Microbial resistance happens when bacteria or fungi that cause diseases become less sensitive, or completely resistant, to the products used to manage them, like fungicides or antibiotics. This makes diseases harder to control over time. While many may have heard about this in human medicine, microbial resistance can also occur in plant pathogen populations. Fungicide resistance is often discussed in commercial agriculture; however, it can also affect home gardens and hobby farms as well.
Here’s what small-scale growers should keep in mind:
1. Avoid Repeated Use of the Same Product
Using the same fungicide or bactericide over and over gives microbes a chance to adapt. Instead, rotate products with different modes of action. The Fungicide Resistance Action Committee (FRAC) and Insecticide Resistance Action Committee (IRAC) are acronyms for organizations that classify pesticides by how they work, which helps pesticide users avoid resistance. These committees group fungicides or insecticides into “mode of action” classes, meaning how the product affects the target pathogen or pest at the biochemical level. Each mode of action is assigned a group number, which is listed on the product label (e.g., Group 3 fungicide or Group 1B insecticide). This number helps users rotate between different classes to prevent resistance. Look for the FRAC or IRAC codes on the label (e.g., M5, 11, 3) and alternate between them.
2. Follow Label Instructions
More isn't better! Neither is only using part of a recommended rate. Over-using or under-dosing can both contribute to resistance. Apply only when needed and at the correct rate. If a product says to use it every 7–10 days, follow that guidance and don’t spray every 3 days "just in case." Low level exposure can allow for the selection of plant pathogen and insect individuals that can withstand the product. Over-using a product can also lead to resistance through over exposure selecting for individuals to overcome the product.
3. Focus on Prevention First
Cultural practices can greatly reduce disease pressure, such as spacing plants to increase airflow, watering early in the day to let leaves dry, removing diseased plant parts promptly, and choosing disease-resistant varieties when available. Cleaning tools and equipment is also essential. Dirty pruners and sprayers can spread disease from one plant to the next. Disinfect tools between uses, especially after working with infected plants or cutting into something that looks suspicious.
4. Stay Informed and Get Involved
Microbial resistance is an evolving issue and staying current with research and local disease trends helps you make smarter decisions in your own backyard or farm. An example would be to follow the ISU Plant and Insect Diagnostic Clinic on social media or look for local Extension events focusing on disease and insect trends.
If you grow grapes or make wine, please join us at our upcoming Field Day: From Grape to Glass: Microbial Resistance in the Vineyard and Winery Field Day on June 30th, 2025, to learn more about microbial resistance in both vineyards and wineries, with practical tips for all scales of growers and winemakers. This program is supported by Iowa State University Extension and Outreach and a grant from the Sustainable Agriculture Research and Education program, grant #ONC24-137. For more information and to register please see this vineyard and winery field day news article.
Photo credit: Martina/AdobeStock
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