Pictures came from Patrick Connors (UGA Pecan Breeder) website
So why are we seeing so much scab in treated orchards? It could be due to diseases becoming resistant to certain fungicides. We are not seeing complete resistance like we see with resistant pigweed, but we are seeing a shift in sensitivity to certain fungicides.
Several local growers have participated in a program offered by UGA. They have been submitting samples of scab from orchards across their farms and the results are pretty interesting. Across many of the test sites there was at least some level of insensitivity to several of the fungicides.
There was low to moderate insesitivity to the triazole fungicides like propiconazole and tebuconazole. There was low to high insensitivity to strobilurin fungicides like azoxystrobin, and TPTH (Tin). In the samples submitted there was little to no insensitivity to Dodine and Thiophanate Methyl.
This does not mean that we have widespread disease resistance. It does make me want to recommend growers submit disease samples for analysis. If we can find insensitive scab then we can change our spray program to better control the disease. The only thing that it will cost you is postage and the time it takes to get the sample.
You can get instructions for collecting and submitting samples at the following link