Too hot for fireblight?
Scientists in Washington State are reassessing fireblight control strategies following the 2006 season, when there were surprisingly few outbreaks.
In theory, after widespread fireblight in 2005 that resulted in carryover bacteria in the orchards, the state"s tree fruit industry was set up for another bad fireblight year if the weather was hot and wet during bloom.
North central Washington experienced high temperatures in mid-May when there was extensive secondary bloom on Bartlett pears. Then it rained for several days.
"The model indicated we had severe infection conditions," recalled Tim Smith, Washington State University Extension educator for north central Washington. "These were the most severe fireblight conditions I had ever seen. What happened? Absolutely nothing. There was not one strike that resulted from that particular infection period."
Smith now thinks that the temperatures were too hot for fireblight to develop. The bacteria live on the tip of the flower stigma and multiply rapidly if the weather is warm. To cause an infection, the bacterial colony has to grow to a certain size and be washed into the nectary before the stigma tip dries up. His theory is that in the 99°F temperatures last May, the blossoms died so quickly that the bacteria didn"t have a chance to cause infections.
"We have never had this scenario before," Smith told growers at the Lake Chelan horticultural meeting in January. "The bacteria are transferred from one flower to the next and the next, and this seems to fall apart when it gets extremely hot."
Bacteria grow slowly in cool weather, and more rapidly as the temperature increases, but it"s thought that growth of the bacteria then slows down in temperatures over 90°F. Typically, fireblight is not a problem in the intense heat of the summer.
Smith said the Cougarblight fireblight risk assessment model is being updated to reflect this.
If there is moisture during bloom when the temperature is 60°F, there is nothing to worry about, he said. At 75°F, growers should be watching the model. At 85°F, someone is likely to get fireblight in their orchard. With several days of temperatures around 95°F, there probably won"t be an infection. However, if the temperature quickly drops from the 90s into the 80s, there is reason to be concerned.
Smith said dew during a warm period can provide enough moisture to cause a fireblight infection. Growers need to be particularly vigilant in newly planted orchards. Nursery trees on Malling 9 rootstocks usually come with buds on them and are eager to bloom during their first summer in the orchard. But they don"t bloom at the normal time"they bloom later.
A fireblight outbreak can turn a beautiful new orchard into a management problem or lost investment overnight, Smith warned. "It happens in a flash, and infection can occur when you"re thinking of other things. Stay on top of it and try to avoid an infection."