Dr. Ken Johnson, plant pathologist at Oregon State University, reminded growers at the Washington State Tree Fruit Association Annual Meeting about risk factors for fire blight, including the vigor of the tree (a vigorous tree can spread the disease more quickly) and among some of the newer cultivars, such as Jazz and Cripps Pink.
But the biggest risk factor for fire blight is a tree’s age — and Washington growers are planting a lot of young apple blocks.
“It’s just about impossible to keep it out once it starts cooking in a young block of trees,” he said. “Programs need to be a little bit more intensive in young blocks. There’s a lot invested there, and if you mess it up, you mess it up big.”
A few things to remember:
Model thresholds for action should reflect the orchard’s risk — whether there was fire blight in the orchard or in the neighborhood the previous year, the age of the block and the cultivar, among other things.
Johnson recommends that growers lower their risk thresholds on the models if they have younger trees that are more susceptible, even if there was no fire blight in the area last year.
A fire blight infection can come late in the bloom period. While it may be difficult to find the pathogen at full bloom, it becomes more abundant as bloom progresses, and by petal fall, can be found easily.
Any interpretation of moisture depends on the orchard location and should be considered when determining risk level. Likelihood of dew and rain are both considerations, as are irrigation, both in the grower’s orchard or from a neighbor’s field.
– by Shannon Dininny