Cotton plants infected with Rhizoctonia (soreshin), which is a perennial fungicide resistance challenge.
The constant quest for healthier crops and higher yields through investment in premium seed traits, varieties and hybrids has triggered the use of fungicides as an increasingly common agronomic practice in field crops. With increased fungicide use comes the need to manage the threat of disease resistance to fungicide chemistries.
Resistance development is a concern because crop protection products may become less effective, or even useless, for controlling resistant pathogens and pests. Maintaining the efficacy of all crop protection chemistries in fungicides, insecticides and herbicides is a critical concern for maintaining grower profitability and sufficient world food supplies.
Fungicides must be sprayed with a volume sufficient to provide coverage of plant tissues. Fungicidal products can be degraded by time and weathering. Therefore, reapplication of fungicides to corn, soybeans, cotton and wheat during the growing season may be advisable to protect plant growth. It’s important to remember that every time a fungicide is applied, rotation of modes of action or applying mixtures of chemistries with different modes of action should be considered to manage disease resistance.
How Fungicide Resistance Occurs
Fungicide resistance arises from a complex interaction among the fungicide mode of action, frequency of fungicide use, application practices, cropping system, and the genetic diversity of the fungus. The more diverse the genetic makeup of the pathogen, the more likely that a strain exists in a field that is less sensitive to the particular chemistry. The less sensitive strains may proliferate and eventually dominate a population due to the following factors:
- Repeated use of the same fungicide mode of action
- Incomplete coverage due to factors such as low spray volume
- Off-label or insufficient application rates
Many fungi produce spores in multiple cycles throughout the growing season. Each reproductive cycle presents another opportunity for resistant spores to replicate. Some diseases pose a greater risk for resistance development, including rusts, powdery mildews, leaf spots and blights, according to University of Tennessee research.
For example, in some soybean fields across the southern United States, frogeye leaf spot populations have become resistant to strobilurin chemistries. While strobilurin fungicides remain effective tools for managing diseases, it’s important to use multiple modes of action from different, effective chemistry classes to manage resistance development.
There are various mechanisms that lead to reduced sensitivity to a fungicide. The University of Wisconsin lists several mechanisms: A change at the target site, active export of the fungicide out of the fungal cell, breakdown of the fungicide active ingredient and reduced fungicide uptake.
Fungicide resistance happens naturally. Every time a farmer treats a field with a fungicide, the treatments kills sensitive individuals, while less sensitive isolates may survive and reproduce. Over time, the scenario is repeated until resistant pathogens may become the norm rather than the exception.
While at the University of Illinois (now at the University of Kentucky), plant pathology professor Dr. Carl Bradley noted in a July 2014 United Soybean Board report, “Continued use of a fungicide will continue to select out the fungicide-resistant individuals in the population. Eventually, resistant individuals dominate the population and the effectiveness of the fungicide will be reduced.”
Risk Factors and the Disease Triangle
Higher fungal disease pressure, which can influence development of fungicide resistance, occurs under favorable conditions that plant pathologists describe using the “disease triangle.” Disease development depends on the interaction of these three factors:
- Weather and environment – Weather is the part of the triangle that cannot be controlled. Environment refers to factors such as row spacing, plant population and other agronomic practices impacted by weather, including soil fertility and irrigation. This element of the triangle will determine the severity of the infection.
- Host and agronomic practices – For the pathogen to survive, it needs a host. This can be a crop plant or vegetation in fencerows and ditches.
- Pathogen characteristics – This part of the triangle refers to the disease-producing agent, such as a fungus, that must be present for the disease to develop. It can be in the soil or crop residue in the field, survive on an alternate host, be blown or carried into the field by wind or machinery, or be introduced with the seed.
Many fungal diseases favor moisture, such as anthracnose, while others are more aggressive under drier conditions, such as powdery mildew and Fusarium stalk rot.
Almost every agronomic practice can impact disease development. Paul Vincelli, Extension plant pathologist at the University of Kentucky, has outlined some common agronomic factors that affect disease development: site selection, previous crop, variety selection, planting date, tillage program, fertility, irrigation practices, proper field drainage and plant spacing. Inadequate coverage or improper timing of fungicide application, such as after fungus sporulation, can also result in poor pathogen control and result in reapplication, potentially exposing the fungal population to multiple treatments with the same chemistry. Not using full label rates can also lead to inferior control and additional applications of the same fungicide. Additional factors can include seeding depth, harvest practices, seed treatment and soil compaction. Anything that increases disease pressure increases the risk of fungicide resistance.
Some fungi pose a greater risk for resistance than others. Common resistant disease factors to consider include:
- Rusts, powdery mildews and leaf spots can go through multiple cycles of infection, spore production, and reinfection during the growing season, resulting in multiple generations per year. Others, such as Fusarium head blight in wheat and smut diseases, only have one infection event per season.
- Fungicide resistance in airborne pathogens pose an even greater threat than soilborne fungi. Air currents can carry fungal spores long distances from fields or overwintering sites, including across the county and even across states. A less sensitive colony of a soilborne pathogen, such as white mold, tends to move slowly – perhaps only a few feet per year–depending on how far soil is moved by equipment or other means.
- The genetic tendencies of some fungi appear to help them adapt quickly to fungicides. For example, some species of Cercospora are very genetically diverse. The greater the genetic diversity, the greater the likelihood of the presence of an isolate in the field that is more difficult to control with a given class of chemistry. For example, the frogeye leaf spot pathogen in soybeans has exhibited resistance to strobilurin chemistries in some parts of the country because of the spread of resistant strains.
Guidelines for Fungicide Resistance Management
The Fungicide Resistance Action Committee (FRAC) is an organization composed of agricultural chemistry and agronomic experts. The group’s mission is to provide fungicide resistance management guidelines to prolong the effectiveness of fungicides and limit crop losses should resistance occur.
Many states develop recommendations for preventing fungicide resistance in field crops using FRAC guidelines, such as the following from the University of Wisconsin:
- Plant disease-resistant hybrids and varieties whenever possible.
- Maintain proper soil fertility.
- Scout fields on a regular basis, noting disease incidence and severity. Use the information to map individual field history for future disease management decisions.
- Avoid sites with a history of high disease incidence.
- Use a crop rotation that fits your area and field mapping.
- Tank mix high-risk fungicides (those with documented resistance, such as strobilurins and triazoles) with fungicides that have (1) different modes of action, (2) active ingredients effective against targeted diseases and (3) similar lengths of residual activity. Do not use reduced rates of fungicides.
- Alternate or tank-mix fungicides with different modes of action when multiple applications are required.
- Apply fungicides preventively or early in the disease cycle and when a disease threat is warranted.
- When possible, avoid curative fungicide applications, especially with fungicides that are at high risk for disease resistance.
Fungicide Chemistry Groups
Knowing which fungicides belong to a particular chemistry group significantly aids farmers in stewarding fungicides wisely. Fortunately for farmers, who have so many management considerations to weigh, resources exist to help them learn about fungus resistance to various fungicide chemistry groups, including the FRAC website. Farmers can also find additional fungicide resistance management tips on this site.
Crop Science Solutions
A well-thought-out disease-management program, including best management practices, proper seed protection and selection and fungicide applications using multiple modes of action, should be implemented to sustainably manage diseases. The following Crop Science solutions are valuable tools to consider for your program.
Before purchasing seed, selecting a seed treatment or applying any fungicide, please read the entire label for the best possible results and to confirm that the product is effective on the disease you need to control. Every product is not suitable for every situation, and correct application technique will ensure the best results.