Pathogenic diversity of avocado and mango isolates of Colletotrichum gloeosporioides causing anthracnose and pepper spot in Australia

Colletotrichum gloeosporioides is a major fungal pathogen of avocado and mango fruit in Australia and overseas. It causes anthracnose and stem-end rot in these crops but has also been identified as the causal pathogen of pepper spot of avocado and tear stain of mango. Research was initiated to determine the pathogenic diversity of pepper spot, with emphasis on avocado. Eighty C. gloeosporioides isolates obtained from avocado and mango fruit showing anthracnose and pepper spot symptoms were screened for pathogenicity, comparative aggressiveness and cross-infection potential by inoculating onto detached avocado and mango fruit, avocado leaf petioles and branches of young, grafted nursery trees, as well as avocado fruit and pedicels still attached to the tree. On detached, ripening avocado and mango fruit in the laboratory, it was found that pepper spot isolates were as capable as anthracnose isolates of causing anthracnose lesions. However, avocado isolates were significantly (P < 0.05) more aggressive than mango isolates on avocado fruit and mango isolates were significantly (P < 0.05) more aggressive than avocado isolates on mango fruit. In field inoculations, pepper spots were formed on developing avocado fruit and pedicels on the tree. Likewise, pepper spots developed on petioles and branches of nursery avocado trees, but not on their leaves. When all isolates were grouped according to symptom or host of origin, significant differences in lesion severity were demonstrated between isolates on avocado petioles in the glasshouse, with avocado pepper spot isolates being the most aggressive, followed by avocado anthracnose isolates then mango isolates from both anthracnose and pepper spot, respectively. On unripe avocado fruit in the field, the pattern was generally similar with the mango isolates being the least aggressive. There were more and less pathogenic strains present in the pathogen populations from both mango fruit and avocado fruit but neither were restricted to anthracnose or pepper spot groupings. Generally, a higher percentage of the most aggressive isolates was from avocado pepper spot. When isolates were grouped according to the orchard of origin, there were significant differences in aggressiveness to avocado both in the glasshouse and the field.