Morphological Variability in Rice Blast Pathogen Pyricularia oryzae (Cooke) SACC

Abstract

Rice is a staple food crop for the majority of the human population worldwide. Significant grain yield losses are reported due to blast disease caused by Pyricularia oryzae across all rice-growing areas of the world. Although presently available blast management strategies reduce disease incidence significantly, blast epidemics are still common, causing devastating yield losses. The present study was aimed at determining the variability among Pyricularia oryzae isolates. Leaf blast samples were collected from the southwestern regions of Bangladesh, and the causal pathogen was isolated following the tissue planting method in the Plant Pathology Laboratory of Agrotechnology Discipline, Khulna University, Khulna, during the period from September 2020 to September 2021. Morphological variability was determined based on nine morphological characteristics, namely front mycelial color, reverse mycelial color, growth behavior, conidial length (µm), conidial width (µm), conidial color, radial growth at 12 days (mm) after incubation, radial growth at 14 days (mm) after incubation, and radial growth at 16 days (mm) after incubation. Seven types of mycelial colors were recorded: light brown, deep brown, light gray, olivaceous gray, gray, deep gray, and black. Two types of growth behavior, smooth and rough, were observed among the P. oryzae isolates. Two types of conidial color, blackish and transparent, were also recorded among the 19 isolates. Cluster analysis based on the nine morphological characteristics grouped the isolates into five distinct clusters. Three isolates were placed in each of Clusters I and III, Cluster II contained seven isolates, Cluster IV contained five isolates, and Cluster V contained only one isolate (PO-H). Isolate PO-H exhibited distinct morphological variation among the 19 isolates and originated from AEZ-11. Principal Component Analysis (PCA) biplot demonstrated a wide range of variation and revealed a strong negative correlation between two groups of variables. One group consisted of conidial length, conidial breadth, and growth behavior, while the other group consisted of reverse mycelial colony color, front mycelial colony color, conidial color, and radial growth at 12, 14, and 16 days after incubation. Among the nine morphological characteristics, front and reverse mycelial color, growth behavior, and conidial length contributed more than 80% of the total variation observed among the tested isolates.