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The study entitled “Host range studies and management of anthracnose of nutmeg
caused by Colletotrichum spp .” was conducted at Department of Plant Pathology, College
of Agriculture, Vellayani during 2018-2020 with the objectives to characterize the causal
agent, study the host range of Colletotrichum spp. causing anthracnose and to develop
effective management strategy to control the disease by using new generation fungicides.
As a part of the study, anthracnose infected samples were collected from four nutmeg
growing districts of Kerala viz., Thiruvananthapuram, Kottayam, Ernakulam and Idukki. For
the infected sample collections, three locations from Thiruvananthapuram (Vellayani,
Karamana and Palode), two locations from Kottayam (Kumarakom and Vaikom), six
locations
from
Idukki
(Myladumpara,
Pampadumpara,
Adimali,
Kambilikandam,
Panickankudy and Kattapana) and one location from Ernakulam (Kadungalloor) were
surveyed. Disease incidence and severity were assessed from the surveyed locations. The
highest disease incidence and severity were observed in Kadungalloor (DI - 90 % and PDI -
56.40 respectively) followed by Kumarakom (DI - 80 % and PDI - 41.33 respectively) and
the lowest disease incidence and severity in Myladumpara (DI - 20 % and PDI - 15.53
respectively).
The symptoms of the anthracnose on nutmeg appeared as small necrotic spots with a
prominent yellow halo on the leaf lamina. Several lesions coalesced together resulted in leaf
blight, shot hole and defoliation. In Kambilikandam and Panickankudy. fruit rot was also
observed along with leaf spot. The cultures of Colletotrichum spp. were isolated from the
infected samples from different locations. Eighteen pure cultures of Colletotrichum sp. (C1 to
C18) were obtained. Seven isolates of Colletotrichum sp. were selected for further studies
based on the days taken for symptom development and rate of lesion development. The
pathogenicity of the seven isolates of Colletotrichum sp. from different locations were
proved by Koch postulates.
The morphological and culture characters of the seven different isolates were studied
in potato dextrose agar (PDA) medium. The isolated cultures of Colletotrichum sp. produced
whitish to greyish radiating mycelium; later turning to off white to pink coloured fluffy to
sparse mycelium with regular margins. Days taken to grow the entire Petri dish ranged from
7 to 10. The mycelium of the fungus was hyaline and septate; and its width ranged from 0.46
μm to 2.48 μm. The conidia were single celled with an oil globule at the centre and wereoblong or dumbbell shaped. The conidial size varied from 7.87 to 19.97 μm x 3.26 to 5.68
μm. The isolates were morphologically identified as C. gloeosporioides.
The pathogenic variability of the seven isolates of C. gloeosporioides was assessed on
detached nutmeg twigs by virulence rating. The isolate C4 was identified as the most virulent
isolate which produced lesion size of 11.89 cm and 16.81cm at 7 DAI and 9 DAI
respectively. The isolate C4 produced symptoms within two days after artificial inoculation
and had a higher rate of lesion development of 4.12 cm day -1 . The other isolates took 3 to 4
days for symptom appearance on artificial inoculation of the pathogen.
Host range of the most virulent isolate of C. gloeosporioides (C4) obtained from
nutmeg was studied in perennial tree spices viz., clove, cinnamon, all spice, betel vine, black
pepper and coconut. C. gloeosporioides isolate of nutmeg is capable of infecting the above-
mentioned host plants. The isolate produced symptoms in all the hosts within 2 to 4 DAI and
the symptoms developed varied from brown lesions, brown lesions with a shot hole to
necrotic spots with prominent yellow halo. The maximum lesion size of 2.43 cm was
observed in clove and minimum lesion size of 1.31 cm in all spice.
In vitro screening of new generation fungicides revealed that triazole group fungicide
propiconazole 25EC at 100 ppm and combination fungicides, carbendazim 12% + mancozeb
63% at 25 ppm; and Trifloxystrobin 25% + Tebuconazole 55% WP at 100 ppm concentration
were the most effective in completely inhibiting the mycelial growth of the pathogen.
The present study revealed the wide host range of the C. gloeosporioides isolate of nutmeg
and also the effectiveness of new generation fungicides in managing the pathogen. The future
line of work should include molecular variability between various isolates, cross infectivity
among the isolates in other perennial hosts, and the efficacy of new generation fungicides
under field condition.