We probably shouldn’t complain about central Washington’s weather, because it tends to be better than many places. In fact, the local weather makes it possible to grow almost any deciduous tree fruit without many disease problems common in other tree fruit production areas. The low occurrence of rain especially helps us avoid tree fruit disease problems that thrive in wet climates.

Unfortunately, that is less true these past few years. You may have read in articles in The Wenatchee World about something called “Fire Blight” that Wenatchee Valley pear growers are burning much later into the growing season than usual.

What is “Fire Blight,” and why are those growers burning it?

Fire Blight is a disease caused by a specific bacteria attacking an apple or pear tree through its flowers. It is native to North America, causing a minor disease of Hawthorn trees. It found the apples and pears planted by the European settlers to be quite susceptible.

The Fire Blight bacteria moved across the USA to the Pacific Northwest in 1948, where it settled in, mostly in pears. What kept the disease under control for years was the weather, and that Red Delicious is quite resistant to the disease. This gave Wenatchee-area growers an advantage over eastern growers due to the cool, dry conditions, especially while apples and pears are blooming.

In order for this disease to do its job, the bacteria must be transported from last year’s infected wood to open flowers by insects. Daily temperatures must be abnormally warm for a few days. By then, the colony of bacteria may have developed to a number numerous enough to infect the flower if the flower is wetted by rain or dew. This must occur in order — bacteria on flower first, proper abnormally warm temperatures, then wetting.

Once the bacteria gain entrance to the flower, they move into the wood and kill a portion of the younger wood and leaves, leaving them brown and appearing burned, hence the name.

This exact infection process is not very common in central Washington, but it has been for the past three years, and the disease is causing grave damage in many pear and apple orchards.

It rained on April 28 this year, when apples and pears were blooming across central Washington. There was a plentiful population of the blight bacteria carried over from last year, and temperatures had been abnormally warm, mid- 80s for the past three days — near perfect conditions for the bacterial development.

Then rain occurred across the entire state, washing the bacteria into the flowers, causing infections that started appearing about two weeks later. Most growers recognized the extreme infection weather conditions, but some were not able to treat their entire orchard in time to prevent infection. Some growers were able to apply controls in a timely manner, and report good results.

The orchard owners are burning the blight cuttings because the only treatment for infected trees entails cutting the infected part off the tree and removing the cutting out of the orchard. Part of the sanitation process includes the burning of the bacteria-infested wood, to reduce the number of bacteria in the orchard environment.

Further new infections are possible this year through “rat-tail blossoms” and wounds on the tree caused by hail or high winds with rain. Sanitation efforts will continue until next spring.

Ornamental flowering pears, crab apples, and shrubs such as Pyracantha and Cotoneaster are also susceptible. Numerous fire blight samples have been brought in the Master Gardener plant clinic for identification this year. 


A WSU Master Gardeners of Chelan County column appears weekly in The Wenatchee World. Tim Smith is one of four columnists featured.