Typical shepherd's crook symptom due to fire blight on an ornamental pear shoot.
PHOTO BY DON HERSHMAN, UNIVERSITY OF GEORGIA, BUGWOOD.ORG.
Dr. Ken Johnson, professor of plant pathology at Oregon State University, knows that growers are concerned with the potential loss of antibiotics for fire blight control.
"An organic equivalency standard has been set between the EU and the U.S.," said Johnson. "One of the bones of contention in the final agreement was the fact that we use antibiotics in pears and apples. Last year, the National Organic Standards Board set a new expiration for streptomycin and oxytetracycline for 2014." Although this action may still be halted, growers should be prepared to initiate alternative controls.
Johnson's research is focused on finding solutions for organic growers, since a larger percentage of the cultivars in acreage currently under organic production are susceptible to fire blight. However, host resistance would be the ideal solution, so the long-term goal is to come up with fruit cultivars that are resistant to fire blight while meeting consumers' preferences for quality and flavor.
One product Johnson is testing is Regalia, an extract of giant knotweed, which acts as a resistance inducer. "It isn't directly toxic to pathogens," he said, "but if you put it on a plant, it causes the plant to turn on defense genes, which makes the plant more resistant to fire blight infections." Johnson added that Regalia probably won't be a stand-alone, and he will research it further this season.
Other possible products are copper and organic acid preparations. However, Johnson says it's unclear whether these products will receive National Organic Program (NOP) approval. One product is Phyton 27AG, a food crop form of Phyton that has been used in the nursery industry. The other is Gowan GWN 9979. The amount of copper in these products is much lower than in previous products, so the pounds of copper applied are lower. Johnson noted that several years of research showed no fruit russeting.
It's important to understand the disease cycle of fire blight in order to control it effectively. Fire blight overwinters in old cankers, and cells become active along the margins of those cankers. "Orchard sanitation is extremely important in fire blight control," said Johnson. "The more of these old cankers you can get out of the orchard, the better off you are." From the old cankers, the pathogen jumps to flowers via rain, wind and insects. During the floral epiphytic phase, the pathogen population is influenced by temperature and the number of blossoms. "The pathogen is not causing disease at this point, but is building up and driving the amount of pathogen cells to cause infection. When the pathogens get to a certain number on a stigma, they slide down the style into the floral cup, enter the natural openings in the base of the flower, and start the infection," explained Johnson. He noted that the entire process is temperature-driven, so it happens more slowly when temperatures are lower.
Fire blight on apple leaves and tip shoots.
PHOTO BY MARY ANN HANSEN, VIRGINIA POLYTECHNIC INSTITUTE AND STATE UNIVERSITY, BUGWOOD.ORG.
"It's important for growers to pay close attention to fire blight models, which model the speed of the floral epiphytic phase," said Johnson. "If you had disease in the orchard last year, the probability of having it (and how soon it shows up) is higher this year. On warm spring days, when temperatures are in the 70s and 80s, fire blight risk builds quickly."
Johnson noted that in the past growers could pay less attention to the pathogen buildup phase, apply streptomycin at high-risk levels and get good control because it effectively killed the pathogen. However, resistance is a severe issue in the West, and a potential issue throughout the U.S.
"If we're going to use nonantibiotic control successfully, we're going to have to work more on the pathogen buildup phase with products," said Johnson. "It's probably going to take a couple more applications of material to get control."
Johnson is currently conducting research on a systems approach to address four questions: When is the fire blight pathogen detectable/active in orchards? Does delayed dormant copper affect pathogen activity? How does bloom thinning affect control? Can we have effective control without antibiotics?
A molecular test has been developed to detect the DNA of the fire blight pathogen. Blossom clusters were collected and sampled at mid-bloom, full bloom and petal fall. Research results showed that the overall probability of finding fire blight pathogen in flowers was 15 percent. At mid-bloom the probability was 9 percent, but at petal fall that figure rose to 30 percent. "There is an easier level of detectability later," Johnson said. "We went into 'dirty' orchards with a high level of disease and found that the disease gets going faster. In a clean orchard, it usually doesn't show up until later."
So, does delayed dormant copper affect pathogen activity? A 2009-2011 study in a California pear orchard showed that by petal fall more than half the samples had the fire blight pathogen. "Delayed dormant copper slows down the initial number of cells that start that epiphytic phase," said Johnson. "The copper on the surface of the tree slows the rate at which bacterial cells are making the jump into the flowers." Johnson noted that the research showed no effect on fruit finish due to delayed copper treatment.
In addressing the issue of blossom thinning, he said that blossom thinning is essential for obtaining good fruit size. He noted that lime sulfur is now used more frequently for blossom thinning in both organic and conventional production, and it is also fairly effective for scab control.
"Two percent lime sulfur plus 2 percent fish oil is a good thinner; first applied at 20 to 30 percent bloom, then at 70 to 80 percent bloom," Johnson said. He added that lime sulfur is not compatible with other fire blight products.
Is it possible to gain effective control of fire blight without antibiotics? Johnson said, "There are two phases of fire blight control. One is the protection of the stigma, which is where the floral epiphytic phase occurs (when the pathogen is growing and spreading among flowers), and protection of the nectary. If we go to nonantibiotic control, we have to worry about this phase more, and that means doing something early in bloom. It could be as simple as putting on bloom thinners or using gram-negative bacterial products such as Bloomtime Biological or BlightBan A506. They colonize the stigmas and slow down pathogen growth on the stigmas." Johnson noted that those products are not stand-alone and must be followed with something to fill the nectary to prevent infection such as Blossom Protect, Serenade MAX or a copper/organic acid product.
Johnson said, "We put on products early that target the stigma and pathogen buildup in flowers, then we put on products late that will prevent infection when the pathogen slides down and goes into the floral cup."
The author is a frequent contributor and freelance writer who farms and raises Great Pyrenees in south-central Pennsylvania. Comment or question? Visit www.farmingforumsite.com and join in the discussions.