MSc.

The study entitled ‘Integrated management of Alternaria leaf spot of cabbage
(Brassica oleracea var. capitata (L.))’ was performed in College of Agriculture, Vellayani
during 2016-2018 with the objectives to study the leaf spot disease of cabbage caused by
Alternaria spp. and to develop management strategy to control the disease using fungicides
and biocontrol agents. As a part of study, survey was conducted in cabbage fields of three
districts of Kerala viz., Trivandrum, Kollam and Idukki and the maximum per cent disease
index (57.14%) was recorded in the cabbage fields of Muthuvankudy region in Idukki
district. Seven isolates of Alternaria spp. were isolated and purified. Pathogenicity tests
were conducted on the potted plants and the results revealed that the C6 isolated from
Muthuvankudy of Idukki district was found to be most virulent and selected for further
studies.
Microscopic characters of different isolates of Alternaria spp. obtained during the
survey were studied and showed that the mycelia were septate and conidia were produced
in chains from the conidiophores. These conidia consist of both longitudinal and transverse
septa and did not have beak which is the peculiar character of A. brassicicola.
The morphological characters of C6 isolate were studied and the average conidial
size ranged from 8-70 μm × 8-19 μm. The conidia were cylindrical in shape and did not
have beak. Based on the molecular characterization of the C6 isolate through ITS
sequencing, blast analysis in ClustalX2 software and dendrogram constructed using
TreeView software confirmed as A. brassicicola with the reference number SR- 1058.
Physiological studies were studied and determined the optimum temperature, pH and
the light intensity for the growth of A. brassicicola. The optimum temperature for the
growth of pathogen was 250C and the growth was less at 150C. which revealed that the
harvested cabbage can be stored safely at the lower temperatures without any post-harvest
damage. The maximum growth of the pathogen were recorded at pH 5.5 and 6. The fungi
exposed under normal day and night condition showed maximum mycelial growth
followed by dark and the growth and sporulation was declined with increase in the light
intensity.
The predominant fungal antagonists isolated from the phyllosphere of cabbage were
Myrothecium sp, Penicillium sp. and a non sporulating fungus which exhibited 54.4%,
50% and 46% inhibition of the pathogen respectively under in vitro conditions. Molecular
characterization of the most effective fungal antagonist was done with ITS sequencing and
identified as M. inundatum with reference number SR-1082.
The in vitro antagonism of five bioagents viz., Pseudomonas fluorescens (KAU),
Trichoderma viride (KAU), T. harzianum (KAU), Bacillus subtilis (KAU) and M.
inundatum were evaluated against the pathogen by dual culture technique and the results
revealed that the maximum percentage inhibition was recorded with T. viride (82.2%)
which was on par with T. harzianum (80%).
The efficacy of fungicides were evaluated against A. brassicicola at three different
concentrations viz., 0.05%, 0.1% and 0.2% concentrations with propiconazole,
hexaconazole, tebuconazole, azoxystrobin and 0.1%, 0.2% and 0.4% concentrations with
propineb, mancozeb, copper oxychloride by poisoned food technique and observed that
four fungicides viz., propiconazole, hexaconazole, tebuconazole and copper oxychloride
showed cent percent inhibition of pathogen at all the three concentrations.
The compatibility of four effective fungicides viz., propiconazole, hexaconazole,
tebuconazole (0.05%, 0.1%, 0.2%) and copper oxychloride (0.1%, 0.2% and 0.4%) were
tested with the effective bioagents viz., T. viride, T. harzianum and B. subtilis by disc
diffusion method and found that the triazole fungicides viz., propiconazole, hexaconazole
and tebuconazole were found to be compatible with B. subtilis and incompatible with T.
viride and T. harzianum at all the three concentrations under in vitro conditions.
A pot culture study was undertaken with cabbage variety NS-183 and evaluated the
efficacy of effective fungicides, bioagents and their compatible combination viz.,
propiconazole
(0.05%),
hexaconazole
(0.05%),
tebuconazole
(0.05%),
T.viride
(106cfu/ml), T.harzianum (106cfu/ml), B.subtilis (108cfu/ml), hexaconazole (0.05%) +
B.subtilis (106cfu/ml) against the Alternaria leaf spot of cabbage and the maximum disease
suppression over control was recorded with combined spraying of hexacoazole + B.
subtilis (79.4%) followed by hexaconazole (68.3%), tebuconazole (65.1%) and
propiconazole (63.6%).
Economic analysis on alternaria leaf spot of cabbage using fungicides, bioagents
and their combination showed that maximum yield was recorded with hexaconazole + B.
subtilis which was 29.5 q/ha. This was followed by hexaconazole (27.4 q/ha),
tebuconazole (26.1 q/ha) and propiconazole (24.2/ha). Benefit cost ratio was assessed and
showed that it was maximum with treatment hexaconazole + B. subtilis which was
2.09:1.0. This was followed by hexaconazole (2.02:1.0), tebuconazole (1.89:1.0) and
propiconazole (1.76:1.0).
The present study highlighted the importance of integrating hexaconazole with
bioagent B. subtilis for effective management of Alternaria leaf spot of cabbage as it
prevents the detrimental effects like development of resistance against a particular
fungicide by the continuous use of the same fungicide under field conditions and for the
long lasting protection even under storage conditions.