Washington State University is working to help growers be better prepared to fight fireblight.

Washington State University is working to help growers be better prepared to fight fireblight.

Washington State University’s Cougarblight model is being updated to improve its ability to predict when conditions are conducive to fireblight.   

The model uses information on temperature, wetness, and presence of fireblight bacteria to predict infections and was developed using data on the rate of growth of fireblight bacteria in a growth medium in a laboratory. This could be very different from what actually happens on the stigma of a flower, where the bacteria compete with each other and other organisms for nutrients, said Tim Smith, Washington State University Extension educator, who developed the model.

Dr. Lawrence Pusey, plant pathologist with the U.S. Department of Agriculture in Wenatchee, Washington, recently developed a technique to calculate the growth rate of bacteria on flower stigmas at a range of ­temperatures, and this data is now included in the model, which was previously updated in 2000.

Pusey found that the growth rate of bacteria is very low during normal spring temperatures below 72°F. The bacteria multiply rapidly when the temperature is in the mid 80s to low 90s, but growth slows and then stops as the temperature exceeds 97°F.

“For years, we noticed that when we got to July and the weather was hot, we’d rarely see new infections,” Smith said during the annual North Central Washington Pear Day this winter. “Flowers dry up quickly and the bacteria are not proliferating very fast, either.”
Infections noticed in July probably began in June, he said.

The model is also being revised to include new information about wetting, which is one of the conditions necessary for a fireblight infection to develop. Smith said it takes a gentle wetting to wash the bacteria down from the stigma into the nectary where the infection can begin. This wetting could be in the form of a light rain or sprinkling or dew. A more thorough wetting, such as from a heavy rainfall, would be more likely to wash the bacteria off the blossom than cause an infection, Smith said.

The model has been basing wetness on leaf readings from weather station leaf wetness sensors, and the wetness sensor is the least reliable piece of equipment on a station, Smith said. Stations that are part of Washington’s AgWeatherNet are not located in orchards, where they would be irrigated, but alongside orchards, where dew is less likely to form.

German researcher Dr. Esther Moltmann has developed a method to calculate the probability of dew forming, based on the relative humidity, temperature, and wind speed. Smith said he also plans to incorporate this new information into the Cougarblight model.

2010 season

Around full bloom to petal fall, bacteria can get into the flowers, and, if conditions are right, an infection can begin. Despite an unusually cool, wet spring in Washington in 2010, fireblight did develop in some Washington orchards last season, Smith said. One outbreak, which happened in early bloom and affected pears in Wenatchee, Monitor, and the lower Entiat Valley, can be traced to a warm period in mid-April. Another outbreak originated during a hot period in mid-May.

Symptoms of primary-bloom infections usually show up by mid-May. However, primary bloom infections are relatively rare because the temperature during bloom does not often reach the high 70s and 80s—the temperatures conducive to infections. Most of the infections seen in Washington arise from secondary blossoms that develop later in the season. Some apple varieties, such as Cripps Pink and Jazz, can develop blossoms the whole season long.

When conditions are conducive to fireblight, the bacteria must get into the blooms for an infection to begin. Smith said he’s working on a method that growers could use to test whether bacteria are in an orchard or not.

If there was a fireblight infection in the orchard the previous season, it’s important that every canker was removed, Smith said. Otherwise, there will be a zone around any remaining canker that will be contaminated with more bacteria than normal, and it won’t take as much warmth for the bacteria to increase to dangerous numbers if there are high levels to begin with.

The Cougarblight model on WSU’s Decision Aid ­System Web site has a setting where growers can indicate if there was fireblight in the area last year, so that it can more accurately predict the risk of infection.


Smith has for many years been testing the efficacy of fireblight management products. One of the most promising lately has been a copper product that is still under development for fireblight and citrus canker. When or whether it will be registered will depend on how well it works on citrus, Smith said.

In trials, Smith applied it three times—during early, mid-, and late bloom. It provided better control than antibiotics. “I think it might play an important role in ­controlling fireblight in the future,” he said.

The product caused singeing of apple blossoms, but no fruit marking in either apples or pears.

Smith is also testing a new biological product that is sold in Europe as Blossom-Protect, whose active ingredient is the yeast Aureobasidium pullulans. It is applied three times during bloom. Smith said he would be recommending it after it is registered in the United States in 2012. “It will probably be the only real viable option for organic growers in the future, other than the new copper product,” he said.

Unless the National Organic Standards Board decides otherwise, organic approval of the antibiotic Mycoshield (oxytetracycline) could sunset next year.

Smith has also been testing a product called Actigard (acibenzolar-S-methyl) that appears to protect the tree from infection. He applied it twice—the first time at ­cluster bud and again just before the flowers opened—followed by an antibiotic. Even when blossoms were inoculated with the fireblight bacteria, very few infections developed, he said. And, in d’Anjou pear trees that were infected, the fireblight didn’t run much past the spur. It had a similar effect on apples. Results were better than with applications of streptomycin alone (applied to ­strep-susceptible bacteria).

“We’re going to pursue this,” he said, noting that it might be useful in blocks that previously had fireblight and are at high risk. During this year’s trials, he will use it alone, without an antibiotic and see how much damage there is.