Tackling scab resistance
Hit scab hard with protectant fungicides.
Apple growers in the Midwest who stuck by the “old ways” of applying fungicides have not faced the problem of apple scab becoming resistant to fungicides. The old ways employed protectant fungicides like captan and mancozeb; the new ways used curative chemistries.
Now what should growers do? Dr. Janna Beckerman, a plant pathologist at Purdue University, Indiana, and Dr. George Sundin at Michigan State University are working on a peer-reviewed paper more fully exploring the topic.
“In Indiana and Michigan, growers have relied on systemic fungicides that provided kickback and gave them some wiggle room as to when to spray. Those fungicides aren’t working as well, and in some instance, aren’t working at all, and growers need to recognize this before they have a widespread failure in their orchard,” Beckerman said.
"The impact of fungicide resistance in apple scab has been significant, with fungicide resistance having been identified in all classes of curative fungicides used for the control of scab,” Beckerman said. “Soon after adoption of each of these newer fungicides, fungicide resistance became a recognized problem, starting in the late 1960s with dodine (Syllit).”
In 1971, after three years of intensive benomyl (Benlate) usage, resistance was detected. After intensive use of sterol inhibitors, such as Rally (myclobutanil) and Nova (myclobutanil), resistance was first described in 1985. In 2009, field resistance to the quinone outside inhibitors Flint (trifloxystrobin) and Sovran (kresoxim-methyl) was reported. The two scientists found resistance to four major classes of systemic fungicides used by growers in the Midwest, including many orchards with a significant proportion of their scab population composed of multiresistant isolates.
“Thus, for the first time since the introduction of dodine in 1958, apple growers must consider the possibility that any mistake in prebloom scab control can result in season-long scab problems and a high incidence of scabby fruit—and possibly 25 applications of fungicides throughout the growing season for scab control alone,” they reported.
On the good news side, in studies last year, Sundin and Beckerman found that only 12 percent of the 195 scab isolates they studied were resistant to all the major fungicide classes. Some were resistant to one or two or three. That means growers have curative fungicides they can use—they just need to find which ones work.
The old fungicide dodine, for example, was largely left behind by growers years ago. Beckerman found that scab in many Indiana orchards is once again susceptible to dodine, and she worked with two growers last year who got excellent scab control using it, in addition to using it in the Purdue experimental orchards.
Unfortunately, she didn’t find the same result for thiophanate methyl (Topsin M). Many growers still use this fungicide to control summer diseases, and indirectly that is still impacting the apple scab population.
One problem appears to be that growers suspecting resistance problems need a complicated analysis to find what will work on their farm. That means gathering over 100 samples per orchard, streaking out the fungi, and hoping to obtain at least 35 isolates to test for fungicide resistance for each class of fungicide. That’s what Sundin and Beckerman did with the 195 Michigan and Indiana isolates they examined last year. In 2009, Beckerman and Sundin received a National Institute of Food and Agriculture grant to develop better methods to reliably and quickly test for fungicide resistance in the orchard.
On the bad news side, Beckerman said, “We don’t have anything new coming down the pipe. The best new fungicide we have is Inspire Super, a second-generation SI, and we know resistance to that can develop as well,” although Beckerman and Sundin noted that it has not been yet observed in either state.
The pathologists also found that there is no “fitness penalty” for resistant scab organisms. It was long thought that resistant organisms gave up something in return for resistance, but that’s not true, she said. In the studies, resistant isolates were just as powerful, just as capable of infecting apples, as those without resistance to fungicides. Much to their surprise, only 1 percent of the isolates tested were susceptible to all systemic fungicides applied in the orchard.
The bottom-line advice from Beckerman is:
“Start early. Don’t wait for the onset of scab to apply protectant fungicides.”
A tank mix of captan and mancozeb not only protects from scab infection, it is effective on other diseases, she said. And captan is somewhat rainfast, in that it redistributes after a light rain. That is important because all green tissue needs protection as it emerges.
She is telling growers in Indiana to spray early and keep their orchards fully covered with protectant fungicides—like a tank mixture of captan and mancozeb—during the primary scab season. Growers in Michigan, the Midwest, and New York are also being advised to take measures like spraying 40 pounds of greenhouse fertilizer-grade urea in 100 gallons of water per acre to fallen leaves and then to mow and mulch leaves in either fall or early spring to disrupt overwintering scab organisms.
Growers who hit scab hard and early have been able to use protectant fungicide programs with captan and mancozeb (Dithane) without having scab problems. In her investigations, she found there was no scab at all to be found in these Indiana orchards, and less of a problem with resistance. Growers with a history of resistance problems reported excellent results using only protectant fungicides. Other protectant materials include copper and sulfur, often used by organic growers.
Beckerman and Sundin addressed this topic during a session at the Great Lakes Fruit, Vegetable and Farm Market Expo in Grand Rapids, Michigan, in December.