Identification of stem canker fungi that pose a threat to the deciduous fruit industry

Research has revealed that fungi are more often responsible for serious stem canker diseases in pome and stone fruit than previously believed. Prior to the commencement of this project, producers and extension specialists were unable to distinguish between fungal and bacterial cankers, and therefore also unable to apply the correct control strategies. As part of this project, fungal isolates collected from the various hosts in South Africa were compared to a worldwide collection of isolates from fruit trees using DNA sequence data from the nuclear rRNA operon. The ITS region was amplified by PCR and sequenced for representative isolates. The major fungal pathogens responsible for stem canker formation and die-back of pome and stone fruit trees have been identified. Results attributed the canker disease of apple, pear, and plum rootstocks to Diaporthe ambigua. All Leucostoma isolates were screened for a HinfI restriction site in the ITS and for a corresponding 268 bp fragment indicative of the species Leucostoma persoonii. Botryosphaeria isolates from pome and stone fruit trees in South Africa grouped in the Fusicoccum luteum and Botryosphaeria obtusa clades, with B. obtusa as the dominant species. Although listed as a pathogen in South Africa, no isolates of Nectria galligena could be found. This project, like similar phylogenetic studies worldwide, has not been able to clarify all the taxonomic questions. However, it could delimit strongly supported groups within Diaporthe, Leucostoma and Botryosphaeria and devise identification tools to define them. These can now be used in further epidemiological, pathogenicity and selection experiments, as well as to address quarantine issues.