Colletotrichum horii causing fruit rot of nutmeg (Myristica fragrans Houtt.) in Malaysia

Abstract

Nutmeg (Myristica fragrans Houtt.) is widely cultivated in Malaysia, with Penang being the leading state. During field inspection in 2022, fruit rot disease was observed in several nutmeg orchards, and local growers reported reduced yields. Affected fruits showed black sunken lesions on the surface of nutmeg fruit. To date, this disease has not been reported in Malaysia. A total of 10 nutmeg fruit showing fruit rot disease symptoms were collected from several nutmeg orchards in Penang for fungal isolation. Morphological and molecular approaches, along with pathogenicity tests, were conducted to determine the causal fungal pathogen. Based on morphological characteristics, the fungal isolates were tentatively identified as Colletotrichum species, and molecular analyses using internal transcribed spacer (ITS), glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and actin (ACT) gene loci confirmed the species identity as C. horii. Phylogenetic analyses using both maximum likelihood (ML) and Bayesian inference (BI) methods provided strong support for the species identification, with high bootstrap values (100 %) and Bayesian posterior probabilities (1.0). Pathogenicity tests on 32 healthy immature nutmeg fruits showed that only wounded fruits developed disease symptoms similar to those observed in the field, while non-wounded fruits remained asymptomatic, fulfilling Koch's postulates. This is the first report of C. horii causing fruit rot of nutmeg in Malaysia. These findings provide a foundation for developing targeted disease management strategies to mitigate yield losses in nutmeg cultivation.