In a lesson about apple scab on the American Phytopathological Society (APS) website, authors Lisa Vaillancourt and John R. Hartman of the University of Kentucky note that the presence of apple scab can be detected in 15th and 16th-century still life paintings of apples.

Centuries later, the disease is still a problem. “Apple scab is the most economically destructive disease of apples in the world,” professor Wayne F. Wilcox notes in a Cornell fact sheet. He continues, “In the Northeast, it is usually not possible to produce apples commercially without some program to control this disease.”

The scab fungus overwinters in infected leaves on the orchard floor. In early spring, sexual reproduction takes place, and the primary spores (ascospores) develop. When spread by wind and rain, the ascospores germinate and infect the unfolding leaves and buds. The lesions usually appear first on the undersides of the leaves in blossom clusters. Ascospore production occurs only once in a season.

In about one and a half to three weeks, depending on the weather, asexual reproduction within the lesions produces conidia. This causes secondary infection on the leaves and fruit. Conidia spores are much more numerous, and many cycles of this secondary infection can occur within a growing season.

Temperature and moisture impact the development of both primary and secondary apple scab infections.

Management involves both prevention and control. Unfortunately, the pathogen, Venturia inaequalis, has developed resistance to several fungicides in some areas. In addition, numerous fungicides are classified at high risk for resistance. The spotty pattern of fungicide resistance – even within a specific orchard – complicates the situation. For these reasons, along with the need to monitor local weather conditions, growers should consult their extension educators for the latest information on controls and their ideal timing.

The Mills table, presented in the APS lesson cited above, charts the rate of infection by temperature and duration of leaf wetness. For instance, at 65 degrees Fahrenheit and nine hours of wetness, a light infection is likely. At 65 degrees Fahrenheit, with a wetting period of 18 hours, expect a heavy infection.

Many land-grant universities track the weather and recommend controls throughout the season based on their regions. For instance, Keith Yoder, professor and pathologist at Virginia Tech, reports current conditions and the likelihood of disease incidence on his tree fruit disease update blog. Kari Peter, assistant professor and pathologist at the Penn State Fruit Research & Extension Center, periodically issues disease updates, which include appropriate controls. Underscoring the necessity for area-specific data, Peter advises, “When controlling for disease, weather and tree growth conditions need to be monitored at a local level within one’s own orchard.”

The lesions on these apple tree leaves were created by the apple scab pathogen.
Photo courtesy of Dr. Kari Peter, Penn State.

Growers should focus on prevention if possible. In Cornell’s “Scaffolds Fruit Journal” from March 12, 2012, professors David Rosenberger and Kerik Cox outline four proven options for reducing ascospore loads: treating the orchard floor with urea in late fall or early spring; flail chopping the leaf litter; applying dolomitic lime; and raking or vacuuming the leaf litter and removing it.

In the APS lesson, Vaillancourt and Hartman also suggest destroying fallen leaves in the fall, as well as pruning trees regularly to enhance air movement and allow sunlight to speed leaf drying. In addition, they write, “Overhead irrigation should be avoided, especially when weather conditions are favorable for scab development.”

The importance of wetness, including humidity, should not be overlooked. In drier climates, apple scab is not as common. Dr. Mark Mazzola, pathologist at the USDA-ARS Tree Fruit Research Laboratory in Wenatchee, Washington, remarks, “Apple scab is not much of a problem here.” Gary Grove, professor of plant pathology at Washington State University, reports, “When we do see apple scab, it’s because of over-the-canopy sprinkling.”

In a Michigan State University Extension post dated May 8, 2014, professor George Sundin says, “Controlling primary scab is critical for preventing fruit infections and keeping scab from becoming a full-season problem.” He also describes some of the newer fungicides and tank-mixing them for control and resistance management.

Alan Biggs, West Virginia University professor of plant pathology and extension specialist, rates risk development potential in the chart “Fungicide Resistance Management Guidelines for Apple Scab and Powdery Mildew Control in the Mid-Atlantic United States.” The chart also provides management guidelines, gives the FRAC code designations, lists the active ingredients and ranks the efficacy of 25 fungicides, including the newer mixes.

Apple powdery mildew management often overlaps apple scab controls. In his paper “The Disease Management Challenges of 2012,” Cox evaluates fungicide performance against those diseases, as well as fire blight, in New York orchards. Besides resistance situations, Cox has noticed in some instances that certain mixes of preventive and curative fungicides have diminished their powdery mildew effectiveness.

As noted, primary apple scab infections commence in spring and should be controlled at that time. Biggs reports, “Disease management becomes more difficult after primary lesions have appeared, with each lesion potentially producing tens of thousands of spores. Even a few primary lesions can be problematic if the weather is favorable for the pathogen. Growers with established primary lesions will need to continue to apply fungicides on a seven to 10-day schedule until summer weather shuts down the production of spores from lesions. Some fungicides can inhibit or eradicate spore production from primary lesions, although this ability can be affected by the presence of less sensitive or resistant isolates of the pathogen to the fungicide being used.”

Sundin says, “As we get later in the season, the growing trees have unprotected tissues between sprays.” He urges growers to keep on top of spray timing and to tank-mix broad-spectrum protectants with fungicides that have a single mode of action. He also notes that the spray interval may need to be shortened.

If left unchecked, apple scab will reduce yields and weaken the trees. Moreover, undetected scab, particularly in late fall, will manifest itself on stored apples, rendering them unsalable for the fresh market.

Apple cultivars vary in their susceptibility to apple scab. Cox reports that McIntosh and varieties with a McIntosh background are susceptible in New York. These include Cortland, Empire and Macoun. Wilcox includes Golden Delicious and Paula Red among those that are moderately resistant. For organic markets, Wilcox lists Liberty, Florina and GoldRush as resistant. However, he cautions that because there are different races of the apple scab fungus, regional variations among cultivars may apply.

The author is a writer/researcher specializing in agriculture.


American Phytopathological Society lesson on apple scab:

Cornell apple scab fact sheet:

“The Disease Management Challenges of 2012”:

“Fungicide Resistance Management Guidelines”:

Michigan State University Extension:

Penn State Extension Fruit Times:

“Scaffolds Fruit Journal”:

Virginia Tech Tree Fruit Disease Update blog:

West Virginia University Kearneysville Tree Fruit Research & Education Center:

Graphic by Siarhei Makhnach/