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Fungicide Resistance Concerns

Dr. Katherine Stevenson’s fungicide sensitivity testing program is a wonderful service for Georgia’s pecan growers and I would encourage all of our growers to take advantage of this opportunity. It is free to the grower and provides orchard-specific information about the potential risk for scab resistance to the most commonly used fungicides for pecan. Instructions for sampling and forms can be found here: http://www.caes.uga.edu/commodities/fruits/pecan/documents/PecanScabSamplingInstructions-REVISED.pdf

Dr. Stevenson is getting a wealth of highly valuable information from this survey. The more orchards she is able to get information from, the better picture we will have about what exactly is going on in the state with fungicide resistance and how we need to manage it and still get the best level of scab control possible.

I have had many calls and have been hearing a lot of talk concerning rumors that “we have resistance to tin (TPTH) and we can’t use it anymore”. This is the furthest thing from the truth.

Dr. Stevenson says “We are seeing some unusually high levels of insensitivity to TPTH this year in some of the sampling locations – higher than we have seen in the past.  However, we have seen a gradual increase in insensitivity to TPTH over the past 20 years, which is not unexpected based on the heavy use of this product.  The unusually high levels we are seeing this year may be because insensitivity has increased over the past 5 years, but it may also be due to the fact that we are sampling more locations this year than we ever have in the past, which provides the opportunity to see the full range of sensitivity values that may not be readily observed when sampling from a small number of locations.”

I have heard many people comparing this to the resistance observed several years ago with Benlate. Dr. Stevenson indicates “It should also be noted that resistance to TPTH is not the same as resistance to Benlate.  Resistance to Benlate is qualitative and is controlled by a single mutation in the fungus.  The mutation leads to complete resistance in that individual fungal isolate.

Unlike Benlate, resistance to TPTH is quantitative and as far as we know, is not conferred by a single mutation, but more likely, the accumulation of many small mutations or other mechanisms.  It is not an “all or nothing” type of resistance, like resistance to Benlate.  This means that even if some insensitivity to TPTH is detected in a population, it generally happens very gradually and the fungicide may continue to be effective, especially when applied at a higher rate.”

There has been a lot of talk about half-rate mixes regarding resistance development. This is based on speculation and may or may not be true. Preliminary results indicate that resistance is also being found in orchards that have always used  only full rates.    Dr. Tim Brenneman offers the following comments:

  1. Anytime a fungicide is used there is some risk of selecting for less sensitive strains of the pathogen. This is particularly true for a perennial crop like pecans that harbors the same pathogen population year after year, and is also such a large tree that makes uniform spray coverage very difficult. In terms of exposure, we have used multiple tin applications for about 50 consecutive years on most of the pecans in the southeast, so it is not surprising to see some resistance. Tin resistance has been found in foliar pathogens of other crops, and has been manageable. Most pecan scab isolates are still showing reasonable sensitivity to tin, and we should be able to use it in integrated spray programs for years to come.
  2. To reduce selection pressure on any fungicide, other modes of action are used, either as alternated full rates or as reduced rates of combined complementary products (ie. Enable/Tin, Orbit/Tin, Stratego, Absolute, Quilt, etc). Both strategies are accepted by the industry and have been widely used on multiple crops. In fact, many new fungicides are only available as mixtures. Some chemistries even work better as mixes, which is the case with Tin/Elast for pecan scab.   There is a strong link between efficacy and resistance management, which makes this combination even more attractive. Mixtures have provided growers reliable disease control at an affordable cost, and they have helped us to stay within the law in terms of allowable amounts of fungicide per year.
  3. Please remember that these sensitivity numbers are relative to scab populations never exposed to fungicides, and do not mean the fungicides no longer have any activity. In orchards with scab showing high levels of resistance to a fungicide, it would be advisable to not rely on stand-alone sprays of that chemistry. However, we have much to learn about the relationship between these lab tests and the disease control expected in the field.

This survey is providing critical information that will help us manage scab, but it is only a first step.  Yes, we may be changing some recommendations, and may be spraying some other  fungicides that we have not been using on pecans.    It will take the combined efforts of UGA, USDA and industry to combat this emerging problem.

The best thing growers can do to learn where they stand with regard to this issue and how best to manage scab in their orchard is to have the scab from their orchards surveyed. In the meantime, avoid using the same fungicides over and over and rotate chemistries as suggested .

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About Lenny Wells

I am a Professor of Horticulture and Extension Horticulture Specialist for pecans at the University of Georgia. My research and extension programs focus on practical cultural management strategies that help to enhance the economic and environmental sustainability of pecan production in Georgia.