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Concorde pears in a healthy block. Fireblight can become systemic in Concorde trees.

Concorde pears in a healthy block. Fireblight can become systemic in Concorde trees.

The 2008 season was a serious fireblight year for pears in Washington’s higher elevations, reports Tim Smith, Washington State University Extension educator for north central Washington.

Generally, it was a cool spring, with just a couple of periods of warm weather. One warm period, on May 7 to 8, was responsible for outbreaks in pears that were blooming at that time in the northern parts of Washington, including the Okanogan Valley and the upper Wenatchee Valley, Smith said. There was no rain during that period in the Leavenworth area, but in low-lying areas and frost pockets, dew on the trees can provide sufficient moisture to trigger an infection.

Bartlett pears became infected during a warm blast from May 17 to 19, which coincided with secondary bloom and was followed by rain across the whole state, Smith said. Though it was a strong infection period, heat and moisture are less dangerous when there are fewer flowers on the trees.

All pear varieties were affected, with serious infections on Concorde orchards. Smith said he is beginning to believe that fireblight becomes systemic in Concorde trees. It gets into the tree, and then shows up later. "It doesn’t exactly attack the tree and then move in," he said. "It shows up on the trunk first and then moves out, and moves very rapidly. That’s very odd for pears."

Smith said the amount of carryover bacteria in the area is an important factor in the development of fireblight. The 2007 season was not a bad fireblight year, but those areas that had it were much more prone to infections in 2008. With carryover bacteria around, it takes less heat to create a problem.

Growers will need to pay close attention to fireblight next year, he warned. The normal threshold for applying fireblight treatments, according to the Cougarblight model, is 500 degree days. However, for areas with carryover bacteria, the threshold is 300 degree-days.