Identifying, Mitigating and Preventing Fungicide Resistance

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You have, no doubt, heard the saying that one bad apple spoils the whole bunch. When it comes to the development of fungicide resistance, that one bad apple comes in the form of a small percentage of fungi population that develops lack of sensitivity to certain types of fungicides. If those resistant fungi are not controlled with fungicides to which the population isn’t resistant, they will reproduce a new generation of resistant fungi and spread throughout the field. This new generation will have a higher frequency of resistant fungi in the population, as the resistant fungi tend to survive fungicide treatments, leading to poor fungicide efficacy.

Fungicide resistance is an increasing problem across the agricultural production spectrum and especially disconcerting and costly in high-value crops like grapes. Early detection of fungicide resistance is critical to decide which products to use in order to achieve successful control. It is the first step toward not only limiting crop losses but also preserving crop protection chemistries and, ultimately, a vineyard’s economic sustainability.

Managing fungicide resistance, its causes, and the impact on crop health and disease management options require a specific and proactive plan of action. A good foundation for that plan should start by visiting the fungicide resistance webpage at

“As a former licensed pest control advisor (PCA), I understand the value that quality resources offer a grower when developing an effective fungicide program,” says Lindee Love, strategic account manager, Corteva Agriscience. “There are limited tools in the toolbox. An effective fungicide plan is an invaluable asset in being good stewards of the limited tools we currently have, as well as future fungicides as they become available.”

The fungicide resistance webpage links to another resource, the Fungicide Resistance Action Committee (FRAC). That website offers a downloadable app from the Apple Store that provides information about the global status of the resistance issues to fungicides, herbicides, and insecticides in one convenient application. You also may search for active ingredients by name, chemical class, and mode of action (MOA).

The repeated use of similar MOA products leads to the development of resistance. Among PCAs, growers, consultants, and the research community, this scenario is well known as the biggest culprit. However, when making a fungicide product recommendation, they recognize and encourage growers to remember it is not the number of MOA that matter but use of effective MOA in rotation programs. The chemistry selected must provide adequate control of the fungi population.

Given the phenology of the crop and pressure from weather changes, some growers scout their vines biweekly, or even weekly, to monitor crop growth and to look for diseases like powdery mildew, caused by the fungus Erysiphe necator. Fungicide resistance is rarely identified until there is a lack of efficacy of the commonly used products. Growers should be proactive and apply preventive measures to avoid the development of the disease as curative applications are less effective and will lead to the selection of resistant fungi in the population.

To estimate when powdery mildew will start to develop in the orchards so preventive measures can be applied, many viticulturalists today still use the Gubler-Thomas risk assessment model made popular in the 1990s. This model has been used to successfully control grape powdery mildew throughout California and grape-growing regions around the world.

“We are in dire need of innovative tools to manage disease development in the field and alleviate some of the weight that’s being put on the current fungicide groups,” Love concludes.

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