MSU Extension Cherries

Excerpt from Michigan Fruit Management Guide by Diane Brown-Rytlewski, MSU Extension, Berrien County, and Annemiek Schilder, Department of Plant, Soil and Microbial Sciences

What is fungicide resistance?

Fungicide resistance is an inherited change in the sensitivity of a fungus to a fungicide. Fungicide resistance usually starts with a gene mutation that changes the target site of fungicide activity in the fungal cell. The fungus then becomes insensitive (i.e., resistant) to the fungicide. Such mutations are random and occur naturally at low levels in pathogen populations. Once a mutation has occurred, however, repeated use of that fungicide allows the buildup of resistant pathogen strains because they are able to survive each fungicide application while sensitive strains are killed. Eventually, the population of resistant strains will cause sufficient disease for the grower to notice a fungicide control failure. Other possible causes of control failure must also be considered, such as higher-than-normal disease pressure, poor fungicide coverage, fungicide wash-off, inappropriate timing and use of an ineffective material (e.g., due to wrong disease diagnosis). Overuse or misuse of certain fungicides can lead to the development of fungicide resistance.

The risk of resistance development varies among chemical classes. Usually, fungicides with a specific or single-site mode of action (e.g., sterol inhibitor and strobilurin fungicides) are more prone to resistance development than broad-spectrum fungicides. For instance, captan, a broad-spectrum fungicide, has been used extensively without problems for decades, but resistance to sterol inhibitor fungicides is widely reported. When pathogens develop resistance to a specific fungicide, they may also express resistance to fungicides with the same mode of action even if they were never exposed to those fungicides. This is called “cross-resistance”. The risk of fungicide resistance also varies by pathogen; pathogens with many generations per season, sexual reproduction and high spore production and dispersal capacity, have the highest risk. Powdery mildews are an example of high-risk pathogens.

How to recognize fungicide resistance

Resistance may gradually increase over time, resulting in partial loss of disease control. This is usually the case with the sterol inhibitor fungicides (e.g., Rally, Elite, Bayleton). In the case of strobilurin fungicides (e.g., Abound, Flint), resistance may appear suddenly with significant or complete loss of control. Resistance may develop in as little as 2 years after the introduction of a new product.

Strategies for managing resistance

In general, the risk of fungicide resistance development is correlated with the mode of action (single-site vs. multisite) and the number of fungicide applications. Rotating fungicides with different modes of action can limit the buildup of fungicide-resistant strains. An international organization, the Fungicide Resistance Action Committee (FRAC), has grouped fungicides by mode of action and estimated the risk of resistance development. Each mode of action has been given a code number. You can select fungicides for rotation by looking at the group code and choosing a registered fungicide with a different code from the one used previously. The fungicide group code should soon be available on all fungicide labels. Many new fungicide labels contain fungicide resistance management guidelines.

More information on fungicide modes of action and resistance management can be found on the FRAC website and in the Michigan Fruit Pest Management Guide (E0154).