Weather and fireblight in England

SUMMARY A method for assessing potential fireblight activity in the field in southeast England is described. It is based on potential doublings (PD), derived from in vitro growth rates of Erwinia amylovora at different temperatures, combined with a rain score (R) derived from daily precipitation values. Where the incubation period (I) was known, the coefficient of correlation between I/R and PD was 085 (P < 0–001). When I was regressed on PD and R, the multiple correlation coefficient was 0–97. The low level of fireblight in England precluded direct testing of the predictive value of the relationship between PD, I and R but its application to growing season weather of 1955‐74 highlighted those years when fireblight activity was high in a particular host. Whilst within the range 65–86 oF (18–30 oC) there is good agreement between accumulated potential doubling values and accumulated degree days, the rain score would probably require modification in other climates.