I was out looking at an orchard yesterday which just received some needed rains. With the dry weather, aphid populations were rising and were sprayed. Mites were also present and now increased after aphid spray. The rain also helped to wash off sooty mold growing on leaves from honeydew of aphids. We observed some Desirables showing a much lighter crop than we were hoping – especially for the lower pressure from scab at this time. Pollination of this year’s crop has been negatively impacted with cooler nights and cloudy, rainy days this Spring. The crop is behind, and we’re still in mid nut sizing period.
UGA Pathologist, Dr. Katherine Stevenson, has been testing many leaflet samples for insensitivity of our fungicides. There has been concern over resistance found in tin (TPTH), leading some to believe we cannot use it anymore. This is not the case. Below is a summary from Dr. Stevenson:
“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.
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.”
UGA Extension Pecan Specialist, Dr. Lenny Wells says the best thing growers can do is learn where you stand with regard to this issue and how best to manage scab in an orchard and to have scab surveyed. In the meantime, avoid using the same fungicides over and over and rotate chemistries as suggested.