.

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