MSc

Yellow Vein Mosaic Virus (YVMV) disease is a devastating disease infecting okra (Abelmoschus esculentus (L.) Moench), at all stages of crop growth, causing 50 to 94 per cent crop loss. The best way to tackle this disease is the use of resistant varieties developed by interspecific hybridization. There is no source of resistance to this disease in the species Abelmoschus esculentus. Hence a study was undertaken in the Department of Olericulture, College of Horticulture, Kerala Agricultural University, Vellanikkara during 2014-2015 for the evaluation of F12 selections of the cross between Abelmoschus caillei variety Susthira (YVMV resistant) and Abelmoschus esculentus variety Salkeerthi (high yielding, widely adapted but YVMV susceptible), with the objective of identifying promising lines with high level of resistance to YVMV.
Okra genotypes consisting of eight F12 selections along with their parents and Punjab 8 were evaluated for qualitative and quantitative characters, pollen fertility and reaction to YVMV in RBD with three replications. Variability and correlations were worked out for all the characters studied. Screening for YVMV resistance was done by creating artificial epiphytotic conditions in field, white fly transmission and graft transmission techniques. Four F12 selections (F12-1, F12-2, F12-6 and F12-7) exhibited high level of resistance to YVMV.
Evaluation of quantitative characters in the F12 selections showed significant variation among the genotypes for traits like, plant height, petiole length, days to first flowering, days to first harvest, length of fruit, number of fruits per plant, crop duration, yield per plant and coefficient of infection to YVMV.
The maximum values for both PCV and GCV were noticed for coefficient of infection of YVMV. Most of the traits possessed high heritability especially for the coefficient of infection of YVMV. High genetic advance could be noticed for plant height, yield per plant and coefficient of infection to YVMV. Correlation analysis indicated that fruit yield displayed positive genotypic association with length of fruit, number of fruits per plant and crop duration.
Pollen fertility studies indicated high level of pollen fertility in F12 selections. Mucilage extraction analysis revealed that only low amount of mucilage was present in F12 generation lines compared to the parent A. caillei variety Susthira.
Four F12 selections showed positive characters such as lower number of ridges per pod, longer fruit length, reduced width of epicalyx segment and less mucilage content similar to the parent Salkeerthi. Based on its promising fruit characters tending towards A. esculentus, selections such as F12-1, F12-2, F12-6 and F12-7 were identified. These selections expressed high resistance to YVMV and high yield.
Molecular characterization of promising selections (F12-1, F12-2, F12-6 and F12-7) and their parental varieties was carried out using RAPD and ISSR markers. RAPD produced a total of 71 amplicons of which 58 were polymorphic giving a polymorphism of 81.69 percent. The number of amplicons produced ranged from three to thirteen with an average of 7.1 amplicons per primer and a mean of 5.8 polymorphic bands per primer. The ISSR primers produced a total of 92 amplicons of which 68 were polymorphic giving a polymorphism of 73.91 percent. The number of amplicons were in the range of six to thirteen with an average of 9.2 markers per primer and a mean of 6.8 polymorphic bands per primer. In RAPD assay the highest percentage of polymorphism was given by OPA 02 (92.37%). In ISSR assay, the primer UBC 811 produced highest polymorphism percentage of 91.89 %. The ISSR marker systems produced more amplicons as compared to RAPD system with more number of markers per primer and more polymorphic amplicons per primer. The amplification patter observed in Selection 1 and Selection 2 was peculiar in both the marker systems. Molecular marker analyses could assess the variability among advanced generation selections and their parents evaluated in the present investigations. The study could locate some markers in the resistant genotypes which on further indepth study will aid in marker assisted selection for YVMV resistance. Further, molecular data generated in the present investigations will serve as a base for fingerprinting the elite genotypes for varietal registration.