Etiology and management of bacterial wilt of yard long bean

Abstract

The study entitled “Etiology and management of bacterial wilt of yard long bean” was conducted at College of Agriculture, Vellayani and Integrated Farming System Research Station (IFSRS), Karamana during 2020-2022 with an objective to identify and characterize the bacterium inciting wilt disease in yard long bean and management of the disease. Bacterial wilt infected yard long bean plant samples were collected from Vamanapuram (8.7226° N, 76.8971° E) and Nanniyode panchayat (8.6988° N, 77.0262° E) of Thiruvananthapuram district, Kerala. The disease incidence was 85 to 90 per cent. The disease symptoms included drooping and wilting of green leaves, collapse of stem followed by death of plants. The bacterium was isolated on triphenyl tetrazolium chloride (TZC/TTC) medium and three methods of pathogenicity tests viz., root dip, root dip and stem injection as well as soil inoculation were undertaken to prove Koch's postulates. The symptoms were observed within three to seven days after inoculation and complete wilting was observed in 7, 10 and 14 days in root dip, root dip and stem injection and soil inoculation respectively. Cultural studies revealed the bacterial colonies to be white or cream, circular, smooth, convex in casamino acid peptone glucose agar (CPG) and white, circular, smooth, convex, fluid like with pink colouration at centre in TZC medium which were the best media for the bacterial growth. Morphological and biochemical studies revealed the bacteria as gram negative, rod-shaped and facultative anaerobic. Molecular characterization using the universal primers of 16S rRNA viz., 27F/1492R revealed 99.83 per cent identity with several isolates of Enterobacter spp. and Kosakonia oryzae and the gene amplified using CM7F/CM31b primers of rpoB, one of four house-keeping genes of Enterobacteriaceae revealed 99.72 per cent identity with Kosakonia oryzae strain Ola51. The 16S rRNA and rpoB sequence were deposited in NCBI database (Acc. no. OP321041 and Acc. no. OP321041 respectively). Further, phylogenetic analysis and biochemical characterization using Enterobacteriaceae specific kit confirmed the pathogen to be K. oryzae. 142 In vitro evaluation of various chemicals by poisoned food technique and filter paper assay revealed that streptocycline (250 ppm) was the most effective bactericide resulting in a mean inhibition zone of 27.67 mm, followed by copper oxychloride 50 % WP (0.30 %) (17.00 mm) and copper hydroxide 77% WP (0.20 %) (15.33 mm). Among the various botanicals screened for antibacterial effect in vitro by paired plate technique, bulbs of Allium sativum revealed the highest inhibition of bacterial growth followed by leaves of Mansoa alliaceae. In agar well diffusion method, bulbs of A. sativum (15 %) followed by leaves of Boerhavia diffusa (20 %) and M. alliacea (20 %) resulted in maximum inhibition (inhibition zone of 20.33, 18.83, 16 mm respectively) of the bacterial growth. An in vivo study was undertaken at IFSRS, Karamana for the management of bacterial wilt disease in yard long bean (var. Geethika) using the best three treatments viz., bulbs of A. sativum (10g per pit), copper oxychloride 50% WP (0.30 %) and copper hydroxide 77 % WP (0.20 %) selected from in vitro studies along with streptocycline (250 ppm), bacteria inoculated and uninoculated treatments as checks. The inoculated control revealed 100 per cent disease incidence whereas soil application of crushed garlic bulbs (10 g/pot at one week before transplanting) followed by immediate covering with cow dung and soil mixture, seedling treatment and soil application of crushed garlic bulbs at 10, 20 and 30 days after transplanting (DAT) registered the minimum disease incidence (13.33 %) with the maximum number of pods (43), pod length (46.82 cm), pod weight (21.67 g) and pod yield (815.13 g) followed by soil application of copper oxychloride 50 % WP (0.3 %) (754.77 g pod yield) with 20 per cent disease incidence. Treatment application at 10, 20 and 30 DAT with 10 days interval (DI) was more effective than at 15, 30 and 45 DAT with 15 days interval. Biochemical studies on identifying the mechanism of disease tole