GENETIC STUDIES OF DOWNY MILDEW RESISTANCE IN INDIAN SLICING CUCUMBER (Cucumis sativus L.)
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Date
2018
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Abstract
Cucumber (Cucumis sativus L.) is one of the most important Cucurbitaceous
vegetable crop grown extensively in tropical and sub-tropical parts of the world. However,
cucumber production is limited by the most devastating disease downy mildew. The
present work was started with the objectives to identify the resistance source for downy
mildew disease in cucumber and to study the inheritance of downy mildew resistance along
with yield traits. Biochemical and molecular analysis were also under taken to develop
DNA marker for resistance. The experiments were conducted at the experimental farm of
Division of Vegetable Crops, ICAR-IIHR, Hessaraghatta, Bengaluru, Karnataka, during
2015-2018. In screening experiment forty-one cucumber genotypes were screened under
natural condition and also under artificial epiphytic condition using detached leaf assay
method. Based on the screening of genotypes under field conditions, sporulation of
pathogen, progress of disease, detached leaf assay and non-linear statistical analysis it was
observed that none of genotypes were immune to downy mildew but genotypes IIHR -438
and African horned cucumber were resistant to downy mildew disease. Ten genotypes
recorded susceptibility PDI and rest of the genotypes recorded highly susceptibility with
PDI of > 60. The six generation populations of two cross combinations were planted in
randomized block design with three replications. Result indicates that resistance in both
crosses was controlled by two pairs of dominant and recessive interacting genes (inhibitory
recessive epistasis). In generation mean analysis the epistatic interaction was greater
importance than main effects in Swarna Agethi x IIHR-438 cross whereas, dominance x
dominance epistasis effect was higher and duplicate epistasis was prevailing in IIHR-431x
IIHR-433 cross. The estimates of gene effects of yield traits through analysis of six
generations revealed that the magnitude and direction of the gene effects varied in crosses.
In Swarna Agethi x IIHR-433 cross showed preponderance of both additive and dominance
effects for number of fruits per plant and epistatic interactions were observed for yield per
plant. However, scaling test was non-significant for fruit length, diameter and weight. In
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IIHR-431 x IIHR-433 cross fruit length, average fruit weight and yield per plant showed
duplicate type of interaction and it hampered the selection at early generation. Epistatic
interactions were observed for node to first flower, number of fruits per plant, fruit
diameter.
Total phenols and enzymatic activity of peroxidase, polyphenol oxidase and super
oxide dismutase were higher in both resistant parents (IIHR-438 and IIHR-433) compared
to susceptible (Swarna Agethi and IIHR-431) parental lines from 0 DPI (days to post
inoculation) to 16 DPI. A total of 143 SSR primers were employed to identify polymorphic
primers using 225 F2 populations of Swarna Agethi x IIHR-438 through BSA, SSR 3-5 and
SSR 4-13 are co-segregated with disease reaction in BSA. The ‘dm’ genes were tagged
with SSR 3-5 and SSR 4-13 markers loci with distance of 16.6 cM in linkage group 3 and
18.1 cM in linkage group 4, respectively. The backcross breeding along with markerassisted selection (MAS) can be employed for early selection of downy mildew resistance
gene in cucumber and its introgression to develop a resistant variety or hybrid.
Keywords : Cucumber, Genetics, Resistance, Downy mildew and Mapping
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t-10004
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