Stone fruit production profitability relies upon managing insects and diseases. Cornell recently hosted a webinar on the topic, featuring Dr. George Sundin with Michigan State University and Dr. Brett Blaauw with the University of Georgia and Clemson.
Sundin said American brown rot (ABR) and cherry leaf spot are “two very important diseases we deal with each year.”
Peaches, sweet cherries, plums and other stone fruits are susceptible to ABR. The fungus grows very quickly so the fungus has high virulence.
Weather plays a role in the fungus’s spread. “We’re getting more rain and multi days of rain,” Sundin said.
Unfortunately, growers in Michigan are dealing with fungicide resistance in products they’ve used for decades.
“If you do sustain some blossom blight, it’s more inoculum when fruit is starting to ripen,” Sundin said. “It’s bad all around.”
As fruits start to ripen and sugars increase, they’re susceptible to brown rot infection. It doesn’t need wounds to infect, but microscopic wounds facilitate infection. It can also spread after harvest, Sundin said. Some fruits may have brown rot you can’t see and those spores can spread in storage.
ABR favors warm, humid weather, such as 18 hours of moisture at 50º F or five hours of wetness at 77º to initiate infection. Once above 80º or below 55º, infection rate is slower. In optimal conditions, mature fruit can decay in as little as two days.
Fungi in stone fruits overwinters in “mummies,” the fruit ensconced in white fungal material which can produce more than 50,000 spores. The highest release period is between petal fall and harvest, with peak sporulation closer to harvest, when there’s more material to infect than at any other time.
Lesser sources include twig and branch cankers and aborted fruit.
Fungi overwinters in peach “mummies” like this. Photo by Deborah J. Sergeant
Sundin believes that optimal protection from ABR relies upon a protective strategy – applying fungicide on fruit surfaces prior to the arrival of fungal spores.
For decades, producers have relied upon Indar® (demethylation inhibitor), which has resulted in resistance levels that are too high to make this fungicide useful.
Alternatives include a succinate dehydrogenase inhibitor (SDHI) premix with strobilurin (Cabrio®); Miravis® (SDHI solo); Luna® Sensation (SDHI premix with strobilurin Flint® Extra) and Cevya® (second generation DMI).
“These are all quite good for brown rot,” Sundin said. “Cevya [has] demonstrated that it does control DMI-resistant brown rot fungus.”
In MSU’s field trials in 2023, spray treatments at 12, six and two days prior to harvest showed reduced disease incidence at harvest.
“We run the trials on peaches as it’s much easier to rate,” Sundin said. “Any disease occurring is because of a lack or reduced efficacy of the fungicide.”
A wet, rainy season and the researchers’ inoculation created an ideal scenario for the fungi.
“Nearly all the fruit was infected,” Sundin said. “It was a really high-pressure situation. We had good results. Miravis lagged a little bit, I’m thinking because of the surfactant we had … We had a really good result with Howler. In our trial from 2024, it did not look as good.”
He advised using the maximum rates of Merivon® (6.7 fl. oz./acre), Miravis (5.1 fl. oz.) or Cevya (5 fl. oz.). Broad spectrum fungicide helps with resistance management, including Captan® or sulfur. For Howler®, applying 2.5 lbs./acre in less conducive weather conditions helped.
Farmers spraying for blossom blight should consider Rovral® their best option to save other fungicides for pre-harvest use.
“Then you’re not using your other modes of action early in the season,” Sundin said. “The inoculant pressure is lower.”
Cherry leaf spot affects sweet cherries. Sundin called it a “relentless disease.” Only two to three weeks after the initial infection, the leaves will defoliate. Trees can’t photosynthesize and will be at risk for death.
More than 50,000 spores can emerge from each lesion.
Keeping at least 50% of the leaves by mid-September can allow enough to photosynthesize, which helps the roots protect the trees over winter and have enough energy base to carry the tree through the cold in good health.
“If we time shift this defoliation so it’s not starting until the end of July, we can have the defoliation not happening until mid-September,” he added.
Cherry leaf spot has shown fungicide resistance in Michigan. Sundin said that several sterol demethylation inhibitors all are ineffective in Michigan.
“It doesn’t leave us with very much but we still have some broad-spectrum fungicides,” Sundin said.
These include Captan, recommended at 2.5 lbs./acre; coppers at 1.2 lbs. metallic cu./acre; and Cabrio and Flint Extra with a half rate of Captan for resistance management.
In his 2024 trials, untreated control trees were almost defoliated by mid-July. “You need two healthy leaves per fruit to ripen the fruit,” Sundin said.
Badge® SC fungicide/bactericide combines well with copper oxychloride and copper hydroxide.
“That combination seems to work,” Sundin said. “The slow release works quite well. We used slightly less metallic in Badge X2 but it has more slow release. We like both, but under intense pressure we like BX2.”
Blaauw also spoke about managing San Jose scale.
“It’s one thing we can creatively market, as ‘dinosaur eggs’ or ‘Dalmatian fruit,’” he said.
San Jose scale affects peaches’ appearance with dark, round speckles. “They’re blobs of an insect that feed on the tree and drain the sap,” Blaauw said.
Armored scales have a hard, waxy shell. Soft scales have a less-defined waxy coating. The pests move across a plant to find a feeding place, secrete the waxy layer and then settle.
“It makes it kind of unappetizing to most people, but the bigger issue is when they’re feeding on the tree … if it’s not managed, this tree will die,” Blaauw said. It can take five to six years if the tree has no management.
Various scale pests are invasive and feed on a variety of hosts, including fruit-bearing trees like apples, cherries, plums, peaches and persimmons.
Current treatment options include chemical controls and in-season options, although biological control and mating disruption are two areas of future research.
