Submitted to: Phytopathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/16/2005
Publication Date: 6/19/2006
Citation: Park, E., Mcevoy, J.L., Conway, W.S., Sams, C.E., Solomos, T. 2006. The effect of calcium on the virulence and polygalacturonase activity of colletotrichum acutatum on apple fruit. Plant Pathology Journal. 5:183-190. Interpretive Summary: In order to provide apples to consumers throughout the year, it is necessary to store fruit in a cold, humid environment for months at a time. However, the associated loss of apple fruit in storage is substantial due to decay caused by postharvest fungal pathogens. Although fungicides can control growth of many of these pathogens, public concerns about health and environmental impact will limit their future application. To reduce storage losses and provide a better source of plentiful food for the consumer, a better understanding of the mechanism by which postharvest fungal pathogens decay fruit is important in order to develop alternate methods of control. The results of this research indicate that there are two major types of a certain protein that helps the fungus to attack and grow on apple fruit. The fungus we studied was Colletotrichum acutatum. It is an important postharvest apple pathogen. We also showed that increasing calcium content of apple fruit, which is also good for people from a nutrition standpoint, results in less decay and postharvest losses, probably due to decreased activity of the two fungal attack related proteins. Apple packing and storages facilities may use calcium treatment to reduce the amount of fungicides used to maintain fruit quality during cold storage. This will in turn benefit consumers by providing higher quality fruit at a lower cost.
Technical Abstract: 'Golden Delicious' apples were treated postharvest with 0, 2 or 4% solutions of calcium chloride and stored at 0oC for two months. Fruit were then inoculated with Colletotrichum acutatum, held at 20oC, and lesion sizes measured over a 16-day period. Fruit treated with 4% calcium chloride had lesions that were significantly smaller (~50%) than those treated with 0% calcium chloride. Fungal growth and polygalacturonase (PG) expression were determined by growing C. acutatum in Richard's solution supplemented with cell walls extracted from 0 or 4% calcium chloride treated fruit. During the first 18 hrs PG activity was not detected. However, within 24 to 27 hrs, the PG activity increased, and was approximately 45% greater in medium containing cell walls from 0% calcium treated apples as opposed to medium containing cell walls from 4% calcium treated apples. After 72 hrs, PG activity produced in both media was similar. These results correspond with pg gene expression studies using the Reverse Transcription Polymerase Chain Reaction (RT-PCR) of RNA isolated 24 hrs post inoculation. Zymogram analysis showed that minor PG isozymes were expressed differently depending on whether the medium was amended with cell walls from calcium treated or non-treated apples, although the four major PG isozymes detected were produced to approximately the same extent under both growth conditions.