Occurrence of Botrytis cinerea and Monilinia laxa on nectarine and plum in Western Cape orchards, South Africa

The occurrence of grey mould (caused by Botrytis cinerea) and brown rot (caused by Monilinia laxa) in the major stone fruit regions in the Western Cape Province was studied over a 3-year period by sampling reject fruit from the Unifruco Quality Evaluation Scheme and from 11 stone fruit orchards. Flowers and fruit from the orchards were treated to facilitate disease expression by surface conidia and latent mycelia (unsterile vs. surface sterilised; untreated vs. paraquat-treated). B. cinerea was found to be the most important pathogen causing blossom blight and postharvest decay on stone fruit. The pathogen was most prominent on early and mid-season cultivars. Surface conidia and latent mycelia consistently occurred on fruit in each orchard, although at fluctuating levels. The amount of B. cinerea on fruits was generally higher during spring than summer. Disease expression on fruit was not governed by the amount of B. cinerea occurring on fruit, but by the ability of fruit to resist disease expression. Blossom infection did not contribute directly to postharvest grey mould. Brown rot was exclusively caused by M. laxa and no evidence was found that M. fructicola had been introduced into the Western Cape Province of South Africa. The pathogen was most prominent on the later maturing cultivars. Immature fruit were generally pathogen-free and disease expression only occurred on maturing fruit. Long-term latency therefore does not seem to play a prominent role in M. laxa fruit rot.