Submitted to: Acta Horticulturae
Publication Type: Proceedings
Publication Acceptance Date: 7/1/2004
Publication Date: 2/10/2006
Citation: Norelli, J.L., Brandl, M. 2006. Survival and growth of Erwinia amylovora on apple leaves. Acta Horticulturae. 704:121-126.
Technical Abstract: Populations of Erwinia amylovora (Ea) dropped from 10-4/leaf to below detectable limits within 48 hrs on 'Royal Gala' apple shoots inoculated with Ea and incubated in a growth chamber at 24ºC and high relative humidity (80-95%). Low Ea populations (<10/leaf) were detected 6 and 14 days after inoculation. Under orchard conditions in June 2002 and in 2003, Ea was detected on leaves after rain events but were short lived. However, in July 2002, Ea populations recovered from leaves significantly increased following a thunderstorm that occurred on a hot day (35ºC). The day after the storm, July 10, low numbers of Ea were detected. On July 15, higher numbers of Ea were detected in leaf washings plated on media, but Ea was not detected in leaf prints on media, suggesting that bacteria were within the leaf. Following 4 cm of rain on July 16, Ea 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 35ºC resulted in more shoot infection than incubation at 24ºC. Pre-inoculation incubation temperature did not have a significant effect on shoot infection. Inoculating plants with bacteria at 4ºC resulted in more shoot infection than with bacteria at 24ºC. When plants were inoculated with cold bacteria (4ºC) and incubated at high temperature (35ºC), Ea quickly became established within young leaves but rapidly declined on the surface of older leaves. Microscopic observation indicated that under these conditions, Ea 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 Ea within the leaf.