N. K. B.
I. S. K.
Department of Botany, University of Peradeniya, Peradeniya 20400, Sri Lanka
Department of Plant Pathology, Agricultural Research Organization, Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel
Mango malformation disease (MMD) is one of the most devastating diseases causing severe economic losses to this crop worldwide. MMD has not been reported in Sri Lanka although the disease was reported in neighboring India over a century ago. Abnormal, thick, and fleshy mango panicles (40%) and proliferating stunted shoots (<1%) showing characteristic malformation symptoms were observed in Peradeniya-Kandy area (7°17'4.15” N, 80°38′14.08” E). Malformed inflorescences and vegetative shoots were collected during January to March and September to November, in 2008 through 2012. Pieces of malformed tissues were surface sterilized in 1% sodium hypochlorite and transferred to potato dextrose agar (PDA). The plates were incubated at 26 ± 2°C for 7 days. Monoconidial cultures of 41 isolates that resembled Fusarium spp. were obtained. Colonies showed white sparse aerial mycelium and magenta-dark purple pigmentation on the underside. Growth rate of the isolates averaged 3.67 mm/day in the dark at 25°C on PDA. To stimulate conidia development, Fusarium isolates were transferred to carnation leaf agar (CLA). Sympodially branched conidiophores bearing mono- and polyphialides with 2 to 3 conidiogenus openings originated erect and prostrate on aerial mycelium. Oval to allontoid, abundant microconidia were produced in false heads on mono- and polyphialides. Dimensions of aseptate conidia were 2.5 to 12.5 (6.47) × 1.25 to 3.8 (2.29) μm. Macroconidia were long and slender, 3 to 5 celled and 27.5 to 47.5 (38.59) × 2.5 to 5 (2.94) μm. Chlamydospores were absent. These characters are consistent for F. mangiferae. DNA was extracted from 30 monoconidial Fusarium isolates (1) and amplified with species-specific PCR primers 1-3F/R (forward: 5′-TGCAGATAATGAGGGTCTGC-3′; reverse: 5′-GGAACATTGGGCAAAACTAC-3′) (3). Eight isolates from malformed inflorescences (I6, I13, I15, and I16) and malformed vegetative tissues (V1, V2, V3, and V4), were identified as F. mangiferae based on a 608-bp species-specific amplified DNA fragment. Pathogenicity of F. mangiferae isolates, I15 and V2, was tested on 1-year-old seedlings cv. Willard planted in 10-liter plastic pots. Conidia suspensions (107 conidia/ml of 0.1% water agar) were obtained from 10-day-old monoconidial cultures. Each isolate was inoculated onto 15 apical buds by placing drops (20 μl) of conidia (2). Both F. mangiferae isolates, I15 and V2, on artificial inoculation produced typical floral malformation symptoms in 40% of the buds, up to 10 weeks after inoculation. The Fusarium isolates recovered were identical in colony and mycelia morphology and conidia dimensions to the original F. mangiferae isolates. No Fusarium species were recovered from control flower buds. To our knowledge, this is the first report of MMD in the inflorescence and the vegetative shoots caused by F. mangiferae in Sri Lanka. Isolation of other Fusarium spp. that were not identified as F. mangiferae in this study suggests that additional Fusarium spp. may be associated with the MMD in Sri Lanka. Further studies are needed to confirm the identity of these Fusarium isolates, their role in MMD, and the distribution over the island. Since the disease is likely to drastically reduce productivity, measures will be required to protect 12,160 ha of mango cultivation from this devastating disease.
References: (1) S. Freeman et al. Exp. Mycol. 17:309, 1993. (2) S. Freeman et al. Phytopathology 89:456, 1999. (3) Q. I. Zheng and R. C. Ploetz. Plant Pathol. 51:208, 2002.