Orchards that have secondary bloom or have later-blooming varieties are more at risk for fireblight infection.
Controlling fireblight without antibiotics is doable, but it requires an integrated approach combining delayed dormant copper sprays with frequent applications of biological agents, says a plant pathologist from Oregon State University.
The antibiotics streptomycin and tetracycline are currently allowed under the National Organic Program for use on organic pears and apples, but a sunset review could eliminate their use on organics in the next few years, according to Dr. Ken Johnson, OSU plant pathologist. Antibiotics are not allowed on organic pears or apples in the European markets, he said.
Johnson advocates an integrated approach to fireblight control (combining antibiotics with biological agents) as a way growers can help prevent resistance to antibiotics. But for growers interested in organic production, there are ways to make the integrated strategy work without antibiotics.
Knowing what puts an orchard at risk is the first step of an integrated control program, he said. If fireblight was present in the orchard the previous year, growers should prune out old cankers to the extent that it’s cost effective. Orchards that have secondary bloom, later-blooming varieties, or are at higher elevations, are more at risk than orchards with uniform bloom or lower elevations.
Using a delayed dormant spray of copper appears to suppress the pathogen’s jump from the cankers to the flowers, Johnson said. He found in past research dating back 20 years or more that delayed dormant copper sprays provided a range of controlup to 70 percent control in two different trials and 35 percent in another trial. “It does work, it does do some good, but I’m not sure that it always works.”
Johnson encouraged growers to use fireblight-risk models like Washington State University’s CougarBlight, which he has found to be very accurate in correlating rising temperatures and degree-days with rising infection levels. In 2009, Johnson collected flowers and analyzed them for fireblight using a quick molecular test. “The positives and negatives matched well with CougarBlight,” he said. “You can trust CougarBlightwhen it says it’s warming out there, that’s also when the pathogen is getting active.”
However, he advises growers who aren’t using antibiotics and who had previous fireblight in the orchard or in nearby orchards to consider using a lower threshold than recommended by CougarBlight and begin applying biological agents earlier than the model recommends.
In trials with both apples and pears, Johnson compared several biological agents to antibiotics to learn about their effectiveness against fireblight. By themselves, biological products like BlightBan (Pantoea agglomerans strain C9-1) and Bloomtime (Pantoea agglomerans strain E325) have had limited effectiveness. A new biological called Blossom Protect (Aureobasidium pullulans) is showing promise, but is not yet registered in the United States. Serenade Max (Bacillus subtilis), marketed by AgraQuest, has antibiotic characteristics, he said, and should be considered as an organic substitute for Mycoshield (oxytetracycline).
In a pear trial he conducted last year, two applications of BlightBan, followed by two applications of Serenade Max, plus Nu-Film-P, provided fireblight control as effective as two applications of Mycoshield. The same was true in apples. With both apples and pears, biologicals need to be applied early in the life cycle and before the build-up phase of the pathogen.
“To the degree possible, practice integrated control,” Johnson said during the Washington State Horticultural Association’s annual meeting. “Delayed dormant copper, followed by biologicals, then Serenade Max, all at higher frequencies of applications, appear to improve fireblight control.”
He believes that applications of the apple bloom thinner lime sulfur plus fish oil are probably helping to suppress fireblight during early bloom. In trials with 2 percent lime sulfur and 2 percent fish oil, which he described as standard bloom thinning protocol in apples, three applications made at 30 percent bloom, 70 percent bloom, and full bloom provided good control. But he cautions that the bloom thinner is not compatible in tank mixes with the biological fireblight control products of BlightBan and Bloomtime. Those products should be applied separately in a later treatment, two to three days after the lime sulfur/fish oil applications.