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Fire Blight

By Tianna DuPont, WSU Extension. April 24, 2017.

As Fire Blight season approaches watch the model and check out our new updated Fire Blight page.

With the cool weather this spring fire blight pressure is generally low. However, with warm temperatures risk could increase. Due to the high innocula from last year it is critical to be on top of things.

It is critical that growers are aware of fire blight risk from when primary bloom begins until the end of secondary (rat tail) flowering. Secondary bloom, typically 1 to 3 weeks after full bloom is generally a prime period for infection. Compared to non-dwarfing rootstocks, dwarfing rootstocks tend to promote secondary flowering. Many modern cultivars (e.g., Bartlett pear and Cripps Pink apple) also tend to produce secondary flowers.

At temperatures of 60-70° F, the fire blight pathogen grows slowly. When temperatures reach 70-75° F, it is important that growers watch the weather forecast and blight risk warning models, and prepare to apply preventative spray treatments. 75-90 °F is fire blight weather. The bacteria will replicate quickly at these temperatures (Pusey and Curry 2004).

Example Organic Spray Program

(Johnson and Temple 2013; Johnson et al. 2014)

  1. Prebloom (just prior to green tip): Fixed copper sanitation if fire blight was in orchard last year (5 to 6 lb/A).
  2. Early bloom apple (crop load thinning): Lime sulfur (plus oil) during early bloom (20 and 70% bloom). Reapply biological if lime sulfur goes on after biological.
  3. Early bloom pear and apple Blossom Protect: One full, two half applications, or two full applications if blight was in orchard last year. In apple, apply Blossom Protect immediately after 2nd Lime Sulfur. In smooth-skinned pears in wetter areas, russet risk might be unacceptably high. Bloomtime Biological is an alternative, fruit-safe biological material
  4. Full bloom to petal fall: Depending on the cultivar russet risk and the CougarBlight model risk apple:
    1. Serenade Optimum every 2 to 5 days (most fruit safe);
    2. Or for improved control mix Serenade Optimum with Cueva, or use Cueva (or Previsto) every 5 to 6 days (3 to 4 qts/A) (This option is least fruit safe for russet).

Example Conventional Spray Program

  1. Early bloom. Apply biological controls. If blight was in the orchard last year use two applications of the biological. There is some russet risk for smooth skin pears.
  2. Full bloom to petal fall. Apply antibiotics only as indicated by risk level in the CougarBlight model.

For Blight Hazard Factors and Control Principles visit the Fire Blight page.


Tianna DuPontImg1380

WSU Extension Specialist, Tree Fruit

(509) 663-8181 ext 211

Washington State University