It’s considered a weak pathogen. Still, with favorable conditions Nectria can kill a tree.
Opportunistic, Nectria develops cankers on trees under stress or weakened by fire blight, black stem borer, sunscald, frost and ice, storm damage, hail, improper pruning, herbicide applications and mechanical damage. Excavation nearby that prunes roots can also lead to weakened trees.
Professor Kari Peter, a tree fruit pathologist at Penn State’s Fruit Research and Extension Center in Biglerville, Pa., characterizes Nectria as a “wound invader.”
Apple, cherry, pear and quince trees can be attacked by Nectria spp. Other hosts include aspen, birch, elm, filbert, hickory, maple, oak and walnut.
Nectria cankers often appear as small, discolored sunken areas around wounds or damaged tissue. The infection frequently occurs at leaf scars, branch stubs, cracks in the branch axils and other lesions that expose the cambium. The tree responds to the infection by forming rounded, corky rolls of callus tissue and bark. If the fungus infects healthy tissue beyond the callus ridge the following year, the tree forms another barrier. Subsequent cankers then develop into concentric ridges that resemble a target.
Random branches and twigs that do not leaf out in the spring are symptomatic of the fungus infection. Repeated infections make the tree more susceptible to decay and diminish the tree’s longevity. If the cankers ultimately girdle branches or small trees, the branch or tree may die.
In the spring and early summer, small cushion-like fruiting structures colored cream, pink, or salmon form in the openings over the canker. These asexual structures turn brown or black with age. During summer and fall, red to orange-red sexual fruiting structures form in the canker area. Wind and water disperse the spores.
Since Nectria canker is most severe on stressed trees, numerous cultural practices can reduce the damage. Young, 2-year-old trees should be protected from drought and freeze damage. Regardless of age, trees should be kept growing vigorously. Infected branches should be removed and must be destroyed away from the orchard or grove. Burning may be the most preferred destruction method for any tree debris. Proper pruning to preserve a sound branch structure should be employed – but avoid pruning in wet conditions. A planting site should minimize wetness duration. The proper moisture and fertilizer levels should be maintained. Avoid wounding the tree bark.
Peter noted, “Disease incidence is greatest in years when wet harvest weather is followed by winter weather that causes cold injury.” The excessively cold winter, plus fire blight, hail, heavy thunderstorms and borer damage combined with the wet season provided ample opportunities for apple trees to be weakened this year in some regions.
Professor Kerik Cox, plant pathologist at Cornell University, reported in late summer 2014 that he had been sent numerous samples of Nectria canker on apples. He also observed it in all of his research orchards from both young current-season plantings to older 15-year-old blocks. Except for his newly planted orchard, he only found Nectria on shoots compromised by physical or chemical injury, or by fire blight.
In New York State, Cox said he saw a lot of the Nectria cinnabarina and a few other weakly pathogenic species which emerge on damaged shoots from mid-summer to early fall. (Nectria galligena causes European canker, and is also important in South America and has been spotted in California. N. galligena is considered more aggressive than N. cinnabarina.) In Pennsylvania this year, Peter observed N. cinnabarina.
In contrast, Dr. Gary Grove, director of the Irrigated Agriculture Research and Extension Center in Prosser, Wash., said Nectria isn’t a problem in that state’s apple orchards due to the dry climate.
According to the U.S. International Trade Commission, the state of Washington produces the most apples followed by New York while Michigan and Pennsylvania rank third and fourth respectively. Both Cox and Peter advise pruning the N. cinnabarina infections from the tree to reduce the number of spores and curtail new infections.
“Pruning out shoots infected with Nectria should occur on a cool day following a 24-hour period of dry weather with two days of dry weather forecasted following pruning,” Cox said.
He stresses that pruning should not be performed in wet weather.
“If the infection has reached the leader or trunk, it is probably best to remove the tree,” Peter added.
She also emphasized removing the cuttings and infected plant parts by burning or physically removing them from the orchard to prevent the still-viable spores from causing new infections during the coming season.
Noting that even the most effective fungicides are only locally systemic and would not penetrate deeply into woody tissues, Cox cautions that management begins in the fall by protecting leaf scars resulting from leaf fall. Copper fungicides at the recommended 20 to 80 percent leaf fall timing can be adjusted for local fall rains.
He warns that applications only protect against new infections, and will not arrest established infections in the woody tissue. If labeled in their states, growers can protect with Kocide 3000 and Badge SC in the fall leaf drop season.Warmer winters, less rainfall and fewer cases of fire blight may minimize Nectria infections next season.
COVER PHOTO COURTESY BY BOB FERGUSON