Fire blight is caused by bacteria. Therefore, antibiotics are the most effective tools to control it. But that doesn’t mean organic growers are simply out of luck. They have weapons with which to fight the blight, too.

“I do think the organic programs work, they’re just a little bit more strategic,” said Ken Johnson, an Oregon State University plant pathologist, at the Washington State Tree Fruit Association’s annual meeting in December in Yakima, Washington.

The past two years saw more fire blight than usual in Central Washington, an arid growing area typically with lower blight pressure than the more humid regions of the Midwest and East Coast. An entire session of the annual meeting was devoted to the disease and how to manage it. Johnson’s presentation related directly to organic control of blight, with an eye toward fruit safety.

Ken Johnson, Oregon State University, speaks during the Organic Controls that Work panel at the Washington State Tree Fruit Association's annual meeting in December 2018 in Yakima, Washington. (TJ Mullinax/Good Fruit Grower)
Ken Johnson, Oregon State University, speaks during the Organic Controls that Work panel at the Washington State Tree Fruit Association’s annual meeting in December 2018 in Yakima, Washington. (TJ Mullinax/Good Fruit Grower)

First, his disclaimer: Antibiotics are not allowed on organic fruit and none of the organic alternatives work as well, he said. And even those that do work may cause russeting in certain varieties.

Instead, organic growers must follow a season-long, phenology-based program of copper sprays, lime sulfur and Bacillus-based biorationals that lasts through petal fall, and beyond.

In fact, that was one of his main messages: Continue spraying a week past petal fall, especially if temperatures spike. In most of his trials in Corvallis, Oregon, he has found the highest populations a week after petal fall, when flowers are all but gone. In one old Bartlett block, his inoculum count jumped from 200 cells per flower at petal fall to over 1 million cells per flower a week later.

Eventually, the tree will reach a point at which sprays will do no good, but researchers are uncertain exactly when that point is.

“So, I’ve been trying to get people to extend their programs, and I think it makes a difference in fire blight control,” he said. He has been telling applicators and conventional growers the same thing.

As for when to start, he recommends applying 5 pounds to 6 pounds per acre of fixed copper just prior to green tip.

Early during bloom, his research has found that lime sulfur sprays used to thin flowers also suppress fire blight. After early lime sulfur sprays for thinning, make sure to apply Blossom Protect (Aureobasidium pullulans).

Lime sulfur and Blossom Protect have a tricky dynamic. Lime sulfur kills bacteria, but it also kills yeast.

He recommends applying Blossom Protect right after the second round of lime sulfur. Try to get at least one full application or two half applications and cover every row. Just remember that pears, especially Anjou and Comice, are more susceptible to russeting than are apples, and the risk from Blossom Protect might be too high, especially in wetter climates.

From full bloom to petal fall, when the weather is warming up, he recommends watching the Cougar Blight model, a computer-based risk assessment developed by Washington State University Extension, to determine spray intervals.

Serenade Opti, a Bacillus-based biorational, applied every two to four days would be the safest for fruit. To improve control, mix it with Cueva (copper octanoate). An effective rate is 4 quarts per acre.

Previsto (copper hydroxide) or other soluble copper compounds also are good choices. The effective rate for Previsto is 3 quarts per acre. Just remember, copper chemicals increase the risk of russet, though Previsto and Cueva have additives to mitigate that risk.

“We like Previsto because it generally makes our trees look fairly happy after we’ve applied it,” he said, comparing photos of a leafy, flower tree treated with Previsto and a more scraggly neighbor treated with another form of soluble copper.

Then, at petal fall and even after, he recommends following up with lime sulfur again, 2 percent to 4 percent, to clean up bacteria, yeast, mildew and fungus — at least on red apples such as Fujis and Galas that don’t russet easily.

On yellow apples, such as Goldens or Opals, and certainly on pears, lime sulfur carries its own russeting risk.

Consult weather reports and the Cougar Blight model for your areas. At this time of the year, temperatures can spike and you may need more than one treatment per week. •

—by Ross Courtney

Related:
—Managing fire blight for the conventional grower
—Stopping shoot blight
—Midwest faces strep-resistant fire blight
—Video: Fire blight management tips from the 2018 WSTFA Hort Show