Opportunistic fungi

Disease organisms invade injured tissue and develop cankers that release spores. Some red strains of McIntosh are susceptible, for reasons unknown, to opportunistic diseases that kill branches.

Photos courtesy of george sundin, Michigan State University

Starting in 2006, an epidemic of sorts started in apple orchards in Michigan. It was a planting epidemic, as some 272,000 McIntosh apple trees of the LindaMac strain and 242,000 of the RubyMac strain were planted. These strains become super-red early and are attractive to growers and consumers for this reason.

As these trees moved into their first years of heavy production, a mysterious ailment struck in many of these orchards. Cankers developed that killed branches and even some whole trees.

Facing an unknown disease problem, growers and Extension fruit educators in the field—Amy Irish-Brown in the Fruit Ridge area north of Grand Rapids and Dr. Nikki ­Rothwell in the Traverse City area—called on Michigan State University plant pathologist Dr. George Sundin.

After studying the situation for nearly two years, Sundin has concluded that the major problem is a horticultural one—something about the super-red Mac strains is making them unusually susceptible to ­relatively ordinary diseases.

He has found six disease organisms involved—black rot, white rot, Anthracnose, Cytospora, coral spot (Nectria twig blight), and Alternaria.

“These new cultivars are somehow predisposed to attack by these opportunistic fungi,” he said. “The symptoms are frequently seen the year after a stress. Not surprisingly, most of these trees are in their fourth or fifth leaf and have just produced their first big crop.”

Epidemic

A few strikes were reported two years ago and then more in the fall of 2009. Last spring (2010), following the previous year’s big crop, many strikes were seen.

It may have seemed like an epidemic, Sundin said. Growers on Fruit Ridge and in the fruit-growing area around Traverse City began planting these new strains heavily about five years ago, so there are a lot of young orchards.

The disease strikes some branches, causing dieback at the tips first and progressively killing the branch. It is not known whether these diseases can act systemically, like ­fireblight, Sundin said.

At first, cold injury was suspected—and it may be part of the problem, Sundin said. These trees may not harden off properly. The symptoms were sometimes worse in frost pockets. Still, many of these disease organisms occur mainly in the Southeast, Pacific Northwest, and West—in warmer climates than Michigan and in areas where it is wet. Michigan has been experiencing warmer, wetter summers and colder winters the last few years.

Parasites

The disease organisms are considered weak parasites that invade pruning wounds and other injuries. Several of them have a fruit-rot phase. The diseases first infect fruit, which provides a source of spores that invade foliage and leaves, causing limb cankers that can girdle and eventually kill branches.

Management can be difficult, Sundin said. With so many disease organisms involved—each with its own cycle of spore production and infection—Sundin says it will be difficult to use fungicides effectively. He is evaluating the various organisms to determine how they act in Michigan orchards and screening fungicides to find those that are effective against each one. If they each have different sensitivities and different timings, a very complicated spray program may result.

Growers at the Great Lakes Fruit, Vegetable, and Farm Market Expo, where Sundin spoke in December, had plenty of questions. But Sundin had few answers. The fungicides have not yet been tested for efficacy—and, obviously, these diseases are not being controlled by the fungicide programs growers are now using in the spring to control apple scab and those used to control summer rots and diseases.

Do the infections become systemic, as with fireblight? Do infections spread from infected trees to nonstressed trees? These questions as yet have no answers.

His advice: “Prune out dying limbs as soon as you see them. Get the infected tissue out of the trees and out of the orchards.” Sundin recommends that dying limbs and cankers be burned to destroy the disease organisms. The cankers can sporulate even after the limbs are removed, he said.

Other pieces of advice: Discourage late-season growth and encourage the trees to harden off, he said. Keep nitrogen applications under control. Late-season copper sprays may play a part—in that they sometimes cause defoliation and earlier hardening.

But a core question remains to be addressed by horticulturists, Sundin said. “What’s going on with these trees? Why are these ­varieties ­susceptible?”