G Swart

University of Pretoria, Pretoria, South Africa

Background and objectives Subtropical fruit producers are responsible for more than one-quarter of the gross fruit export earnings in South Africa. Avocados and mangos comprise a large portion of this industry with total earnings for the 1995/96 season being $22 million and $8 million respectively. Colletotrichum gloeosporioides (Penz.) Penz. & Sacc. is the most important post-harvest pathogen on avocado and mango and forms quiescent infections in the orchard. Basic orchard sanitation practices such as skirting of trees and removal of dead branches are not always followed, resulting in a buildup of inoculum. Anthracnose incidence and severity varies throughout the growing season, with some export consignments being virtually disease-free and others completely unmarketable. The purpose of this study was to detemine the fluctuating patterns of postharvest diseases of mango and avocado, the role of C. ;gloeosporioides in these infections, and to compare isolates obtained using cross-infection studies.

Results and conclusions
A total of 6327 avocado and 1682 mango fruit were evaluated and 6056 isolations made. More anthracnose was generally observed on avocados than stem-end rot, whereas levels of anthracnose and soft brown rot on mangoes fluctuated. No significant differences were observed in the isolation frequency of C. ;gloeosporioides from anthracnose and stem-end rot lesions. Typical anthracnose lesions were produced on avocado by most isolates, although isolates from certain areas were found to be more virulent than others. Isolates obtained from eating-ripe fruit were significantly more virulent than isolates obtained from over-ripe and post-over-ripe fruit. No significant differences were observed in virulence between isolates from stem-end rot or anthracnose lesions. Avocado isolates induced significantly larger lesions in avocado than in mango. Preliminary comparative data obtained in these experiments, indicate clear differences between isolates of C. ;gloeosporioides. Similar findings were reported with isolates from mango, citrus and papaya, but with greater variability in pathogenicity [1]. Furthermore, isolates obtained from a specific host were found to be more pathogenic on that specific crop than on others [2]. Molecular analysis has shown that genetically distinct strains exist within the C. ;gloeosporioides complex [3]. All isolates obtained in this study have been compared using morphological and physiological characteristics. Comparison on a molecular level is in progress and a cluster analysis will be carried out using all factors investigated as variables in order to determine relationships, if any, between South African isolates of C. ;gloeosporioides from avocado and mango.

1. Quimo TH, Quimo AJ, 1975. Plant Disease Reporter 59, 221-224.
2. Alahakoon PW, Brown AE, Sreenivasaprasad S, 1994. Physiological and Molecular Plant Pathology 44, 93-103.
3. Hayden HL, Pegg KG, Aitken EAB, Irwin JAG, 1994. Australian Journal of Botany 42, 9-18.