STUDIES ON ALTERNARIA BLOTCH OF APPLE AND ITS MANAGEMENT
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Date
2018-10
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UHF, NAUNI
Abstract
ABSTRACT
Alternaria blotch caused by Alternaria alternata f. sp. mali is one of the economically important disease of apple particularly in
Himachal Pradesh. Therefore, the present investigations were undertaken with the objectives to record the prevalence of this disease; study
the variability of causal agent Alternaria isolates and evolve effective disease management strategy through the use of fungicides, plant
extracts, bioproducts, biocontrol agents and their combinations as well as host resistance during the year 2015-2017. Alternaria blotch of
apple was found to occur in moderate to severe form in different apple growing areas of district Shimla of Himachal Pradesh. The mean
disease incidence ranged from 11.00 to 50.00 per cent with maximum at Summerkot and minimum at Khadrala of district Shimla, while
highest disease severity was recorded at Chaithla and lowest at Khadrala in the same district with an average of 6.20-27.57 per cent.
Pathogenicity test revealed that symptoms appeared after 4 to 7 days of inoculation. Eleven isolates of Alternaria spp. were isolated from
apple cv. Royal Delicious and fiveisolates were isolated from tomato, cabbage, marigold, carnation and pear. These isolates differed in their
ability to produce typical symptoms. Morphological, cultural and molecular studies were made to ascertain the prevalence of variability
among the isolates. Colonies varied in their cultural behaviour ranging from velvetty to cottony, mostly appressed, with regular to irregular
margins. Colour of the colonies varied from light to dark olivaceous. The mean ,of the isolates was between 32.90 to 66.92
mm after 8 days of inoculation with highest growth in Isolate 1 on potato dextrose agar medium and lowest in Isolate 8 on Dimmick’s agar
medium. Morphological studies revealed variations in their size, shape and septations of conidiophore and conidia. Maximum conidial size
was 19.38-38.8× 9.90-20.7 μm and minimum was 9.9-23.8× 5.3-13.6 μm in Isolate 10 and Isolate 9, respectively. Isolates when tested for
their pathogenic behaviour showed variation in incubation period, latent period and disease reaction. A positive reaction was observed on
the susceptible apple cv. Royal Delicious during the leaf necrosis bioassay, demonstrating the host specific nature of the toxin. Molecular
characterization of the seven isolates of Alternaria spp. showed 99-100% homology with Alternaria alternata, Alternaria mali and
Alternaria spp. Phylogenetic tree depicted that all the isolates were closely clustered to Chinese isolates deposited earlier in the NCBI. The
sixteen isolates of the Alternaria spp. were differentiated on the basis of location, host, cultural and morphological characteristics using
RAPD markers. Leaves collected during autumn produced the maximum inoculum in the month of August i.e. 3.7 ×103conidia/ml whereas,
the production of primary inoculum was least (1.8×103 conidia/ml) on the leaves during April. Alternaria alternata f.sp. mali infection on
host physiology revealed that reducing and non reducing content were more in uninoculated plants in all the test cultivars with time lapse.
Total phenolics content, polyphenol oxidase, peroxidase, PAL activity and ROS were more in inoculated plant during all the sampling hours
in all the three test cultivars. Screening of cultivars revealed that only one cultivar Top Red Delicious showed highly susceptible reaction
and Starking Delicious showed susceptible reaction. Six cultivars viz., Manchurian Crab, Tydeman’s Early Worcestor, Hardeman, Stark
Spur Golden, Gold Spur and Scarlet Gala were found to be moderately susceptible at RHR&TS Mashobra while, two cultivars viz., Royal
Delicious and Vance Delicious showed susceptible reaction at Nauni. Out of ten systemic and non-systemic fungicides evaluated under in
vitro conditions, two fungicides viz., difenoconazole and propineb exhibited complete mycelial growth inhibition against the test pathogen.
Amongst eight plant extracts tested under in vitro conditions, leaf extract of neem was found to be most effective with 72.89 per cent
mycelial growth inhibition, while in case of bioproducts tested, Neemazal, cow urine and field formulation gave cent per cent mycelial
growth inhibition. Under pot conditions, difenoconazole + propineb gave good disease control as compared to all other combinations.
Among SAR chemicals tested, BABA @2000 ppm found to be the most effective in all the cultivars, with minimum disease severity
rangingfrom 5.24-7.15per cent and maximum disease control rangingfrom 69.71-75.85per cent. In case ofSAR chemicaltreatments, there
was an increasein reducing sugar and non reducing sugar over the untreated controlin all three cvs. of apple with maximum in salicylic acid
and minmum in calcium carbonate treated leaves, respectively. Total phenol content in apple leaves after treatment with SAR inducers was
found to be maximum in salicyclic acid treated leaves and minimum in calcium carbonate. Polyphenol oxidase, Peroxidase and PAL
activity was found to be highest in salicyclic acid treated leaves in all the three cultivars. Production of Reactive oxygen species was found
to highest in salicyclic acid treated leaves in all the three cvs. Among biocontrol agents, Pseudomonas flourescens @105cfu/ml found to be
best in all the cvs. tested with minimum disease severity ranging from 1.90-13.33 per cent and maximum disease control ranging from
63.79-89.01 per cent. Maximum suppression of the primary inoculum was observed when the leaf litter was dipped in urea (50000 ppm)
followed by difenoconazole (1000 ppm) and hexaconazole (1000 ppm). Under field conditions, the combination of difenoconazole and
propineb was most effective(95.30%)in disease control, whileBacillus subtilis was found least effective (58.66%).
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