Alternaria alternata Causing Postharvest Fruit Rot of Mangifera indica in China

Mango (Mangifera indica L.) is currently ranked fifth in total production of the world’s major fruit crops and is commonly known as “the king of fruits.” It has been cultivated for more than 1,300 years in China. In 2017, China’s mango yield reached 1,982,200 tons, with Guangxi, Hainan, Yunnan, Guangdong, Sichuan, and Fujian provinces as the main producing areas. However, in May 2018, a postharvest fruit rot was observed at an incidence of 8 to 10% on M. indica cultivar Tainong at the supermarket in Wuxi, Jiangsu, China (31.48°N, 120.28°E). Symptoms on fruits initially appeared as small dark brown spots that later enlarged and became necrotic. To isolate the pathogen, symptomatic tissues were surface sterilized for 30 s with 75% ethanol followed by 0.1% mercuric chloride, rinsed in sterile distilled water, and plated onto potato dextrose agar (PDA). The plates were incubated at 25°C with a 12-h photoperiod for 7 days. The morphology of the cultured fungi was observed to be dark brown with whitish edges and abundant aerial mycelia. Conidiophores were septate and measured 22.4 to 53.2 × 1.8 to 3.3 μm. Conidia were borne in chains or singly and were obclavate or ovoid, measuring 8.6 to 37.2 × 7.3 to 15.7 μm (n = 50) with two to five transverse septa and zero to three longitudinal septa. Based on the morphological characteristics, the pathogen was identified as Alternaria sp. (Simmons 2007). Total genomic DNA of the isolated fungus was extracted with a Plant/Fungi DNA Isolation Kit (Sigma-Aldrich, Oakville, ON, Canada). Translation elongation factor 1-alpha, internal transcribed spacer, actin gene, Alternaria major allergen, plasma membrane ATPase gene, and calmodulin gene were amplified with primers EF1-728F/986R (Carbone and Kohn 1999), ITS1/ITS4, ACT512F/783R, Alt-for/Alt-rev, ATPDF1/DR1, and CALDF1/DR1 (Lawrence et al. 2013), respectively. Results of sequences were deposited in GenBank (accession nos. MK733276, MK733276, MK791190, MK791191, MK791192, and MK791193, respectively). BLAST analysis showed 99 to 100% homology to ex-type sequences of A. alternata (KC584634, KF465761, MG736306, KP123858, JQ811979, and MG736308, respectively). Pathogenicity tests were conducted on surface-sterilized mango fruits; 4-mm-diameter mycelial plugs from colonies growing on PDA (7 days old) were placed on the surface of 24 mango fruits. Plugs of sterile PDA were placed on 12 healthy fruits as a control. The inoculated fruits were maintained in an artificial climate chamber at 25°C and 80 to 85% relative humidity with a 12-h photoperiod for 7 to 10 days. Dark-brown necrotic lesions developed similar to those observed on the original samples, whereas control fruits remained symptomless. The pathogen was reisolated from infected fruits and confirmed as A. alternata based on morphological characteristics and the molecular sequences. To our knowledge, this is the first report of A. alternata causing postharvest fruit rot of mango, and this pathogen has been reported to cause postharvest rot in kiwifruit (Li et al. 2017) and jujube (Alam et al. 2018). Postharvest fruit rot is a serious disease because it reduces fruit shelf life and threatens the sustainable development of fruits.