Fungicide Resistance in Home Gardens
Chemicals have been successfully used for decades to control fungal pathogens. Currently there are fungicides on the market belonging to over 40 different chemical groups. The chemical groups are classified based on what part of the fungal physiology they target. Some chemical groups target a fungus at multiple points in its physiology, others only at one point. Chemical groups that only target one aspect of the fungal physiology are at high risk for resistance development. Numerous fungi have developed resistance to chemicals belonging to the demethylation inhibitors (DMI), QoI fungicides, and others, making these products less effective..
Fungicide resistance develops when fungicides from one chemical group are used continuously without alternating with a fungicide from another chemical group. A mutation in the gene that codes for the fungicide target can alter the fungal metabolism, allowing the fungus to continue its life cycle without interruption after the fungicide was applied. By using chemicals repeatedly from the same group, fungal individuals with a mutation that makes them immune to the chemical are selected, whereas the individuals without the mutation are killed. This may not be obvious initially, but over a couple of years the population of a specific fungus in a yard or orchard will consist of nearly 100% resistant individuals.
I recently received two inquiries from applicators stating that the fungicides they used for necrotic ring spot in home lawns did not work anymore. It turned out that they had used chemicals from the same chemical group for several years. In one case the applicator doubled the dose of the chemical in hopes that it would work again, but it didn’t. The chemical class of fungicides is usually provided on the label. The label will also state the number of times it can be used consecutively in one growing season as well as the maximum amount that can be applied per area per year. This information is provided to prevent the development of resistance.
The benefit of following the label and alternating with fungicides from different chemical groups is not only for the benefit of one homeowner or grower. Fungal spores travel for miles and spores from resistant individuals can travel to orchards and yards further away. The owner of that orchard may use a recommended fungicide not knowing that the isolates in his orchard were already resistant when they arrived. To prevent fungicide resistance it is essential to follow the instructions on the label and alternate between products from different chemical groups. If we lose the effectiveness of good fungicides due to resistance without alternative control strategies, millions of dollars in crop losses could be the result.