Morphological and molecular identification of fungal species associated with postharvest stem-end rot disease of avocado in Sri Lanka

Avocado (Persia americana) is one of the most popular fruits grown in Sri Lanka. Postharvest infections occur wherever the crops are cultivated. Among them, stem-end rot (SER) is the major disease reported. Several fungal species including Lasiodiplodia and Dothiorella spp. have been reported to be associated with this disease in regions where avocado is grown. The aims of this research were to identify the fungal pathogens associated with the stem-end rot of avocado in Sri Lanka based on morphological characteristics in combination with molecular and phylogenetic analysis. Diseased avocado fruits were collected from local markets and causal agents were isolated. Colony morphology and characteristics of conidia were observed using phase-contrast microscopy. Sequence analysis was performed using internal transcribed spacers (ITS) of the ribosomal DNA followed by phylogenetic analysis. Four endophytic fungal isolates were identified and designated as SER 01, SER 02, SER 03 and SER 04 which were distinguished respectively as Lasiodiplodia theobromae, Lasiodiplodia hormozganensis, Diaporthe nelumbonis and Fusarium oxysporum. To our knowledge, this is the first evidence on the occurrence of L. hormozganensis, D. nelumbonis and F. oxysporum associated with SER of avocado in Sri Lanka. Identified pathogens were proven to be collectively pathogenic to avocado following demonstration of Koch’s postulates. The average value of collective disease severity after seven days of inoculation of avocado cv. Pollock was 40% at 28 C and it always fluctuated between 30% and 45%.