S.-P. Huang, and
J.-Y. Mo, Institute of Plant Protection, Guangxi Academy of Agricultural Sciences, Nanning, Guangxi, 530007, China; and
P. Ning, Department of Biotechnology, Guangxi Agricultural Vocation-Technical College, Nanning, Guangxi, 530007, China; and
T. Hsiang, School of Environmental Sciences, University of Guelph, Guelph, Ontario, N1G 2W1, Canada
Mango (Mangifera indica L.) is an economically important fruit in southern provinces of China. In May 2012, field surveys including 2,250 mango trees were done in nine orchards of five different counties in Guangxi Province. An outbreak of gummosis was observed in the province involving over 30,000 ha with an average of 50% disease incidence (DI) and a maximum of 70% in some orchards. Until then, gummosis had been considered a common but not serious disease. In 2012, high temperatures in April and extensive rain in May favored increased disease development. Infected plants showed abundant gum secretion from branches, stems, and main trunks. Some branches died from the disease. During the early stages of infection, branches or stems turned brown followed by xylem necrosis and exudation of a milky sap. The sap turned yellow and finally formed amber gum within several days. Initially, the gum appeared as small droplets, increasing in number, and covering most of the branches and the trunk with bark cracking under severe conditions. On potato dextrose agar (PDA), isolates with similar morphology were consistently recovered from symptomatic plant tissues after surface sterilization in 75% ethanol for 30 seconds and then in 0.1% mercuric chloride for 1 min. Five single-spore isolates from five different locations in Guangxi Province were used to evaluate characteristics of the pathogen. On PDA, cultures were gray with an irregularly distributed, fast-growing, and fluffy aerial mycelium, showing a dark underside as the colony changed from greenish to black after 5 days at 28°C. After 1 month, cultures produced globose pycnidia. Conidia were elliptical and hyaline when immature, becoming dark brown and one-septate, longitudinally striate when mature, and ranged from 20.0 to 28.0 × 10.5 to 16.0 μm (average 23.3 × 13.7 μm). Paraphyses produced within the tissues of pycnidia were hyaline, cylindrical, nonseptate, and up to 61 μm long. The fungus was identified as Lasiodiplodia theobromae (Pat.) Griffon & Maubl. (=Botryosphaeria rhodina (Cooke) Arx) based on morphological and cultural characteristics (1,2). The rDNA internal transcribed spacer region of one isolate showed 100% identity to L. theobromae (GenBank HM346876.2) and was deposited in GenBank (JX982240). Pathogenicity of the five isolates was tested in the field on healthy tissues in June 2012. Five green twigs and five 3-year-old branches were used. Three wounds were made on each twig or branch with a sterilized needle. Mycelial plugs were placed on wounds and covered with Parafilm. Uncolonized PDA plugs were used as controls. Two weeks later, typical brown lesions were observed on inoculated branches, and gum exuded from infected wounds. No symptoms were seen on the controls. Koch's postulates were fulfilled by reisolation of L. theobromae from diseased branches. L. theobromae is well documented as a pathogen of mango. In China, the disease was observed in the 1990s in Hainan Province, and the causal agents were identified as L. theobromae and Colletotrichum gloeosporioides Penz. & Sacc based on morphological observation (3). To our knowledge, this is the most severe outbreak reported from China.
References: (1) V. S. de Oliveira Costa et al. Eur. J. Plant Pathol. 127:509, 2010. (2) F. Wang et al. Plant Dis. 95:1378, 2011. (3) Q. C. Xiao et al. Tropical Crops Research (in Chinese) 2:25, 1995.