Fire blight is a damaging disease, no matter the location. Most production areas of the Pacific Northwest and California experience major outbreaks every three or four years. And while full orchard outbreaks are rarer, annually many orchards experience fire blight strikes on secondary blooms in May or June, during the weeks following petal fall. And with increasingly higher tree density orchards planted to susceptible scions and rootstocks, the risk for damage from strikes increases.
Growers in Western states produce 90% of the fresh organic apples and pears grown in the U.S., according to data from USDA-National Agricultural Statistics Service. In 2014, organic growers lost a valuable tool against fire blight when antibiotics were phased out of certified organic production.
Ken Johnson, Professor of Plant Pathology at Oregon State University, is working on non-antibiotic strategies for growers in California, Washington, and Oregon, thanks to a $476,000 USDA Organic Agriculture Research and Extension Initiative grant.
Johnson’s advice is to shift growers from using a one-punch tactic to fire blight control, and move into an integrated control strategy.
David Granatstein, Sustainable Agriculture Specialist with Washington State University (WSU), agrees.
“Successful non-antibiotic control will rely on an array of different tools combined in different ways for specific crops, cultivars, and conditions,” Granatstein says.
With organic growers reaching new heights – according to a WSU surveys at grower-meetings, certified-organic apple acres in Washington alone increased by 13% to 16,172 acres in 2016 and at least 8,000 more acres in transition – successful strategies are paramount.
For successful fire blight control in organic orchards, an integrated, season-by-season approach is recommended. It’s a change from a time when antibiotics were the one-trick pony. According to Johnson’s research, products are recommended by application windows. The sequence of non-antibiotic materials is dubbed “integrated control.”
“Integrated, non-antibiotic fire blight programs use the disease cycle to time the sequence of materials to target phases of pathogen activity during the season,” Johnson says. “While a given material alone may not be sufficient to manage fire blight by itself, a combination of materials at the right timings can provide control similar to or better than antibiotics.”
For example, growers can spray lime sulfur in early bloom for fruit load management, as it also protects the tree in early bloom thanks to its bactericidal properties.
“In a just-planted orchard, the chance of infection is greatly reduced by removing blossoms (by hand or lime sulfur thinning),” Johnson says. “Many organic growers successfully use the blossom removal method to prevent fire blight from developing in secondary blooms on their young pears and apples.”
Blossom Protect (Westbridge) can be applied in early bloom to help protect the flower from being colonized by the fire blight pathogen.
“The decision to use a biological material such as Blossom Protect is strategic and made independent of weather-based risk determinations. It should be applied after the last lime sulfur thinning spray (a one-day interval is sufficient),” Johnson says.
In order to create a protective residue on the blossom, bacillus-based biorationals such as Serenade Opti (Bayer CropScience) and soluble coppers such as Cueva (Certis) and Previsto (Gowan) can be applied later in bloom. Johnson suggests growers follow disease risk models to help assess when and how often to spray.
“Generally, soluble copper materials show higher efficacy than Bacillus-based biorationals, but the soluble coppers pose a greater risk of causing fruit russeting, the severity of which is dependent on cultivar-sensitivity and environment,” Johnson says.
Variations Among Western States
Lime sulfur can be unpredictable in humid regions, and growers in regions with frost events may also hesitate to chemically thin flowers that open later in bloom, Johnson says. With lower chilling hours – such as in California – bloom can be longer.
“Long bloom periods with untidy endings are especially prone to infection because the fire blight pathogen can build to very high populations when given this additional time,” Johnson says.
Higher humidity can also increase the likelihood of fruit russeting, says Johnson.
“Soluble coppers should be applied when conditions promote drying of the material,” says Johnson. “At least in one case, the yeast material, Blossom Protect, has also been implicated as a cause of fruit russetting in a high humidity environment. As a general rule, russet risk is higher in smooth skin pears than in apples. And, in higher humidity areas, more reliance needs to be placed on fruit-safe materials.”
