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Alternaria tenuissima Causing Alternaria Blight on Pigeonpea in India

Pigeonpea (Cajanus cajan (L.) Millsp.) is a major grain legume of the tropics and subtropics worldwide. In India, pigeonpea is the third most important food legume after chickpea and field pea. Blight symptoms on pigeonpea were observed in alarming proportion during the 2009 through 2011 crop seasons in Andhra Pradesh state in India. Disease incidence ranged from 20 to 80% irrespective of cultivars sown. Infected plants in the field showed symptoms on all aerial parts of the plant (leaves, stems, buds, and pods) irrespective of age of the plant and leaves. Symptoms on leaves were small, circular, necrotic spots that developed quickly forming typical concentric rings (1). Later, these spots coalesced and caused blighting of leaves. Spots were initially light brown and later turned dark brown. On stems, spots were sunken with concentric rings. In severe infection, defoliation and drying of infected leaves, branches, and flower buds was observed. The fungus was successfully isolated from all the infected plant parts (leaves, stem, buds, and pods) on potato dextrose agar (PDA) medium. After 4 to 5 days of incubation at 28 ± 1°C with a 12-h photoperiod, the fungus produced colonies that were regular and flat. The periphery of the colony was olive green with a black center. Monoconidial isolations were used to establish a pure culture of the fungus. Conidiophores were short, arising singly, and were 8.86 mm long and 2.97 mm thick. Conidia varied from 15.78 to 28.70 mm long and 8.03 to 13.47 mm wide. Very small beak (1.6 to 3.2 mm) or no beak was observed. Horizontal and vertical septations of conidia varied from four to six and two to four, respectively. The pathogenicity test was conducted on 8- to 10-day-old pigeonpea plants of cultivar ICPL 87119 by spraying with a conidial suspension (5 × 10⁵ conidia/ml). Inoculated plants were covered with polythene bags and kept in a greenhouse at 28 ± 1°C with a 12-h photoperiod. After 48 h, the polythene bags were removed. Ten days after inoculation, symptoms were similar to those observed in fields. This experiment was conducted twice with two independent sets of plants. No symptoms were observed in water-inoculated control plants. The fungus was reisolated from the inoculated plants. On the basis of the morphological characteristics, the pathogen was tentatively identified as Alternaria tenuissima. The identification was further confirmed by the rDNA and internal transcribed spacer (ITS) primer. The ITS region of rDNA was amplified with ITS 1 and ITS 4 primers. Both orientation sequenced amplicons (481 bp) were submitted to GenBank (Accession No. JQ074094). A BLASTn search revealed 99% similarity to A. tenuissima (Accession No. HQ343444). To our knowledge, this is the first report of molecular identification of A. tenuissima causing Alternaria blight in pigeonpea in India.

Reference: (1) Kannaiyan, J. and Nene, Y. L. 1977. Trop. Grain Legume Bull. 9:34.