Survival and growth of Erwinia amylovora on apple leaves

Populations of Erwinia amylovora dropped from 10(4)/leaf to below detectable limits within 48 hrs on 'Royal Gala' apple shoots inoculated with E. amylovora and incubated in a growth chamber at 24 degrees C and high relative humidity (80-95%). Low E. amylovora populations (< 10/leaf) were detected 6 and 14 days after inoculation. Under orchard conditions in June 2002 and in 2003, E. amylovora was detected on leaves after rain events but was short lived. However, in July 2002 E. amylovora populations recovered from leaves significantly increased following a thunderstorm that occurred on a hot day (35 degrees C). The day after the storm, on 10 July, low numbers of E. amylovora were detected. On 15 July, higher numbers of E. amylovora were detected in leaf washings plated on media but E. amylovora was not detected in leaf prints on media, suggesting that bacteria were within the leaf. Following 4 cm of rain on 16 July, E. amylovora was detected in both washes and prints. The effect of high temperature and rapid temperature change during inoculation were studied under controlled environmental conditions on both 'Royal Gala' and 'M.26' shoots. A post-inoculation incubation temperature of 35 degrees C resulted in more shoot infection than incubation at 24 degrees C. Pre-inoculation incubation temperature did not have a significant effect on shoot infection. Inoculating plants with bacteria at 4 degrees C resulted in more shoot infection than with bacteria at 24 degrees C. When plants were inoculated with cold bacteria (4 degrees C) and incubated at high temperature (35 degrees C), E. amylovora quickly became established within young leaves but rapidly declined on the surface of older leaves. Microscopic observation indicated that under these conditions E. amylovora would colonize hydathodes and glandular trichomes of young leaves. These results suggest that rapid temperature changes during summer storms can lead to the establishment of E. amylovora within the leaf.