SUSCEPTIBILITY OF APPLE HYPANTHIUM TO ERWINIA AMYLOVORA IN RELATION TO FLOWER AGE AND COUGARBLIGHT MODEL

Abstract

Fire blight risk in the northwestern United States is widely determined with the Cougarblight model, involving a 4-day temperature evaluation developed on the assumption that flower stigmas support growth of E. amylovora for only a few days. Studies relating flower age to stigma receptiveness to bacteria did not offer a satisfactory explanation for the effectiveness of the model, and thus, the relation of flower age to hypanthial infection was investigated. Direct inoculations of flower hypanthia of various ages were performed with detached crab apple flowers in the laboratory and with Gala apple flowers in the orchard in 2005 and 2006. In both environments, disease incidence decreased with flower age. Rate of declining susceptibility was dependent on temperature in the laboratory. Regression analysis of orchard data indicated an inoculum dose effect, with steepest decline of susceptibility occurring during the initial days after petal expansion. In 2005. when conditions were most favorable for disease, the pathogen level of 104 CFU (representing a maximum population due to wetting of a few hours according to laboratory tests) caused a disease incidence that decreased from 40 to 18% for flowers 0 to 4 days old and from 12 to 9% for flowers 5 to 8 days old. Cougarblight and other models now incorporating a blossom-age component (e.g., MaryblytTM) may be explained by multiple factors. Our general understanding is that high temperatures during bloom allow E. amylovora to reach maximal levels of population on stigmas within a time period when hypanthia are still highly susceptible to infection upon wetting. Further study of these interrelationships could lead to improvements in fire blight risk assessment.