Fungal pathogens associated with button shedding in coconut and its management using antagonistic microorganisms

Abstract

The research work entitled ‘Fungal pathogens associated with button shedding in coconut and its management using antagonistic microorganisms’ was conducted during 2019-21 at Department of Plant Pathology, College of Agriculture, Vellayani and Coconut Research Station, Balaramapuram with the objectives to isolate the fungal pathogens associated with button shedding in coconut, in vitro screening of the isolated pathogens and spraying of antagonists on the field for the management of the disease. Fallen infected nuts were collected from College of Agriculture, Vellayani and Coconut Research Station, Balaramapuram. The percentage disease incidence and intensity were high during monsoon season with highest value in the month of May in both locations. The disease was highly prevalent in Coconut Research Station, Balaramapuram compared to College of Agriculture, Vellayani. The samples collected were grouped into different age groups based on their maturity and found that the fungal infection was more prevalent in two month old nuts (88.57 %) and 3-4 month old nuts (87.00 %) while five month or above old nuts recorded lower fungal infection (40.00 %). The fungal pathogens isolated from both locations include Phytophthora palmivora, Lasiodiplodia theobromae, Fusarium oxysporum, Colletotrichum gloeosporioides and Thielaviopsis paradoxa. Regardless of the pathogen isolated from the samples, the fallen nuts displayed brown to black lesions extending from the perianth regions to the distal end. The isolated fungi vary in their morphological and microscopical characteristics which forms the basis for their identification. P. palmivora isolates were white to dull white in colour with stellate or irregular pattern. Lasiodiplodia theobromae cultures were initially white in colour which later turned grey and eventually became black in colour. F. oxysporum produced whitish colony growth with yellow or pink colour on the rear side. Colletotrichum gloeosporioides isolates showed white colony growth with white or greyish white colour on the rear side. T. paradoxa isolates had whitish colony growth with black pigmentation. The sporangia of P. palmivora were elongated, papillate and pedicellate while L. theobromae had dark brown, septate conidia with longitudinal striations. F. oxysporum produced oval micro conidia and sickle shaped macro conidia. Spores of C. gloeosporioides were cylindrical in shape with oil globule in the center. Dark brown conidia in chains without any septa were the characteristics of T. paradoxa. When cultured on Potato Dextrose Agar medium, the isolates varied in the time taken to fully cover the 9 cm Petri plate. T. paradoxa and L. theobromae took only three days to attain full growth. P. palmivora and C. gloeosporioides isolates attained full growth within 10 to 12 days. F. oxysporum took more than 12 days to reach full growth. The isolated pathogens when inoculated on two month old healthy nuts resulted in the formation of brown to black lesions. The lesion size varied with the inoculated fungus. T. paradoxa and L. theobromae were the most virulent and caused severe rotting of nuts within 15 days of inoculation. The combined inoculation of isolates obtained from Coconut Research Station, Balaramapuram resulted in much more severe rotting than isolates obtained from College of Agriculture, Vellayani. The isolated pathogens were screened in vitro against antagonistic microorganisms obtained from Department of Agricultural Microbiology, College of Agriculture, Vellayani which include Pseudomonas fluorescens PN026, Trichoderma sp. (KAU strain), Bacillus amyloliquefaciens VLY24 and Bacillus velezensis PCSE10. The antagonists exhibited lysis or overgrowth or antibiosis in dual culture plates depending on the pathogen tested. Among these antagonists, Trichoderma sp. (KAU strain) showed higher inhibition percentage and antagonistic index. Among the bacterial antagonists, B. amyloliquefaciens VLY24 and B. velezensis PCSE10 recorded wider inhibition zone with higher percentage inhibition and antagonistic index. P. fluorescens had the lowest percentage of inhibition and antagonistic index. The antagonistic microorganisms tested in vitro were cultured in liquid broth and adjusted to 108 colony forming units per ml. The antagonists are then sprayed on two to seven month old bunches of button shedding affected palms in Coconut Research Station, Balaramapuram. Copper oxychloride (0.2 %) was used as chemical check. The number of nuts in the treated bunches were noted down at the time of spraying and also at fortnightly intervals. The percentage nut fall for each interval was calculated. After 70 days of spraying, the percentage of nut fall was much lower in bunches sprayed with B. amyloliquefaciens VLY24 (42.71 %) followed by Copper oxychloride treatment (47.54 %). The nut fall percentage exhibited a declining trend which could be attributed to the decrease in fungal infection in mature nuts.