Fighting Fungicide Resistance in Fruit and Nuts
By rotating chemistries and implementing other best management practices, fruit and nut growers can get the most out of their disease-management programs.
Fungicide resistance continues to be an issue for growers of various crops, including fruit and nuts.
Phillip Brannen, Ph.D., Extension Fruit Pathologist and Plant Pathology Professor at the University of Georgia, says fungicide resistance is a “huge problem” among fruit growers in the South and other regions. “It’s a major issue,” Brannen adds, predicting that more fungicide classes will continue to become resistant to various diseases on a variety of crops.
The three main categories of fungicides sprayed on fruits and nuts for which resistance is increasing are demethylation inhibitors (DMIs), succinate dehydrogenase inhibitors (SDHIs), and quinone outside inhibitors (Qols), a group that contains strobilurins.
The development of fungicide resistance is an evolutionary process, says Lindee Love, Strategic Account Manager for agricultural chemical company Corteva Agriscience.
“Resistant fungi develop over time and then reproduce to the next generation,” she explains. The larger fungi population continues to spread, causing resistance to a fungicide’s mode of action, Love adds. With “limited tools in the toolbox” to control the resistant fungi, the resistant population can continue to reproduce.
Themis Michailides, Ph.D., a Plant Pathologist at the University of California, Davis, says resistance depends on the disease. For instance, Alternaria, which occurs in fruit and nuts, can develop resistance to fungicides quickly, as in two to three years after product registration if the fungicide is used excessively. But other diseases, such as Botryosphaeria canker and blights in nut crops, don’t develop resistance, Michailides adds.
It’s not just fungicide resistance that’s a problem, Brannen states. Contact fungicides such as captan, chlorothalonil, mancozeb, thiram, and ziram, which are the backbone of many spray programs because they don’t develop resistance, are in danger of being limited and even banned by the Environmental Protection Agency (EPA).
“There’s a lot of pressure to remove products of that nature, which will put even more pressure on the products that are going to develop resistance,” Brannen says.
What Growers Can Do
Fungicide resistance has been a fight for growers, but those who are fighting back are experiencing success in keeping resistance in check.
Most growers are taking the matter seriously, Brannen says. Fungicide resistance is a popular topic of discussion at meetings. “We talk about FRAC (Fungicide Resistance Action Committee) codes, modes of action, and how to incorporate resistance management into spray programs,” he adds.
Michailides says there are general rules to combat fungicide resistance. “One common aspect is not to use the same chemistry to control disease. You must alternate and rotate different groups of fungicides,” he adds.
While labels often limit growers to the number of chemical applications, alternating between or tank mixing different classes is vital, Brannen agrees. Love stressed that it’s important to rotate chemistries according to different FRAC groups, not trade names. Just because they have different trade names doesn’t mean they have different modes of action, she adds.
Spray timing is also important, Michailides notes. With pistachios, it’s important to spray at bloom time, especially if the weather is wet and cool, to control Botrytis. At the same time, this spray will also provide almost 50% control of Botryosphaeria, he adds.
Love says if growers are concerned they have resistance in the field, they should send samples to a lab to confirm what diseases have developed resistance. It’s critical for growers to act if a fungicide is deemed ineffective to learn if it should be removed or limited in their programs.
Love advises growers to devise a tentative fungicide program to ensure they are rotating the different FRAC groups and to adjust such programs as necessary, based on weather changes, disease pressure, and other factors, so the fungicides they’re using are more efficacious on the diseases being targeted. In addition to rotating chemistries, she advises growers to scout early and often (weekly) to estimate disease pressure. She also says growers need to ensure favorable air flow, proper irrigation, and balanced fertility (too much nitrogen can make plants more susceptible to disease).
Brannen says the UGA Cooperative Extension helps growers determine what fungicides to use and in what order to use them in spray programs, including rotation and tank mixing, to utilize resistance management.
“We take a very hands-on approach, depending on the commodity,” Brannen states. “With peaches, for example, [our approach] is very direct. We’ve had a spray program in place for more than 20 years where we tell producers what they need to spray during a certain time frame for resistance management.”
With grapes, however, the UGA provides workshops for growers to help them develop spray programs. “Grapes are a complicated commodity to grow relative to the number of times a grower has to spray all the different fungicides that need to be sprayed,” Brannen says. “We actually sit down with growers during these workshops to help them develop spray programs that meet all the needs for both disease control and resistance management.”
The key factor with resistance management is growers “need to mix it up,” Brannen says. He tells growers that contact fungicides like captan, mancozeb, sulfur, and chlorothalonil should be utilized as much as possible throughout the season because diseases don’t develop resistance to them, and they provide good resistance management tools. “With the fungicide categories that do develop resistance, growers need to target them during the time diseases are most prevalent,” he adds.
Brannen, Michailides, and Love say it’s important to treat preventively for disease as part of resistance management.
“Growers want to keep disease levels as low as possible for as long as possible,” Brannen says, noting that disease thresholds aren’t the same as insect thresholds. “We don’t want a little bit of disease. A little bit of disease early in the season is always going to be bad.”
Continued Education
Brannen says he and other pathologists continue to study and learn about resistance management. “We certainly have the opportunity to learn more,” he adds. “There’s a lot of information available on how resistance develops, but we still have questions about how to best utilize materials when they come to the market to prevent resistance from occurring. And that, again, is probably related as much as anything to the individual fungus or bacteria because they’re all different.”
The studying and learning have paid off. For instance, Brannen says it has been determined that an SDHI and Qol combination should be used in the first spray to control brown rot in peaches followed by an application of a DMI, even if there is DMI resistance. “This is our standard recommendation,” Brannen says. “You don’t want to spray a DMI first. You can’t reverse it.”
There are few new active ingredients being developed for broad-spectrum control fungicides with new modes of action. Love says the industry is in “dire need” of new chemistries to help alleviate pressure on the current chemistries in the market.
Michailides agrees, adding that a new active ingredient is needed with a different mode of action or different modes of action. “It would be nice to have an active ingredient where the pathogens will not develop resistance,” he adds. “Most fungicides today have a specific mode of action, which is why there is widespread resistance. However, if a fungicide has a wide mode of action, then it’s difficult for a pathogen to mutate in all different gene sites.”
Brannen believes fungicide resistance management will eventually be improved by genetics. “There’s an opportunity to incorporate more genetics into particular plants, fruits, and otherwise,” he says.
Learn more about managing fungicide resistance.