Morphological and Molecular Characterization of Alternaria Isolates Causing Purple Blotch Disease of Onion

Abstract

Purple blotch is the most destructive disease commonly prevailing in almost all onion growing pockets except in very cool production areas of the world. The present study was carried out to understand the diversity in Alternaria population causing purple blotch disease of onion. Out of fifty six geographical Alternaria isolates of Karnataka seventeen were selected as descriptive isolates based on morphological characteristics for understanding the genetic variability. The isolates exhibited Ash, Ashy black, Ashy white, Ashy green and blackish green colour. Sabouraud dextrose agar supported the maximum growth whereas corn meal agar did not support the growth. Asthana Hawker’s medium supported the sporulation of the isolates. The isolates showed variability in shape, conidia length, width, beak length and number of septa. Sequencing and blasting of nucleotide sequence of ITS region of ribosomal DNA of isolates revealed 100% matching with five different species of Alternaria viz., A. porri, A. alternata, A. tenuissima, A. palandui and A. brassicicola. Cluster analysis of ITS rDNA sequences did not provide reliable differentiation between the species and among the isolates. The results put forward that ITS region of ribosomal DNA is inappropriate for taxonomic resolution of Alternaria species infecting onion.