MSc

The research entitled “Etiology and management of fungal root rot of cassava in
Southern Kerala” was conducted in the Department of Plant Pathology, College of
Agriculture, Vellayani, during 2020-2022 with an objective of Identification and
characterization of major pathogen causing fungal root rot in cassava and its
management using different biocontrol agents and fungicides.
During this study, root rot infected cassava plants were collected from different
locations in the Thiruvananthapuram and Kollam districts of Southern Kerala.
Twenty-one isolates of different fungi were obtained from root rot infected parts of
cassava. Various symptoms such as yellowing of leaves, rotting of tubers and collar
region of cassava stems, mycelial growth over the infected tubers and wilting of
cassava plants were noticed in diseased samples from different locations. Based on the
pathogenicity test conducted on potted cassava plants of one - month old Vellayani
Hraswa variety, T3B (isolate from Thalayal of Thiruvananthapuram district) and K1B
(isolate from Ezhukone of Kollam district) isolates were found to cause 100 Percent
Disease Incidence (PDI). Based on the number of days taken for disease development,
the isolate T3B (Fusarium sp.) was found to be most virulent with a PDI of 100,
which took 30 days for symptom development, followed by K1B (Fusarium sp.) with
PDI of 100 and 40 days for symptom development. Hence, T3B was selected for
further studies.
Cultural and morphological characteristics of the T3B isolate were studied. The
colony was white cottony, with irregular margins on the front view and a light pinkish
tinge on the rear view. Microscopic characters of the T3B isolate revealed that
the mycelia were septate and hyaline. Macroconidia and microconidia were produced
abundantly on conidiophores. Macroconidia were hyaline, septate (0-2 septa), slender,
almost straight and elongated with blunt ends, while the microconidia were slender,
hyaline, and single celled with oblong shape. Chlamydospores were absent. Molecular
characterization of T3B isolate was carried out using ITS sequencing using universal
primers followed by comparative nucleotide sequence alignment with the available
database from NCBI. The isolate T3B was confirmed as Fusarium proliferatum with
99.79 per cent homology with F. proliferatum strain OR25F07 (MT476359.1) and the
dendrogram was also constructed as a distance tree which showed a phylogenetic
relationship. The nucleotide sequence of the T3B isolate was deposited in GenBank
and the accession number was retrieved as OQ376433.
As part of the study, microflora was isolated from the rhizosphere of healthy cassava
plants from Vellayani and Venganoor of Thiruvananthapuram district in Kerala.
Among the isolated bacterial and fungal rhizosphere microflora, viz., Aspergillus sp.
1, Aspergillus sp. 2 Bacteria 1 and Bacteria 2, Aspergillus sp. 1 from Vellayani
showed the highest inhibitory effect against the pathogen with 64.84 per cent
inhibition.
The in vitro antagonism of seven biocontrol agents viz., Pseudomonas fluorescens
PN026, Bacillus sp. isolate 1 and Bacillus sp. isolate 2 (RRS, Moncompu),
Trichoderma sp. 1, Trichoderma sp. 2 (KAU), Piriformospora indica (No. INBA
3202001787) and Aspergillus sp. 1 (selected antagonist) were evaluated against F.
proliferatum by dual culture technique and results revealed that highest percentage
inhibition was recorded with Trichoderma sp. 1 (57.57 %) followed by Trichoderma
sp. 2 (51.51 %).
The in vitro evaluation of fungicides through poisoned food technique against F.
proliferatum was conducted using eight fungicides viz., mancozeb 75% WP, copper
oxychloride 50% WP, propineb 70% WP, carbendazim 50% WP, propiconazole 25 %
EC, cymoxanil 8% + mancozeb 64% WP, carbendazim 12% + mancozeb 63% WP
and trifloxystrobin 25%+ tebuconazole 50 % 75 WG at 50, 100 and 200 ppm. The
highest per cent inhibition was showed by carbendazim 50% WP and trifloxystrobin
25%+ tebuconazole 50 % 75 WG at 200 ppm with an inhibition percentage of 87.87%
and 100%, respectively. Carbendazim 50% WP and trifloxystrobin 25%+
tebuconazole 50 % 75 WG were the most effective fungicides used for in vitro assay.
A pot culture study was undertaken with the Vellayani Hraswa variety to evaluate the
efficacy of effective biocontrol agents and fungicides, viz., Trichoderma sp. 1.
Trichoderma sp. 2 (KAU), carbendazim 50% WP and trifloxystrobin 25%+
tebuconazole 50 % 75 WG against F. proliferatum and results shown that the highest
disease suppression over control was recorded with Trichoderma sp. 2 (100.00%)
followed by Trichoderma sp. 1 (88.89%) and trifloxystrobin 25%+ tebuconazole 50
% 75 WG (77.78%) with sett treatment and soil application at one and two months
after planting. The highest yield of 1342.66 g per plant was obtained from treatment
with Trichoderma sp. 2 @108
cfu/ml followed by trifloxystrobin 25%+ tebuconazole
50% 75 WG @1 gL-1 (1106.00 g per plant) and Trichoderma sp. 1 @108
cfu/ml
(1003.66 g per plant).
The present study revealed that Fusarium spp., Lasiodiplodia spp., Colletotrichum
spp. and Rhizoctonia spp. were associated with root rot of cassava in Southern Kerala.
The most virulent pathogen was identified as Fusarium prolifaratum by analysing the
cultural, morphological and molecular characteristics. The sett treatment and soil
application of biocontrol agent Trichoderma sp. 2 (KAU isolate) (108
cfu/ml) or
the fungicide trifloxystrobin 25%+ tebuconazole 50% 75 WG (1 gL-1
) at one and two
months after planting can effectively manage the disease.