Overcoming Apple Scab

Ask researchers and crop consultants what the most prominent and problematic disease is for apples in the Northeast U.S. and you’ll hear “apple scab” more often than not.

Dr. David Rosenberger, professor of plant pathology and superintendent of Cornell University’s Hudson Valley Laboratory, says three critical factors affect the severity of apple scab infections in a given year.

First, be aware of existing inoculum within the orchard. “Growers who had trouble controlling apple scab last year can anticipate that they’ll likely have a tough year again in 2009 because inoculum is still present in the orchard,” Rosenberger says.

Second, watch the weather during snowmelt. “At the lab, I’ve observed that growth of the fungus stops when leaves dry out in the winter, because the fungus needs water to grow,” Rosenberger says. If no rain occurs during and after snowmelt, fungus development remains slow because of the lack of moisture. But if rapid snowmelt is followed by warm spring rains, the spores mature very quickly.

Third, observe the amount of spring rainfall. Apple scab ascospores are only discharged during rain. “The other two critical factors might both indicate a heavy scab year, but if there isn’t any rain from green tip through bloom, it’ll end up being an easy scab year,” Rosenberger says.

Experts agree that fungicides are still the best way to control apple scab, but caution growers to follow a management approach incorporating multiple control strategies in order to stay ahead of scab resistance.

A Multifaceted Approach

“Start by planting scab-resistant cultivars whenever possible,” Rosenberger says. While many of the major apple cultivars are susceptible to the disease, breeding programs throughout the U.S. and in several foreign countries are developing disease-resistant cultivars.

Rosenberger also recommends that growers who have a history of apple scab problems consider spraying orchard floors with 40 pounds of urea per acre prior to green tip. “A urea spray reduces the number of ascospores likely to emerge from leaves, so if you do end up with a wet spring, you’re at less risk because you’ve reduced the amount of inoculum.”

However, fungicides are still necessary for effective apple scab control as well as management of other diseases. “The number one reason growers continue using sterol inhibitor (SI) fungicides is because they are still the best products out there for controlling mildew and rust diseases,” says Rosenberger.

Glenn Morin owns a consulting business, New England Fruit Consultants, which has provided consulting services to commercial tree fruit growers for the past 25 years. One of the top performers in Morin’s fungicide efficacy trials is Indar 2F (fenbuconazole, Dow AgroSciences). Morin reports that Indar has shown efficacy against low to moderate levels of demethylase inhibitor-resistant apple scab in various research trials in the Northeast, because it remains on the fruit surface longer and has a greater intrinsic activity against the apple scab fungus than most other SI fungicides.

But even “super SI’s” — along with all other fungicide classes — should always be used as part of an integrated apple scab control program to manage resistance. “Apple growers should always apply a fungicide like Indar in combination with a fungicide of a different class,” says Morin. “Also, don’t make more than two back-to-back applications of the same fungicide class, and be sure to use labeled rates.”

Rosenberger’s recipe for successful apple scab control starts with a mancozeb fungicide, followed by an application of a strobilurin in combination with mancozeb at pink bud stage and bloom or at tight cluster and pink. “Follow up with an SI fungicide in combination with mancozeb at petal fall and first cover. That generally results in pretty good control,” says Rosenberger.