MSc

ABSTRACT
Management of stem rot and foliar blight of cowpea (Vigna unguiculata (L.) Walp.)
Cowpea (Vigna unguiculata (L.) Walp.) is one of the highly demanding, nutritionally rich vegetable and legume crops cultivated in Kerala throughout the year. Stem rot and foliar blight disease in cowpea was emerged as a serious disease in farmer’s fields especially during monsoon period and resulted in severe yield reduction and economic loss. Hence present study was undertaken during 2018-2020 with an aim of identifying and characterizing fungal pathogen causing stem rot and foliar blight disease in cowpea and to evaluate biocontrol agents and chemical fungicides against it.
Cowpea plants showing symptoms of stem rot and foliar blight disease were collected from six locations of Kasargod district such as Cheemeni, Pallikkara, Nileswar, Trikaripur, Udinur and Periya showing disease incidence ranging from 12 to 80 % and percent disease index of 8 to 51 %. Highest disease incidence recorded at Nileswar and lowest at Periya. Six isolates obtained were named with index ‘Sr’ such as Sr1, Sr2, Sr3, Sr4, Sr5 and Sr6.
Pathogenicity test of six isolates were done in cowpea by soil inoculation and leaf inoculation methods. Isolate Sr3 produced symptom on stem at 2nd day and on leaf at 24 h of inoculation which was earlier than other isolates. Area of water soaked lesion on stem (6.4 cm2) and leaf (9.6 cm2) was also highest for Sr3 and observed fastest radial growth compared to others and covering 90 mm petridish at third day of inoculation. Based on disease development and growth rate on PDA, Sr3 was considered as most virulent isolate.
Pathogen was identified based on cultural, morphological and molecular characterestics. Cottony and fluffy pure white colony with light zonation and presence of two types of fungal hyphae with clamp connection were observed. Two types of sclerotial bodies were detected and cross section of sclerotium revealed the presence of different layers such as outer thick skin, rind, cortex and medullary cells under compound microscope. Molecular identification based on D1/D2 region of LSU revealed 99.61 % similarity with Athelia rolfsii. Based on all these characteristics isolate Sr3 was identified as Sclerotium rolfsii.
Symptoms associated with stem rot disease were water soaked lesion at the basal region of stem followed by wilting, yellowing of the aerial parts, necrosis, girdling and rotting of the stem. White mycelia and sclerotial bodies were formed at the infected portion. Foliar blight disease showed water soaked lesion on the leaf with concentric ring formation. Both mycelia and sclerotial bodies were also produced over the lesion.
Effect of biocontrol agents on S. rolfsii tested by dual culture method explained higher inhibition per cent of Trichoderma harzianum followed by Trichoderma viride. Bacterial biocontrol agents showed least potential of antagonism in which Pseudomonas fluorescens exhibited zero inhibition on pathogen.
Among the fungicides tested against S. rolfsii by poisoned food method, mancozeb 75WP (0.1 %, 0.2 % & 0.3 %) and propiconazole 25EC (0.05 %, 0.1 % & 0.2 %) were found best having 100 % inhibition over the pathogen whereas copper oxychloride 50 WP (0.1 %, 0.2 % & 0.3 %) and carbendazim 50WP (0.05 %, 0.1% & 0.2 %) were not effective. Compatibility of mancozeb, propiconazole and chlorothalonil were tested with T. harzianum and T. viride. Mancozeb (0.2 %) exhibited 100 % compatibility with T. harzianum and 92.39 % with T. viride. Propiconazole (0.1 %) was highly (100 %) incompatible with both of these followed by chlorothalonil (0.2 %).
Field evaluation with most efficient fungicides, biocontrol agents and combination of both were experimented on cowpea (variety: Kanakamani) in pot culture method. Pre-sowing drenching with post sowing drenching and spraying of mancozeb (0.1 %) and propiconazole (0.05 %) at 20, 40, 60 days of sowing were found best for the management of stem rot and foliar blight disease having BC ratio of 1.9 followed by combinations of mancozeb and Trichoderma sp. Among the biocontrol agents T. harzianum were found superior than T. viride.
Hence soil drenching at 7 days before sowing and soil drenching and aerial spraying at 20, 40, 60 days after sowing with mancozeb (0.1 %) or propiconazole (0.05 %) or mancozeb-Trichoderma combinations or T. harzianum (2 %) can be recommended for management of stem rot and foliar blight disease in cowpea. future line of work should be focused on field level study of stem rot and foliar blight disease in other districts of Kerala and to evolve local specific management with native isolates of biocontrol agents and their metabolites.