Journal of Plant Pathology Archive

Abstract:

Potato plants with brown spot symptoms were observed in the Punjab, Pakistan during March 2015, with an approximated incidence of 45.5%. Symptoms were small, brown lesions on leaves with concentric rings coalescing into larger lesions. Infected leaves died. For pathogen isolation, surface sterilized leaves were cut from lesion edges, and incubated at 25 ± 2°C on potato dextrose agar medium for 7 days. Fungal colonies were fast-growing, brownish, and cottony. Conidiophores arised singly or in clusters, usually 2-6, 42-27 µm in length and 4-25 µm in width. Conidia were greenish brown, catenate, ovoid or obclavate, multi-celled, with 2-6 transverse septa, 1-2 longitudinal septa, 12-32 × 6-12 µm in size. Genomic DNA of the fungus was isolated by CTAB method, amplified using ITS1 and ITS4 primers (White et al., 1990), sequenced (GenBank Accession No. LT605000) and showed 99% similarity to the sequences of Alternaria alternata isolates. Pathogen was identified as Alternaria alternata (Fr.) Keissler. Koch's postulates were verified with modified in vitro detached leaves assay (Park et al., 2008). Fifteen potato leaflets were placed with the bottom side up in a Petri dish, then covered with paper towels and 20 ml distilled water. Spore suspension (1 × 105 ) of A. alternata from 7 day-old cultures was drop inoculated on each leaf side; sterile water was used for control leaves. The experiment was repeated twice. Inoculated leaves stored at 25±2°C for 7 days, developed brown lesions and spots similar to those observed in the field, while control leaves showed no symptoms. A. alternata was re-isolated from inoculated leaves and pathogenicity test confirmed it as the casual agent of brown spot disease. Brown leaf spot caused by A. alternata on different hosts was also reported in South Africa and other parts of the world (Van der Waals et al., 2011; Thomma, 2003). This is the first report of A. alternata causing brown spot of potato in Pakistan