New treatment may prevent losses from apple scab

Apple scab, a fungal disease affecting apple orchards in Illinois and worldwide, can significantly reduce fruit quality and yield. In fact, the disease recently damaged more than 50 percent of some apple varieties in Illinois orchards. When samples from those orchards were tested, some strains of the fungus were found to be resistant to traditional fungicides.

“I rushed to do something to prevent this disaster. We did an experiment in 2014 and 2015 and were lucky to get very good results,” reports University of Illinois plant pathologist Mohammad Babadoost.

Babadoost and his team tested a new protocol using combinations of systemic and contact fungicides. Dithane M-45 (mancozeb), a contact fungicide, should be applied at the green-tip stage at 3 to 4 pounds per acre, along with the systemic fungicide Inspire Super (difenoconazole + cyprodinil) at 12 fluid ounces per acre. After seven days, the treatment should be followed up with a combination of Dithane M-45 and Fontelis (penthiopyrad) at 20 fluid ounces per acre. Each treatment should be repeated three times, seven days apart, for the most effective control.

“When we tested this combination of chemicals, we could not find even a single scabby apple,” Babadoost says. “Growers that trialed the treatment in 2015 reported no scab.”

Despite the success of the treatment, Babadoost notes that it should not be seen as a silver bullet. “We are in a battle with the pathogen almost all the time,” he says.

Apple scab causes lesions on leaf and fruit tissue that thicken and take on a scabby appearance. In later stages of the infection, the skin of the fruit can crack, allowing in secondary pathogens that can lead to fruit rot or other symptoms. All growing portions of the tree are susceptible to the fungus.

Babadoost warns, “Any green tissue is subject to being attacked. It starts very early in the season. If growers are able to control it effectively as soon as growth starts in the spring, there will be almost no disease by summer. But if they miss the window in spring, summer will be a disaster.”

In addition to the new fungicide treatment protocol, other control options are available to growers. For example, growers can choose apple varieties that are resistant to apple scab; including ‘Honeycrisp’, ‘Jonafree’, and ‘Gold Rush’; avoiding susceptible varieties, such as ‘Fuji’, ‘Gala’, ‘Honeygold’, ‘Winesap’, and others. A more extensive list of resistant and susceptible apple varieties is provided in Babadoost’s recent U of I Extension Fact Sheet.

Again, Babadoost issues a warning: “Even if an apple variety is resistant, it might not be resistant forever. Resistance might break down.”

Small growers, organic growers, and home gardeners can prevent infection by removing or applying a five percent solution of urea to all dead leaves on the ground, as the fungus overwinters in leaf litter. Removing nearby crab apple trees will also be beneficial. Organic growers can apply organic sulphur- or copper-based fungicides, but Babadoost is not confident that organic fungicides will provide good control of resistant strains of apple scab.

“Production of organic apples in Illinois is not an easy task,” he says.

Growers should monitor and treat trees early and often to prevent widespread infection. With the new treatment protocol in place, the 2016 growing season holds a great deal of promise for apples in Illinois. For more information, read the U of I Extension apple scab Fact Sheet.

— Lauren Quinn, College of Agricultural, Consumer and Environmental Sciences

Source: University of Illinois