Apple, pear growers fight fire blight outbreak

Fire blight bacteria oozes from the branch of an apple tree in an Omak, Wash., orchard on June 6. Insects, birds, wind and water can spread infectious spores.

YAKIMA, Wash. — Fire blight, a bacteria that kills apple and pear trees, has been an accelerating problem the last three springs in Central Washington orchards. 

Growers say last June was the worst month. They cut infected limbs and whole trees and burned them. An East Wenatchee agricultural consultant, Nick Stephens, in June said it was the worst he’d seen in 28 years and orchards young and old were in grave danger. 

Six months later, Dec. 4, at the Washington State Tree Fruit Association annual meeting in Yakima, Tianna DuPont, Washington State University tree fruit specialist, said everyone knows it was a “hard year” for fire blight, but “we don’t know how hard or how much it cost us.”

A show of hands of a couple hundred growers in the session showed about half dealt with fire blight this year.   

Sarah Kostick, a WSU horticultural doctoral candidate, talked about her studies in 2016 and 2017, the first on some cultivars. 

“Most apple cultivars are susceptible to some extent to fire blight. It’s difficult to compare studies because of different methods used,” she said. 

Kostick and her team inoculated up to 30 shoots per tree on numerous varieties for two years and added other methods to create a multiple matrix survey. 

The results show Jonathan, Granny Smith, Gala, Honeycrisp, Cripps Pink, Jonagold, Golden Pinova and McIntosh are highly susceptible to fire flight. Fuji, Cosmic Crisp, Delicious and Rome Beauty were moderately susceptible and Aurora Golden Gala, Empire and Enterprise were of low susceptibility.

Gennaro Fazio, research geneticist at USDA ARS in Geneva, N.Y., said rootstock can become infected in bark wounds at the base or from infection at the top of the tree that spreads to the bottom. He talked about fire blight field trials in Geneva rootstock and said many new varieties, including Jazz and Envy, are sensitive to fire blight. 

“Cosmic Crisp (the new Washington state variety) is moderately sensitive, but that’s still sensitive so it’s a concern,” Fazio said. 

Fire blight can stay subdued in a tree for a long time and it’s hard to get rid of, he said. 

Kerik Cox, associate professor at Cornell University, talked about assessing and minimizing the threat of fire blight following mechanical thinning and hedging and said hedging can be used to remove fire blight.  

He also talked about calcium and biological treatments at pink stage of bloom. 

Kari Peter, research pathologist at Pennsylvania State University, said fire blight has been a significant challenge in young, high-density and older large-tree apple orchards in that state. She talked about low rate calcium applications. 

Fire blight overwinters in trees and reactivates in oozing cankers around blossom time. It is acerbated by extreme heat followed quickly by rain during bloom. It attracts flies and other insects that spread it to blossoms. Within a week or two, infection is ahead of portions of trees that show withering. 

Antibiotics, copper fungicides, lime sulfur, other minerals and biological controls are applied before, during and after bloom but at best are 80 percent effective, growers have said. 

Central Washington field reporter

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