Scab resistance breakdown
Most scab-resistant apple varieties around the world carry the same resistance gene.
Apple scab is the most important disease in the Midwest.
Scab-resistant apple cultivars are more resistant than their nonresistant cousins, but that doesn't mean they are immune from infection, says a Purdue University Extension pathologist.
"Many people assume that resistant means never getting the disease, so they think they don't have to do anything," said Janna Beckerman, extension plant pathologist for Purdue. "These trees do need to be sprayed, just not as frequently [as standard cultivars]."
In the Midwest and other humid areas of the United States, apple scab (Venturia inaequalis) is the most important disease of apples, she said. Losses from the disease can reach 100 percent if weather conditions are conducive and the disease is not properly managed. Standard varieties grown in the Midwest require 10 to 20 fungicide applications to protect the crop from scab. However, repeated application of many systemic fungicides has resulted in the development of fungicide resistance, Beckerman added.
Most of the scab-resistant apples in the world have origins leading back to the PRI plant breeding program, a collaborative effort between Purdue University, Rutgers, the State University of New Jersey, and the University of Illinois. Efforts to breed scab-resistant cultivars began in 1926 when the Vf gene from Malus floribunda 821 was bred into commercial apple cultivars. Since 1970, nearly 90 percent of the scab-resistant varieties that have been released worldwide reportedly carry that Vf resistance gene, according to Beckerman. Such varieties include Williams' Pride, Jonafree, Enterprise, Prima, Pristine, and GoldRush.
"Unfortunately, just as reliance on a few fungicides has resulted in fungicide resistance, reliance on disease-resistant apple varieties in the absence of other management has resulted in the breakdown of Vf-based scab-resistance worldwide," Beckerman said.
A breakdown of the resistant Vf gene has been reported in Europe, England, and, most recently, in the United States. In 1993, scab lesions were first found in a German orchard on the scab resistant variety Prima. In 2007, Beckerman found scab on M. floribunda 821 in what's known as the Old Hort Farm, Purdue's Horticultural Research Farm in West Lafayette. Scab on resistant trees was also found in Illinois and Ohio. Last year, the disease was found in Indiana and Illinois on the resistant varieties of Pristine, Pixie Crunch, and Jonafree.
"With the majority of resistant cultivars possessing single-gene scab resistance from M. floribunda or M. floribunda 821, we've created a situation where all of our eggs are in one basket," she said, adding that reliance on a single Vf gene worldwide places tremendous pressure on the pathogen.
What does the breakdown in resistance mean to the grower?
Beckerman answered that regardless of cultivar susceptibility, apple scab must be managed through the application of fungicides. For organic and sustainable production, it is important, even with resistant varieties, to rigorously apply fungicides during key scab infection periods to prevent primary infection when weather is cool and wet and the tissue is young and susceptible to infection. Applying one to three fungicide sprays to prevent primary infection in the spring should keep resistant cultivars free of scab for the entire season, she noted.
Cultural practices can also help minimize and even prevent scab. Such practices include selecting sites that provide more than six hours of sunlight per day, spacing trees to allow good airflow and light penetration, and pruning to open the tree canopy.
She advised growers to avoid planting scab-resistant varieties next to susceptible apple cultivars to prevent any successful scab infections in susceptible isolates that could ultimately infect a resistant variety.
Orchard sanitation is also key in preventing the disease from overwintering on fallen leaves and developing into fruiting bodies. But she admits that removing all fallen leaves is impossible. However, mulch or flail mowing in late autumn to shred leaf litter can reduce the risk of scab by 80 to 90 percent if all of the leaf litter is shred. Additionally, applying 5 percent urea to autumn foliage can help increase leaf decomposition and reduce the amount of fungus that will survive the winter.
If urea applications are made, care must be taken to avoid overfertilizing the trees, stimulating tree growth and predisposing trees to winter injury. She suggested that urea be applied just before leaf fall, or just after leaves have fallen. Urea could also be applied as a ground spray near the silver-tip stage of growth to avoid stimulating tree growth.