PhD

The study on ‘Phenotypic and molecular characterisation of Phytophthora
sp. inciting leaf fall of nutmeg’ was conducted in the Department of Plant
Pathology, College of Horticulture,Vellanikkara during 2014-2015. The major
objectives were to study the cultural, morphological and molecular characters and
variability of different isolates of Phytophthora sp. associated with leaf fall of
nutmeg and also to study the host range of the pathogen and to chalkout suitable
management strategies
Isolation of the pathogen from 18 samples from different locations revealed
the association of the fungus, Phytophthora sp. and its pathogenicity was
established under lab and in vivo conditions. Inoculation of the pathogen with
culture disc on injured lower leaf surface showed early infection than that with
zoospore suspension. Symptoms observed on leaves, shoot and fruits were almost
same under both natural and artificial conditions. Variation in virulence was noticed
among the isolates collected from different locations. The isolates, PPaL-1 and
PPaF-17, from Parakkadavu, Thrissur and PSrL-10 from Sreemoolanagaram,
Ernakulam were highly virulent. PKoL-2, the isolate from Kodissery was less
virulent and other 14 were moderately virulent.
Cultural and morphological characters of the isolates of pathogen were
studied with different media viz. carrot agar, potato dextrose agar, oat meal agar,
coconut water agar and V8 juice agar. Variation in cultural characters among the
isolates was observed only in carrot agar and the variation in growth rate was
noticed among the different media. Morphologically, mycelia of Phytophthora
isolates from nutmeg were branched, coenocytic and hyaline and the sporangia
were borne terminally /laterally on the sporangiophore in sympodial fashion,
caducous, semi papillate, ovoid/elongated-ovoid/ellipsoid in shape with average
size of 31.9-49.5 x 22.3 – 27.9 μm, L/B ratio of 1.4 – 1.8 and pedicel length of 10.21
– 20.24 μm. Early sporangial production was noticed in carrot agar and potato
dextrose agar and the maximum count was in oat meal agar and carrot agar.
Numerous chlamydospores were observed in all media.
Comparison on the cultural and morphological characters of Phytophthora
isolates of nutmeg with other Phytophthora spp. such as P. meadii, P. palmivora,
P. capsici, P. colocasiae, P. citrophthora and P. ramorum revealed that,
Phytophthora isolates from nutmeg could not be completely fitted into the
phenotypic description of any of these known Phytophthora species. However, they
showed some similarity to P. meadii, P. colocasiae and P. ramorum.
In molecular characterisation, out of 18 isolates of nutmeg Phytophthora,
15 showed maximum homology with P. colocasiae and three viz. PPoL-3,
PMaL-4 and PVaL-15 with P. meadii. Isolates PPaL-1, PKoL-2, PVeL-5, PKtL-6,
PKaL-7, PMtL-8, PThL-9, PTuL-11, PKnL -12, PPaF-17 and PMoF-18 also
showed homology with P. citrophthora and PMaL-4 and PSrL-10 with P. botryosa.
Host range of Phytophthora isolate of nutmeg includes, rubber, vanilla,
rose, Coreopsis, Eucalyptus and Citrus. Nutmeg is also a host of P. meadii of vanilla
and P. citrophthora of Citrus and non host of P. palmivora, P. capsici,
P. colocasiae, P. meadii of arecanut, rubber and cardamom. Rose is also found to
be a host of P. meadii isolates of arecanut, rubber, cardamom, vanilla and
P.citrophthora of Citrus.
The cultural, morphological and molecular characters, host range and cross
infectivity studies of various Phytophthora isolates could not revealed the exact
identity of these isolates, as the distinguishing features overlapped among the
various Phytophthora species.
In vitro evaluation of chemicals / bioagents showed complete inhibition of
the pathogen with 1% Bordeaux mixture, copper hydroxide (2g/l), copper
oxychloride (2.5g/l), potassium phosphonate (3ml/l), combination fungicides,
iprovalicarb + propineb (1.5 and 2.0g/l), cymoxanil + mancozeb (2g/l) and
Trichoderma viride -1, T. harzianum and T. viride -2, the isolates from nutmeg and
the reference cultures viz. T. viride (KAU) and T. harzianum (IISR).
In in vivo experiment, all treatments were superior to control of which,
spraying of 1% Bordeaux mixture + soil drenching of copper hydroxide (2g/l) and
spraying of 1% Bordeaux mixture and soil application of T. viride showed
maximum reduction of the disease. In addition, spraying and drenching of copper
hydroxide and copper oxychloride were also found equally effective.