Kill the pathogen

Apple growers have three new fungicides they can use to control apple scab, and Dr. George Sundin would like to see growers keep them.

“Use full covers and full rates,” he said. “Kill the pathogen. Resistance management is resistance prevention.”

Sundin, who is Michigan State University’s tree fruit pathologist, spoke to growers at several events this winter.

“We can’t afford to lose good fungicides every seven years,” he told them.

“The chemical companies have invested a lot of money in these new fungicides, and they say they don’t have anything new in the pipeline. We want to save these, so they’ll still be effective in 20 years.”

Growers got the strobilurin fungicides (such as Flint) in 1999, and by 2008, Sundin’s lab had detected resistance to them. The industry got the first succinate dehydrogenase fungicide Pristine (pyraclostrobin and boscalid) for cherry in 2004, and by 2011, resistance to it appeared in the cherry leaf spot pathogen.

The new fungicides are: Fontelis (penthiopyrad) from Dupont; Merivon (Xemium and pyraclostrobin) from BASF; and Luna Tranquility (fluopyram and pyrimethanil) from Bayer. All have new labels in 2013. These materials should be used no more than twice during a season, targeting the stage from pink to petal fall. They should be part of a full fungicide spray program, beginning with an early attack on overwintering inoculum, to achieve total suppression of primary scab by petal fall, and continuing with a protectant fungicide cover spray program designed to end the season with low levels of scab on leaves when they fall from the trees.

The program Sundin recommends starts with urea application in fall or early spring. Applying 40 pounds of urea in 100 gallons of water, and 100 gallons per acre, will reduce the spore level. Flail mowing is another possibility, as chopping the leaves disrupts scab lesions on the leaves that will release spores in spring.

Sundin said scab pressure during the growing season depends greatly on the level of inoculum in spring. If 20 percent of the leaves have lesions in the fall, there will be 7,000 times as many spores ready to infect new ­tissue than if the orchard has 1 percent infected leaves, he said.

At green tip, growers should spray with protectant fungicides such as captan and EBDCs like manzate, mixed together at a rate of three pounds of each, with copper, if they choose, which also adds some fireblight protection. Captan or EBDCs should be tank mixed into every spray aimed at apple scab all through the season, he said, being mindful there are limits stated on the label for how many pounds can be applied and how close to harvest they can be applied.

The period from pink to petal fall is the time of highest scab pressure, he said. Temperatures are rising, spore release is peaking, and new tender tissue is emerging and expanding rapidly. Weather may be rainy, very conducive to the growth of scab organisms, and orchard conditions can make spraying difficult.

When growers ask Sundin about curative properties or back action of fungicides, he tells them, “Don’t think about it.”

Where they get into trouble—and jeopardize the effectiveness of their fungicides—is when they allow scab to get away from them and then apply good fungicides to high levels of inoculum, he warned. This gives the organism a good chance to generate potentially resistant spores.

He thinks resistance comes in part from allowing populations to build too high, extending spray intervals, getting inadequate coverage, and teasing the organism with less than totally lethal rates.

Sundin also reminded growers that some of the new materials are premixes the companies developed to make sure growers applied more than a single fungicide, to avoid resistance development. But, since apple scab in Michigan is already resistant to one part of the mixture, growers need to treat them as if they were single products and make their own tank mixes using EBDCs and captan.

These protectant materials have never fostered scab-resistant strains, and some growers still rely on them totally for their apple scab fungicide program.