​Phoma Blight of Soybean in Kashmir: Etiology, Relative Yield Losses and Critical Stage of Management Intervention

Abstract

Background: Soybean leaf blight has been viewed as a possible biotic stress responsible for decreasing yield potential of soybean in Kashmir over the years. Therefore, its etiology and recording associated yield loss were deemed necessary corresponding to baseline findings for deciding any chemical or non-chemical intervention in disease management. Methods: Field and laboratory investigations were undertaken during crop periods of 2019 and 2020. Disease specimens were collected from various locations across valley and the pathogen was isolated on Potato Dextrose Agar medium. To identify the host range of soybean isolate, nine pulse crops were tested under controlled conditions (Tem. 22±3°C and RH greater than 80%). The relative yield loss and critical stage of chemical intervention were assessed by exposing soybean populations to disease at different phenological stages which created five different levels of disease in a field experiment laid in RBD with four replications. Result: Phoma sojicola (syn. Ascochyta sojicola) was associated pathogen of this disease of soybean in Kashmir. Typical disease symptoms on leaves comprised of roughly circular and brown lesions (up to 20 mm diameter) with concentric rings having interspersed pycnidia. Most often the diseased tissue fall apart and gives it a shot hole expression. Radial daily mycelial growth of the pathogen was 3.62 mm colony was grey olivaceous in the centre and light olivaceous towards periphery. Conidia were hyaline, oblong to ellipsoidal and 1-2 celled with average dimensions of 7.98 x 2.87 µ. Chlamydospores were brown olivaceous, globose to sub-globose with average dimensions of 16.89 x 9.30 µ. After artificial inoculation, Phaseolus vulgaris, Vigna sinensis, V. radiata, V. mungo, Cicer arietinum, Vicia faba and Cajanus cajan manifested disease symptoms after variable incubation periods of 8-13 days, while as Lens esculenta and Pisum sativum resisted the invasion. Early appearance of the Phoma blight (V-1 stage) in soybean can reach a terminal intensity of 67.16 per cent besides causing 51.72 per cent yield loss. Tolerable yield loss (5.63%) was achieved only when the crop was maintained disease free up to R-3 (beginning pod development) stage.