Johnson suggests Bloomtime Biological (NuFarm) to be substituted in areas where russeting may be of concern. Also bacillus-based biopesticides such as Serenade Opti (Bayer) are also recommended, however the residual period is short for these biopesticides, typically two days, instead of double that for antibiotics, Johnson says.
Cultural Control Suggestions
Vigor control has long been presented as a key preventative cultural approach and should be part of an organic grower strategy,–Granatstein says.
Rootstock choice is also important, Johnson says, with ELMA.26 and M.9 being highly susceptible. While Geneva rootstocks offer resistance, it is still possible for highly susceptible scions to be infected.
“Damaged scions on resistant rootstock are much more likely to survive and regrow than die from a girdling rootstock infection,” Johnson says.
Johnson also says growers should limit the overhead sprinkler irrigation while flowers are open, which can trigger fire blight outbreaks.
“Several studies have shown pear and apples trees are not stressed by reducing the frequency of irrigation during the bloom and post-bloom periods,” according to Tianna DuPont, regional Extension specialist with WSU in Wenatchee, WA. “A little soil drying is actually beneficial, assuming trees are then well-watered during the more stressful summer period. It is unlikely the trees can be overly stressed during the few days of peak fire blight risk. Keep the intervals between irrigation as long as possible, and let the soil surface dry.”
Growers are encouraged to use weather-based guides such as the CougarBlight or Zoller models to help predict the potential risk of infection and to respond accordingly.
“In organic programs, where biological materials are important components, spray applications based only on the model warnings will likely be too late to achieve effective control,” Johnson says. “Biologicals need to grow their populations on the flowers before fire blight pathogen cells arrive in order to be competitive. This may take two to four days. Additionally, by integrating multiple preventative materials, we are able to target the pathogen at each stage of its life cycle and gain overall better control.”
An Example of an Organic Spray Program
(Johnson and Temple, 2013; Johnson et al., 2014)
- Prebloom (just prior to green tip): Fixed copper sanitation if fire blight was in orchard last year (4 to 6 pounds per acre).
- Early bloom apple (crop load thinning): Lime sulfur during early bloom (20 and 70% bloom) applied for crop load thinning is a strong anti-microbial. The lime sulfur percentage (2-10%) is based on the specific crop thinning goal. Re-apply biological if lime sulfur goes on after the biological.
- Early bloom pear and apple Blossom Protect: One full application (maximum label rate), or if blight was in orchard last year, two full applications. In apple, apply Blossom Protect immediately after the second lime sulfur thinning spray. In smooth-skinned pears in wetter areas, russet risk from Blossom Protect might be unacceptably high. Bloomtime Biological is an alternative, fruit-safe biological material. Note:Biologicals, unlike antibiotics, should be applied preventatively. They need time to build their populations on the flowers. Waiting for the model to predict a high risk of infection is likely too late.
- Full bloom to petal fall. Spray treatments during this period can be guided by weather-based infection risk models. Depending on the cultivar russet risk and the model: Serenade Opti every 2 to 4 days (most fruit safe), Or for improved control mix Serenade Opti with Cueva (2 to 3 quarts per acre), or use Cueva (3 to 4 quarts per acre) or Previsto (3 to 4 quarts per acre) every 4 to 6 days (This option is least fruit safe for russet). Rates on the higher end of labeled rates have been most successful in field trials.
Blossom Protect Application Tips
- Apply to every row. Research has found better colonization of flowers by the yeast strains when it has been sprayed on the whole tree.
- Apply early. Applications during early bloom (30 to 70%) allow sufficient time for the yeast population to grow, which leads to more effective control.
- Use a buffer. Blossom protect applications with the companion material, Buffer Protect, have shown significantly better control than with Blossom Protect alone.
- Re-apply after lime sulfur. Lime sulfur applications are both anti-bacterial and anti-yeast and will affect the populations of an applied biological. Reapplication of the biological after lime sulfur treatment is necessary, or wait to use Blossom Protect until after lime sulfur.