Submitted to: Postharvest Biology and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/4/2015
Publication Date: 3/1/2016
Citation: Janisiewicz, W.J., Evans, B.E., Bauchan, G.R., Chao, C.T., Jurick II, W.M. 2016. Wound responses of wild apples suggest multiple resistance mechanism against blue mold decay. Postharvest Biology and Technology. 117:132-140.
Interpretive Summary: Blue mold is the most destructive disease of stored apples in the US and worldwide. Commercially used apple cultivars have little or no resistance to this disease. Recently, we found that some apples from the wild apple tree collection maintained in Geneva, NY that were originally brought from Kazakhstan have a very high level of resistance to this fruit rot. The strength of this resistance was evaluated in tests where apples were wounded and the wounds were allowed to heal for various periods of time before inoculating with the decay causing fungus at a very high dose. The most resistant apples did not develop any rots even if no time was allowed for wound healing before the wounds were inoculated with decay causing fungus. Examination of the fruit cells surrounding the wounds for deposition of some substances commonly known to be involved in plant resistance to diseases, such as reactive oxygen species, lignin, suberin and callose, indicates that the resistance mechanisms are very complex and cannot be directly explained by any of these substances. Trees of the most resistant/immune apples will be maintained in the collection for future in-depth study of the mechanism of resistance and exploration in an apple breeding program.
Technical Abstract: Blue mold caused primarily by Penicillium expansum and to a lesser extent other Penicillium spp. is the most destructive disease of stored apples in the US and worldwide. It was recently shown that resistance to blue mold exists in wild apple germplasms, Malus sieversii, from Kazakhstan and central Asia maintained as a collection in Geneva, NY. We initiated studies to determine the durability and the mechanism(s) of resistance to P. expansum in select wild apple accessions. Wound responses (up to 96 h in 24 h intervals) affecting P. expansum infection and related cytological changes were determined in accessions with varying levels of resistance. In general, the more resistant the accession the quicker the wound response that prevented the fungus from infecting tissue and causing decay. No decay developed on immune apple accessions even when inoculated immediately after wounding at the inoculum concentration of 10E5 conidia /mL. On moderately resistant accessions, a 24 h interval between wounding and inoculation was sufficient to avert decay. Reactive oxygen species (ROS) were detected at high levels immediately after wounding in the immune as well as susceptible accessions, and callose and lignin/suberin appears to play a minor role in resistance responses. Our results indicate the presence of a high level of durable resistance/immunity in the wild apples which is governed by several mechanism(s). This presents a new challenge for explaining the observed resistance, and at the same time, creates an opportunity for exploiting these resistant mechanisms in breeding programs to incorporate resistance to fruit decays into commercial cultivars.