The Fire Blight (CougarBlight) model is a blossom infection model and intended for use during moderate temperatures (50-86°F) that usually occur during spring and early summer and when flowers are open. This blossom infection model attempts to identify the days when abnormally high daily temperatures occur during the spring of the year, when blossom infection conditions are relatively rare. However, by mid-June, the model almost always indicates that the temperatures are prime for infection, although new infections are not likely during high summer heat. Therefore, the model is turned off on June 21st.
During summer, but not predictable with the CougarBlight model, vegetative infections may occur (as opposed to blossom infections). These vegetative infections are especially triggered by strong rain/wind or hail events that cause minor wounding of the tender growing shoot tips of fruit trees. If a near-by active fire blight canker or oozing infection has contaminated the leaf tissue, the rain can then move the bacteria into these wounds on the very susceptible tissue of the shoot tip, leading to infection. The best hope for prevention and reduction of infection is a copper fungicide applied before or soon after the storm, while antibiotics have not worked well against these sorts of infections.
The Fire Blight model is turned back on August 20th as it can be important in the fall under rare circumstances that we do not understand well.
WSU Regional Extension Specialist