Fresh Perspective on Fungicide Resistance in Berry Crops

Anthracnose and botrytis are two common fruit rot diseases that can alter the quality and yield of blueberry crops. Anthracnose announces itself through shriveled fruit and oozing orange spores, while botrytis presents a telltale coating of gray mold. Although easy to spot, these infections are difficult to control. Blueberry fungi are notorious for developing resistance to several chemical classes at once, which can leave growers grasping for answers.

“Over time, these microbes become more resistant as a group,” Timothy Miles, a plant Pathologist at Michigan State University, said at the Southeast Regional Fruit and Vegetable Conference in January. “There can be a gradual buildup of resistance, or it can be like a light switch where the fungicide stops working.”

UNDERSTANDING FUNGICIDE RESISTANCE

Anthracnose often infects blueberries during bloom, staying dormant until the fruit ripens. Caused by the Colletotrichum acutatum fungus, the disease overwinters on dead twigs, old fruit trusses, and bud scales. Spores from overwintering infections are then dispersed onto developing fruit during early growth.

The Fungicide Resistance Action Committee (FRAC) classified Botrytis cinerea as a high-risk pathogen for its ability to resist multiple chemical classes concurrently. The disease produces millions of airborne spores, usually entering the crop during the cooler days of early spring bloom.

Farmers battling these contagions must first understand the factors that drive fungicide resistance, Miles said.

“Some of it has to do with the type of chemical, or what gene it’s targeting and how it’s inhibiting the fungus,” Miles said. “Sometimes it’s the intrinsic property of the chemical or how it’s applied.”

PROBLEM DISEASES

A 2021 survey of Michigan blueberry growers named anthracnose and botrytis as problem diseases, along with mummy berry, Phomopsis, and Alternaria fruit rot. MSU research indicates 80% resistance in C. acutatum to FRAC 11 materials (strobilurins) and rising resistance in B. cinerea to FRAC 7, 11, and 17. In Michigan it is also common to find B. cinerea that is simultaneously unaffected by FRAC 1 (MBCs), FRAC 7 (SDHIs), FRAC 11 (QoIs), and FRAC 17.

Given the rampant resistance of C. acutatum to FRAC 11, Miles said that growers should reconsider their reliance on these chemistries. This list encompasses products such as Abound, Pristine and Quilt Xcel, he noted.

“It’s not something I recommend to use during the critical periods (of growth), at least in Michigan,” Miles said. “Most of the places that have grown blueberries for a long time had high resistance, but we didn’t find resistance to FRAC 9, 12, and 7. These would be your SDHIs, or things like Switch, which would be a 9 and a 12.”

B. cinerea’s fungicide resilience originates from a highly malleable genome that is constantly shuffling its DNA. In addition, a single blighted blueberry cluster can pump out innumerable spores — in the right conditions, these survivors can re-infect a field in days.

CHEMICAL CLASS CROSSOVER

A research team that included Miles found both anthracnose and botrytis fruit rot in Georgia blueberry crops. Beyond blueberries, botrytis is pervasive across Michigan’s grape and strawberry fields. FRAC 11 materials have demonstrated minimal impact on B. cinerea based on samples collected in 2019, 2020, and 2022.

“We noticed that Pristine had a lot of resistance in botrytis as well, and same with Abound, so FRAC 11 materials look like they won’t work perfectly,” Miles said. “But then we saw some resistance to cyprodinil (Vangard, Syngenta, FRAC 9) and a couple of other chemical classes, too.”

Blueberry growers do have some options to combat this tough pathogen, Miles said. For instance, while B. cinerea has shown resistance to Pristine (BASF) and Luna (Bayer CropScience) products (FRAC 7), Luna enjoys some efficacy against the pathogen.

Cultural practices that promote pruning, limited overhead irrigation, and air circulation are another means of managing the pathogen before it infects your crop, Miles said.

“These practices are very important, but you can also rotate FRAC codes,” Miles said. “Keeping inoculum low in your field — and then using dormant sprays or anything that’s multi-site — would also be helpful.”