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When apple breeders in New Jersey, Indiana, and Illinois came together in 1926 to form the Purdue-Rutgers-Illinois university collaboration called PRI, their ­number-one goal was to create new apple varieties that were scab resistant.

They did it, too, and about 20 resistant varieties have been released. While several varieties have reached commercial production levels, others are just great choices for backyard gardeners and roadside marketers who want freedom from the tedious problem of repeatedly spraying against apple scab. Some of the variety names reflect this desire: Freedom and Liberty, for example.

But about 15 years ago, reports began to come in, first from Europe, that new races of apple scab could overcome the genetic resistance, which was originally derived from genes (Vf) carried in a crab apple, Malus floribunda 821. Since then, a number of PRI varieties formerly labeled scab-resistant have had their resistance broken by the new scab strains.

Dr. Janna Beckerman, a plant pathologist at Purdue University, said the appearance of these strains has changed the way pathologists look at the disease. Disease resistance is not absolute. She now suggests that new scab strains appear over time and that varieties once resistant will not remain so.

Take precautions

“Golden Delicious, when it was first released in Europe, was reported as scab resistant. That resistance quickly broke down. Today, Honeycrisp is noted to be less susceptible to scab than say, McIntosh or Gala. In time, and as Honeycrisp is more widely planted, I suspect we’ll see the fungus evolve the ability to infect it better.”

She now recommends that growers with PRI varieties take precautions to keep them resistant to scab. That means spraying them two or three times early in the spring with protectant fungicides to eradicate primary scab, the time when new strains might emerge from the overwintering leaf litter in the orchards.

Some PRI varieties appear more resistant than others, reflecting their heritage. While all of them carry the Vf gene, some of them, like GoldRush, also carry other resistance-related genes derived from other sources. Golden Delicious, a parent of GoldRush, was once considered quite resistant to scab, and GoldRush’s scab resistance so far seems to be unbroken.

Regardless of a cultivar’s susceptibility, fungicide applications are an essential component of effective scab management and maintaining the durability of the gene, Beckerman wrote in a Purdue publication aimed at helping growers manage scab-resistant apples. For disease-resistant cultivars in particular, the most critical step of the process is controlling primary infection by ascospores in the spring. These spores are the offspring of the previous year’s infection and have the potential to infect even scab-resistant cultivars if they contain the right combination of genes. Using fungicides on scab-­resistant cultivars can help prevent infection by any newly virulent strain of the fungus, protecting the tree from primary scab and any successful secondary scab. You don’t need to apply fungicides like these trees were McIntosh, but you can’t pretend they are immune and will never get the disease.

Apply fungicides in early spring from green tip and continue on a seven- to ten-day schedule until petal-fall (seven days during wet weather, ten days if dry), she recommends. After petal fall, scout trees to confirm that no primary infection occurred. If no scab lesions are observed and primary scab control was successful, no further fungicide sprays for scab are necessary, although fungicide ­applications to manage other diseases may be ­necessary.