Fungicide Application Notes

Fungicide rotations for the Greenhouse to minimize resistance.

An excerpt from an article by Janna Beckerman Department of Botany and Plant Pathology, Purdue University

Fungicides are important tools for managing ornamental plant diseases. There are many different fungicides and numerous methods of classifying them. This publication examines how fungicides are classified and recommends management practices to prevent fungi from developing resistance to these products.

Fungicide classification is one way to classify fungicides by their chemical structures or modes of action —  the specific ways the fungicides affect a fungus. Fungicides that share a common mode of action belong to the same fungicide class (sometimes referred to as a fungicide family). Unfortunately, if a fungus is resistant to a specific fungicide, it is usually resistant to all the fungicides within that fungicide class.

Target site Fungicides are also characterized by their specificity. site-specific fungicides react with one very specific, very important biochemical process, called the target site. For example, a fungicide target site could be the specific proteins involved in cell wall biosynthesis, RNA biosynthesis, or cell division. Site-specific fungicides target these specific processes, which prevents the fungus from growing and ultimately causes its death.

Multi-site fungicides have multiple modes of action, so they affect multiple target sites, and simultaneously interfere with numerous metabolic processes of the fungus. Fungicide resistance occurs when a fungus develops a genetic mutation at the target site that reduces its sensitivity to a specific fungicide. Because they affect multiple target sites, multi-site fungicides have a very low risk of causing fungicide resistance because it is highly unlikely for a fungus to simultaneously develop all of the mutations necessary for resistance. Site-specific fungicides, however, have a much higher risk of causing resistance because a single genetic mutation at the target site can change a fungus’ biochemical process so that it can still perform the needed biological function (cell division, membrane biosynthesis, respiration). The result is a fungus strain that is less susceptible or no longer susceptible to the site-specific fungicide. If a single fungicide continues to be used, the fungicide-sensitive portion of the population is suppressed over time, and only the fungicide-resistant portion of the population remains, which goes on to reproduce and make up the majority of the population. Eventually, the fungicide is ineffective because this majority of the fungal population is no longer susceptible to it.

Minimizing resistance: To minimize the possibility of fungicide resistance from occurring, implement a comprehensive management strategy before resistance develops. Some key tactics to include in your management strategy include:

  1. Follow good plant health practices. Using disease-resistant cultivars, following proper planting and fertilization techniques, and sanitizing equipment reduce the reliance on fungicides, thereby reducing the risk of their over-use and the development of resistant populations.
  2. use the recommended doses as stated on fungicide labels. Many fungicides have been extensively tested to identify the optimal rate. Cutting the rate results in a sublethal dose that is not only ineffective for disease management but increases the risk of resistance.
  3. Minimize the number of fungicide treatments per season and apply only when necessary. Excessive use of site-specific fungicides increases the likelihood of resistance. By reducing the number of site-specific fungicide applications, you reduce the likelihood of resistance development.
  4. Do not rely solely on one fungicide with a site-specific mode of action. Use a diversity of fungicides with different modes of action that provide broad-spectrum disease control. There is no single, best fungicide. There are, however, multiple fungicides with different efficacies for different diseases.

Many single-site fungicides are highly effective by themselves, but you should tank-mix them with another fungicide from a different family or rotate or alternate multiple fungicides to reduce the risk of resistance. The important thing to remember is that you should avoid consecutive applications of site-specific fungicides. tank-mixing and rotating There are two tactics that can reduce the risk of fungicide resistant disease populations: tank-mixing and rotating fungicides. As the name suggests, tank-mixing consists of mixing a fungicide with a high resistance risk with another fungicide with a low or negligible resistance risk.

Rotating fungicides involves alternating products that have different modes of action so that you avoid back-to-back treatments with any one site-specific fungicide. Tank-mixing and rotating are important for two reasons. First, both practices limit the amount of time fungi are exposed to any one product. Second, other fungicides could potentially suppress any resistant populations before they have a chance to reproduce. Selecting the proper tank mix or rotation partners in a fungicide resistance management program is critical. To develop an effective tank mix or rotation:

Use fungicides with different Group Codes which denote different fungicide families. Remember, fungicides with different trade names can belong to the same chemical family.

Always pair site specific products with a multi-site inhibitor fungicide Group Code M.

Carefully read fungicide labels to determine if they can be used together or rotated with each other. Fungicide rotations can be simple or complex, depending on the problem and the pathogen that is causing it. For example, you may control a Septoria, Myrothecium, or Alternaria leaf spot by rotating a fludioxinal based fungicide Group 12 with a chlorothalonil based fungicide Group M thereby minimizing the risk of resistance to Group 12 fungicides. Other diseases may require more elaborate rotations.

The bottom line is that rotating fungicides reduces the possibility of resistance development. This is important as the labels on most newer, site-specific fungicides have strict use recommendations to minimize the risk of fungicide resistance and protect the long-term efficacy of the product. By carefully following these recommendations, and using fungicides with different group codes, diseases and fungicide resistance can be carefully and effectively managed.