MSc

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.