A fungal disease called apple scab recently damaged more than 50 percent of some apple varieties in Illinois orchards. Samples taken from the affected orchards found that some strains of the fungus were resistant to traditional fungicides. University of Illinois plant pathologist Mohammad Babadoost reports, “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.”
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.
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.”
Source: University of Illinois/fruitgrowersnews.com
