Genotypic and seasonal influence on leaf spot disease in amaranth

Abstract

The investigation on " Genotypic and seasonal influence on leafspot disease in amaranth" was undertaken in the Department of Olericulture, College of Horticulture, Vellanikkara, Thrissur during 1996 - 1998. The objectives of the study were to identify amaranth accessions resistant to leaf spot disease, to isolate and identify the pathogens associated with the disease, to study the seasonal influence on leaf spot and yield and to find the possibility for biochemical cataloguing of amaranth accessions. Evaluation of 168 amaranth accessions for leaf spot disease resulted in identification of 14 immune, 15 resistant, 19 moderately resistant, 34 moderately susceptible and 86 highly susceptible accessions. Red types were found highly susceptible as compared to green types. Disease infection occurred within 15 days of planting resulted in maximum percentage of disease severity. Two types of leaf spot symptoms were inflicted and causal organisms identified were Rhizoctonia solani Kuhn. and Colletotrichum capsici (Syd.) Butler and Bisby. The two organisms either alone or in ombination caused the incidence and development of leaf spot disease. Red types were infected by Rhizoctonia solani alone where as the green types were infected by both pathogens. Seasonal influence on disease recorded maximum disease severity in July crop and minimum in April crop. Low temperature, high relative humidity and high rainfall were the favourable weather conditions for disease development. Leaf spot in amaranth was negatively correlated with maximum and minimum temperatures where as relative humidity and rainfall were positively correlated with the disease. Highest yield was realised in March planted crop and lowest in June crop. Yield and yield attributes except plant height were positively correlated with maximum and minimum temperatures where as relative humidity and rainfall were positively correlated with them. High content of total phenols, OD phenol and ascorbic acid were recorded in immune and resistant accessions than susceptible. Total phenol content was higher in rainy season. OD phenol, ascorbic acid and pigments decreased upon infection in rainy season. Oxalates and nitrates had no influence on leaf spot disease development. High negative correlation was found between total phenol, OD phenol, .ascorbic acid, chlorophyll 'a' and 'b' and disease. Generally immune and resistant accessions had more number of PPO and PRX bands than susceptible types. The additional band expressed in diseased condition in certain accessions can be taken as the biological marker for leaf spot resistance.