Pigeonpea (Cajanus cajan (L.) Millsp.) is the most important protein rich grain legume crop being cultivated worldwide. During surveys (2010 through 2012) conducted in major pigeonpea growing states in southern and central India (Andhra Pradesh, Karnataka, and Maharashtra), rapid mortality of pigeonpea plants was observed. This occurred in all of the surveyed areas with disease incidence of 20 to 60% irrespective of cultivar and crop growth stage. Symptoms included chlorosis, drooping and rolling of the leaves followed by rapid mortality of whole plant. Pinkish growth on infected stems and branches was observed and the inner layer of the infected stem had brown discoloration. Xylem vessels of the infected plants were healthy and did not show any blackening. Isolations from infected stem tissues consistently yielded cultures of Fusarium sp. on potato dextrose agar (PDA) medium. Monoconidial isolation from three separate isolates was used to establish pure cultures. The morphological characters of the fungus were consistent with descriptions in Fusarium keys (1) for Fusarium acuminatum (Ellis & Everhart). The mean colony growth was 86 mm after 7 days, with white aerial mycelium developing brownish pigmentation in the center on PDA. The dorsal side of the colony had rose to burgundy pigmentation. Macroconidia were broadly falcate with 3 to 5 septa, and were 3 to 8 × 39 to 64 μm. Microconidia were absent and chlamydospores formed in chains of 20 to 50 μm. Koch's postulates were established on seedlings of pigeonpea (cv. ICP 7119) using root dip inoculation of 10-day-old seedlings. The roots were immersed in a conidial suspension (6 × 106 conidia/ml) for 2 to 3 min; the control plants' roots were immersed in sterilized distilled water in a beaker. Inoculated seedlings were transplanted into pre-irrigated pots (12 cm) containing sterilized vertisol and sand (3:1). Five seedlings were used for each of three replications. Inoculated plants were kept in the greenhouse at 28 ± 2°C and irrigated with sterilized water. Inoculated plants developed symptoms identical to those observed in the field and disease incidence reached 100% within 96 h after inoculation. The experiment was conducted twice with two independent sets of plants. No symptoms were observed in water-inoculated control plants. The rDNA internal transcribed spacer (ITS sequence) was amplified with ITS1 and ITS4 primers (4). The amplicons of both forward and reverse (438 bp) were sequenced and submitted to GenBank (Accession No. JX177431). A BLASTn search revealed 100% sequence similarity to the nucleotide sequence of F. acuminatum (Ellis & Everhart) (GenBank Accession No. HQ443205). To our knowledge, this is the first report with confirmed molecular identification of F. acuminatum on pigeonpea. Occurrence of F. acuminatum on various plant species have been reported by Summerell et al. (3). Presence of F. acuminatum from soils of pigeonpea fields have been reported; however, no information on location, symptoms, plant mortality, and identification of pathogen has been provided (2).
References: (1) J. F. Leslie and B. A. Summerell. Pages 122-123 in: The Fusarium Laboratory Manual. Blackwell Publishing Professional, Hoboken, NJ, 2006. (2) A. P. Singh and S. N. Bhargava. Phytopathol. Z. 100:300, 1981. (3) B. A. Summerell et al. Fungal Diversity 46:1, 2011. (4) T. J. White et al. Pages 315-322 in: PCR protocols: Guide to Methods and Applications, San Diego, Academic Press, 1990.
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