A predictive model for forecasting fire blight of pear and apple in Washington state.
Fire blight outbreaks were sporadic in apples and pears grown in the northwest USA. Pears were the most common host, and infection usually occurred through secondary blossoms from mid-May to Jul. It is suggested that most growers tend to ignore models except during the full bloom period because most damage is relatively minor and all common models are over-predictive. In order to develop a more accurate fire blight predictive system, temp. that occurred prior to known isolated infection periods were analysed. A simple degree hour based method for estimating the relative growth rate of Erwinia amylovora in blossoms was developed. Degree hours over 15.5°C were totalled for the 4 d preceding a blossom wetting event. Daily degree hours peaked at 31° and declined as the daily high approached 40°. Infection of blossoms was rare when degree hours totalled <240 when blossoms were wetted. The risk of infection increased with higher degree hour totals. The most serious fire blight occurred when 4 d degree hour totals were between 270 and 350 when flowers were wetted. The severity of the disease in any specific orchard depends on the relative presence of flowers, cankers and the disease susceptibility of the pear or apple variety. This paper was presented at the Sixth International Workshop on Fire Blight, held in Athens, Greece, 20-23 Oct. 1992.