Histopathological Studies and Enhancement of Resistance Against Purple Blotch of Onion

Abstract

Commercially, among the other cultivated vegetable crops, onion has paramount importance throughout the country. It is valued for its particular sharp taste and is a fundamental element for the cooking in numerous areas. A number of diseases attack onion crop but purple blotch is the most important one and cause huge losses. Genetic variability exists among onion genotypes to exploit resistance against purple blotch. Selection of onion genotypes with higher potential of resistance against purple blotch is considered a relatively better and natural way to combat against purple blotch. In 2014-15, studies on evaluating the resistant source against purple blotch of onion was conducted at the experimental research area of Department of Plant Pathology, University of Agriculture, Faisalabad. Twenty five onion genotypes were grown in randomized complete block design (RCBD) and evaluated against purple blotch on the basis of percent disease index (PDI) and disease rating scale. Experiment of resistance induction was conducted in glass house during 2015 and 2016. Selected genotype was treated with salicylic acid (2 mM) and Bion (125 ppm) by using foliar application and seedling root dipping methods to observed the severity of PB and biochemical attributes (PO, PPO, PAL, total Phenolics) following completely randomized design in factorial arrangement. Histopathological experiment was conducted during 2015-16 in which healthy and diseased onion tissues were compared with respect to time interval under microscope. Results showed that Phulkara, Sunset, Ceylon, TI-172 and XP-Red showed resistant response with rating 1. Five varieties (Desi Red, Early Red, Robina, Dark Red, Mirpurkhas) expressed moderately resistant response with rating 2. VRIO-6, VRIO-1, VRIO-4, Red Nasik and Desi Black express moderately susceptible response with rating 3. Five varieties viz. VRIO-9, Pak-10321, Fsd Red, Pusa Red and Red Imposta gave susceptible response with rating 4, while five varieties (VRIO-3, VRIO-5, VRIO-8, VRIO-7, VRIO-2) exhibited highly resistant response under rating 5 in both years. During 2015-16, severity of PB was reduced significantly with the application of resistance inducers. Foliar application significantly reduced the disease severity in both years as compared to seedling root dipping method. Maximum reduction in disease severity was observed in Bion (52.04%) while in salicylic acid (37.11%) reduction was assessed. Biochemical activity was maximum after ten days of treatment application. Foliar application of resistance inducers enhanced the biochemical activity as compared to seedling root dipping method. Remarkable increase in Total Phenolics (80.40%), Peroxidase activity (25.59%), Polyphenol Oxidase activity (47.29%), Phenylalanine ammonia lyase (88.53%) was assessed where BION + Alternaria porri was applied over control. Similar but increasing trend was observed during 2016. In histopathological experiment, Alternaria porri penetrated through epidermis and stomata in leaf tissue after three days of infection. Penetration of Alternaria porri accompanied with chemical degradation of surrounding host tissue. Host cell walls contained extracellular deposits that were prominent under microscope after six days of infection. Alternaria porri cause damage to host cell due to which cell collapse. Cytoplasm of cell disorganized due to excessive growth of hyphae and secondary metabolites. Whole process from penetration to cytoplasm degradation completed after nine days of infection. Similar results were observed in both years regarding histopathology. Resistance of onion genotype was enhanced in case of Bion against PB whereas foliar application of Bion (125 ppm) + Alternaria porri minimized purple blotch disease more efficiently.