Field Insights Blog | GreenCast | Syngenta
Field Insights Blog | GreenCast | Syngenta

Fungal Disease Resistance

In intensively managed golf course turf managing for or the potential development of disease resistance is a key to developing season-long disease control programs. Currently the turfgrass diseases that have documented cases to at least one group of fungicides includes anthracnose (Colletotrichum cereale), dollar spot (Clarireedia spp), gray leaf spot (Pyricularia grisea), Microdochium patch (Microdochium nivale) and Pythium blight (Pythium aphanidermatum).

In a series of two posts we look at how fungal resistance occurs and then strategies for managing resistance. These posts are based on an article I wrote that appeared in the Eureopean Turfgrass Society newsletter in the fall of 2020. 

Figure 1.  Dollar spot is the most common turfgrass disease associated with fungicide resistance. 

Quality turf systems like those found on golf courses and sports fields are managed for a high degree of uniformity and density for aesthetic and functional purposes. Selecting the proper turfgrass species and cultivars combined with proper cultural practices is necessary to reduce the likelihood and severity of diseases. Where disease incidence is chronic and severe, even under the best cultural practices, fungicides are needed to effectively control diseases. Inherent in the use of fungicides is understanding what fungal resistance is and how to manage it for the long term.

Fungal pathogens have evolved overtime in how they infect their host and survive in nature. An integral part of the evolution of fungi is their sexual and asexual reproduction cycles. Sexual reproduction provides fungi the opportunity to mix genes from two individuals to get new and unique individuals. Another method for developing individuals with new gene combinations is anastomosis - asexual reproduction - which is the union of hypha resulting in an intermixing of contents. The most common form of reproduction for most turfgrass pathogens is asexual.

A fungal population with a natural sensitivity to a fungicide is known as a “wild type.” In such a population genetic diversity occurs resulting in a small number of individuals with resistance to the fungicide, even though that fungicide may have not been applied.

The small number of fungicide resistant individuals begin to dominate the population in the presence of the fungicide as the wild types are selectively removed. The fungicide resistant population, which is not affected by the fungicide continues to grow becoming a greater proportion of the population. The result is a directional shift in the fungal population toward resistance. From a turf management perspective, disease control is lost and the future use of the fungicide is compromised.

Mechanisms that the resistant fungal individuals may have to avoid the toxic effects of the fungicide include:

  1. Decreased permeability: The resistant types do not absorb as much of the pesticide as the wild types
  2. Metabolism: The resistant type detoxifies the pesticide or reduces the release of the compound
  3. Decreased affinity at the site of action: Some fungicides are known to attack specific sites in a pathogen. A modification in the resistant fungus site of action can make the fungicide ineffective
  4. Circumvention of the site of action: Circumvention means that the pest has an alternative metabolic pathway for the one that is blocked by the fungicide
  5. Compensation: A resistant pathogen increases the production of an inhibited substance. For example, if a fungicide inhibits a certain enzyme, the resistant pathogen increases the production of that enzyme

A component in the development of a resistant fungus population is the population’s “fitness”. Fitness is a general term used to describe how adapted the population is to grow and reproduce in the environment. For example, benzimidazole resistant dollar spot populations are highly stable or fit. Benzimidazole resistant populations have been found to dominate the population supplanting the wild type permanently even after the absence of the fungicide.

Conversely, resistance to iprodione (dicarboximides) have weaker fitness. In the absence of iprodione, the wild type or sensitive population returns to a predominant component of the population while the resistant population declines. The population shift back toward the wild type is referred to as fitness penalty, the selection against resistance.

About the author

Dr. Karl Danneberger is a professor of Turfgrass Science at The Ohio State University. Dr. Danneberger's contact information can be found here. You may also follow Dr. Danneberger on Twitter:

© Syngenta. Always read and follow label instructions. Some products may not be registered for sale or use in all states or counties. Please check with your state or local Extension Service to ensure registration status.