Disease Management for Roses

Fungicide Resistance Management

By Ethel M. Dutky
Department of Entomology, University of Maryland

For many years pesticide resistance was seen primarily in insecticides and miticides, but not in fungicides. This was because the earlier types of fungicide acted on many life processes in the fungal cell (multi-site activity), and it was very unlikely that cells could exist with resistance to the many forms of toxicity. However, with the advent of fungicides that act primarily on one specific cellular function (single-site activity), resistance was soon seen. When a pest population is exposed to a poison over many generations the individuals that are sensitive die, leaving individuals that are less sensitive to reproduce. This is possible because within a population there may be individuals that have the ability to tolerate the poison, perhaps because of alternative metabolic pathways, or the ability to metabolize the poison, or an altered receptor site to which the poison cannot attach. Whatever the mechanism, resistance is a serious risk in the use of "single-site" pesticides, including fungicides. Some examples of plant diseases where resistance is now a common problem in the field and greenhouse include Botrytis , powdery mildews, and Pythium root rot.

What Can the Grower do to Prevent Fungicide Resistance?

Several strategies exist to cope with the risk of fungicide resistance. One strategy is to rotate the chemicals you use, so the pest is not continuously exposed to the same type of poison. Rotation usually consists of a few (2 to4) consecutive applications, then switching to a different mode of action. This is the primary strategy used for insecticides, miticides and herbicides. Another strategy is to use a mixture of two fungicides; one component of the mixture is a "single-site" and the other is a "multi-site" fungicide. Some examples of this type of mixture are Zyban, ConSyst, Sprectro 90 and Stature. This strategy is unique to fungicides as we still have several types of fungicide that have "multi-site" activity. Some examples of these multi-site toxicant fungicides are chlorothalonil, coppers and mancozeb. It is also important to apply the fungicide at the label rate, and not to apply at reduced rates. Repeated applications of single site compounds at reduced rates will promote resistance development.

The fundamental basis for disease prevention relies on modification of the environment to reduce the risk of disease. Once the crop is exhibiting symptoms, it may not be possible to avoid losses. Powerful non-chemical tools to reduce diseases include sanitation and environmental management.

Biological Fungicides

Some fungi and bacteria are now available to provide biological control of crop diseases. These products work using a variety of mechanisms including: competition for food sources or infection sites; stimulation of plant defenses; production of antibiotics; and attack and consumption of fungal resting structures. They are broad spectrum. These products work best when incorporated into the production system from the start. They do not provide control once a disease is established. Using biologicals can permit the grower to reduce the number of chemical applications to the crop. Some examples of biological fungicides are RootShield, SoilGard, Trichodex, AQ10, Mycostop, GalltrollA, Companion, Serenade, Kodiak and Deny.

The following table lists some fungicides labeled on ornamentals by their mode of action class. You can use this information to assist you in developing a rotation program.