FIRST REPORT OF ALTERNARIA ALTERNATA AUSING LEAF SPOT OF POLYALTHIA LONGIFOLIA IN PAKISTAN
N. Akhtar, M. Nayab, N. Sattar, F. Qurashi
In October 2014, plants of Polyalthia longifolia, an ornamental growing on the premises of University of the Punjab, Lahore were observed, that showed brown circular spots 1 to 2 mm in size covering 30 to 40% of the leaf blade. Small tissue pieces from a number of spots were surface-sterilized, plated in malt extract agar (MEA) and incubated at 25±2°C. Morphological observations were made on 7-day-old old cultures. Fungal colonies, deposited in the First Fungal Culture Bank of Pakistan (FCBP1518), were olive-green and did not show exudates or zonation. Conidia were olive brown in colour, arranged in long chains, had prominently geniculate beaks with protuberant scars. Some were 4-celled, 10- 15x20-30 μm in size, but the majority were 6-celled, ellipsoidal and up to 10-15x40-50 μm in size. These traits concur with those of Alternaria alternata (Simmons, 2007). Amplification of the ITS region of ribosomal RNA (White et al., 1990) yielded a product (GenBank accesssion No. KT283682) with 100% similarity to comparable sequences of several A. alternata isolates: FL7-PL4 (KP900243.1), EIODSF001 (KJ173524.1), KSH53T (KF380814.1). Pathogenicity tests were performed three times by inoculating autoclaved sterilized soil with 2×105 spores of three different isolates of the pathogen separately. Leaves were also sprayed with the same spore inoculum. Similar necrotic spots appeared 15 days post inoculation on the plants that were inoculated through the soil and after 21 days on those that were sprayed with spore suspension. Control plants remained asymptomatic. The fungus was successfully isolated from symptomatic plants, fulfilling Koch's postulates. To the best of our knowledge, this is the first report of P. longifolia leaf spot by A. alternata from Pakistan.