Growers producing apples in the cool, damp northeast quadrant of the United States need to take a step-by-careful-step approach to apple scab control—starting early and being meticulous—or they can be in for a long summer of expensive spraying and still lose part of their crop to this fruit-disfiguring disease.
Dr. Kerik Cox, a Cornell University plant pathologist who advises growers in western New York, said that he and other plant pathologists in his state—and across the Great Lakes, Midwest, and Mid-Atlantic region—are in close agreement on the advice they give growers and in explaining the reasons for it.
The chief problems are growing resistance of the scab organism to existing fungicides that have curative activity and a dearth of new materials having curative activity. Careful use of protectant fungicides is becoming critical.
Last spring, Cox and fellow Cornell pathologist Dr. David Rosenberger, who serves growers in eastern New York, wrote a paper outlining options for growers faced with managing high-inoculum orchards and sensitive varieties like McIntosh. The year 2009 was a difficult scab year for New York growers, so high overwintering inoculum levels posed a threat in the spring of 2010.
“Apple scab emerged as an expensive problem in apple orchards throughout the northeastern United States in 2009,” Cox and Rosenberger wrote. “Experienced growers who had managed scab effectively for many years found themselves spraying throughout summer to control scab that kept appearing on new leaves.”
Growers were ready to implement good advice last spring, and the weather cooperated as well, so 2010 turned out to be not a difficult year for scab control, Cox said.
“A major factor contributing to summer scab problems in 2009 was the near total lack of hot dry periods during summer,” the scientists reported. “Daytime temperatures above 85°F for several consecutive days cause a decrease in the number of conidia produced in scab lesions. At the same time, heat may slow tree growth, thereby leaving trees with less susceptible tissue since each new leaf is susceptible to scab for only a few days. The absence of heat to shut down scab during summer in 2009 would have been less important if we still had fungicides with the kind of eradicant scab activity that the DMI (demethylation inhibitor) fungicides had before that fungicide class was compromised by resistance issues.”
The current recommendations for growers as they try to contain apple scab start with orchard sanitation to reduce the amount of inoculum their orchards are exposed to in spring:
- Apply urea sprays (40 pounds of urea per acre in 100 gallons of water) to fallen leaves in fall or spring. Applying urea as late as green tip will reduce the level of ascospores. Scab spores travel only about 100 feet, and orchards are basically self-infecting.
- Shred leaf litter with a flail mower. This promotes decay and, when done in spring, reorients leaves and disrupts ascospore discharge.
- Apply dolomitic lime (2.5 tons per acre) over fallen leaves in fall to help increase the rate of leaf decay.
- Remove leaf litter from the orchard by raking, sweeping, or vacuuming. Leaf removal may be a practical option for organic growers.
“None of these approaches will eliminate 100 percent of the ascospores, but any one of them can reduce inoculum production by 80 percent or more,” according to Cox and Rosenberger.
Growers need to start early applying protectant fungicides on a seven- to ten-day interval, keeping new growth covered and replacing fungicides lost after considerable rain. They should apply protectant fungicides at the first sign of bud break.
“In orchards where DMIs, strobilurins (QoIs), and/or Syllit (dodine) are no longer effective, failure to protect against green-tip infections almost guarantees a scab problem through summer,” the pathogists said.
In general, their fungicide recommendations remain the same as in previous years.
They recommend a prebloom schedule involving a tank mix of mancozeb at three pounds per acre plus captan-80 at 1.5 to 3 pounds per acre.
Captan cannot be used in combinations with prebloom oil sprays. “By using Flint (trifloxystrobin) or Sovran (kresoxim-methyl) with mancozeb for two sprays sometime between tight cluster and petal fall, one gains even more activity against scab (so long as it is not strobilurin-resistant) as well as protection against rust and mildew,” they said. “Even where DMIs no longer control scab, they are still recommended in combinations with mancozeb or captan for sprays at petal fall and second cover because they still provide the best overall activity against mildew and rust diseases.
“In cases where a protectant fungicide could not be applied ahead of rains during the prebloom period, Scala (pyrimethanil) or Vangard (cyprodinil) can provide up to 72 hours of reach-back activity. However, these fungicides do not redistribute well, so they should always be used in combinations with mancozeb or captan. Syllit (dodine) combined with mancozeb might also prove useful for early-season sprays if resistance testing has shown that the scab population is not fully resistant to dodine.”
“Not much spraying is needed for scab after second cover, and the recommendation for first cover would work for second,” Cox said. “After second cover, growers will primarily spray captan with Topsin (thiophanate-methyl) or captan alone, or captan with ProPhyt (potassium phosphite) to control summer diseases and any residual scab from third to seventh to eighth cover.”
Fungicide resistance testing
New York apple growers who have persistent scab problems are encouraged to submit leaf samples to Cox’s fungicide-resistance testing program, one of few such programs in the United States. “Results from a fungicide resistance analysis are critical for taking corrective action if scab shows up in the orchard during the growing season,” the scientists said. “Where DMI fungicides or Syllit are still active, these products can arrest a scab epidemic, whereas using them against resistant populations can waste time and money.”
Cox’s resistance-testing procedure takes some time, so samples need to be taken in the current summer season to have results useful the next year.
In summary, the pathologists said, “Control strategies need to be adjusted based on inoculum levels, fungicide resistance issues, and cultivar susceptibility. Some kind of inoculum reduction strategy should be employed in orchards that have severe scab, and inoculum reduction is especially critical for high-inoculum orchards with DMI-resistant scab and scab-susceptible cultivars like McIntosh.”