PhD

Realizing the need to identify a resistant source in okra (Abelmoschus esculentus (L.) Moench) against the shoot and fruit borer (Earias vittella Fab.) 144 geographically diverse germplasm lines spreading over 10 Abelmoschus taxa were assembled through exploration and from ICAR and SAUs. A filed study was undertaken during 2002-03 at the College of Horticulture, KAU, Vellanikkara utilizing the 144 okra genotypes. The germplasm exhibited high variability for qualitative traits like fruit shape, leaf shape, fruit quality, petal colour, epicalyx shape, pigmentation on stem, petiole, leaf and petal, fruit colour, fruit position on main stem, pubescence on stem and fruits, epicalyx number and fruit ridges. Medium to high variability was observed for quantitative traits like number of internodes, single fruit weight, fruit length, marketable fruit yield, plant height, number of branches, number of leaves, internodal length, number of fruits, fruit yield, shoot and fruit borer infestation and incidence of YVMV.

High genetic divergence was observed among the genotypes. Characters like plant height, days to first flowering, fruit length and internodal length have contributed maximum towards genetic divergence. The genotypes were grouped into 13 clusters. To combine high yield with shoot and fruit borer resistance, hybridization was suggested between genotypes in cluster IV (A. caillei) and cluster XII (A. esculentus). Hybridization between cluster V (landraces) and IX (unimproved germplasm) was also suggested to realize heterosis for fruit yield.

Genotypic correlation co-efficient revealed that genotype showing early flowering, enhanced plant height, high internode number, high leaf number per plant and short fruit length were less prone to shoot borer infestation. If shoot borer infestation is less, fruit borer infestation would be minimized, since they were correlated positively. Fruit infestation showed negative correlation with marketable fruit yield, hence by selecting genotype having higher marketable fruit yield, both shoot borer infestation and fruit borer infestation could be minimized.
Two accessions viz., AP 5 (IC 33087) and EC 16 (EC 169500) flowered early. The landrace Maravendai ranked first for plant height and fruit girth while Aarumarasavendai displayed prolonged flowering period and high internode number. Accession KL 21 (IC 282257) and Susthira had short internodal length. Long fruits were observed in Anakomban, KL 26 (IC 140907) and NER 7. Two accessions viz., OR 2 (IC 99746) and WB 1 (IC 52305 B) ranked top for fruit quality.

The high yielding genotypes of A. esculentus were KL 28 (IC 140934 and Arka Anamika and of A. caillei was EC 305760 (228.62 g per plant). Arka Anamika, KL 9 (IC 45818) and KL 28 (IC 140934) and West African okra lines AC 5 (EC 305760) and Thamaravenda and wild species A. angulosus, A. moschatus var. multiformis and A. tetraphyllus var. pungens showed field resistance to YVMV.

The genotype responded differently to shoot infestation and fruit infestation although the pest that inflicts shoot and fruit damage was one and the same. Abelmoschus tetraphyllus was highly resistant shoot borer and A. ficulneus was moderately resistant to shoot borer. Abelmoschus tuberculatus and A. tetraphyllus var. pungens were highly resistant to fruit borer and moderately resistant to shoot borer. Among A. caillei genotypes, AC 5 (EC 305760) was moderately resistant to shoot and fruit borer while Susthira was moderately resistant to shoot borer alone. A. esculentus genotype KL 9 (IC 45818) of was moderately resistant to shoot borer and EC 2 (EC 329365) was moderately resistant to fruit borer. Hypersensitive response to fruit borer infestation was noticed in AC 5 (EC 305760), Susthira and A. tuberculatus.

The inter-specific hybrids of A. tetraphyllus x A. esculentus accession KL 28 (IC 140934) and A. tuberculatus x A. esculentus cv. Arka Anamika were highly resistant to fruit borer. In both cases, resistance was found dominant over susceptibility and controlled by major genes.

A 6 x 6 full diallel analysis involving A. esculentus and A. caillei genotypes revealed preponderance of additive gene action for shoot borer resistance, days to first flowering and flowering period. Non-additive gene action were observed for plant height, internode length, fruits per plant, fruit weight, fruit length and fruit yield. Both additive and non-additive gene action were important for fruit borer resistance. The cross Arka Anamika x KL 9 (IC 45818) and KL 9 (IC 45818) x AC 5 (EC 305760) produced high marketable fruit yield.

Generation mean analysis in the cross Arka Anamika x KL 9 (IC 45818) revealed that shoot and fruit borer resistance involved more complex higher order gene interactions. In the cross KL 9 (IC 45818) x AC 5 (EC 305760), duplicate epistasis govern the inheritance of fruit borer resistance.

Multiple choice test revealed that shoots of A. tetraphyllus and fruits of A. tuberculatus and A. tetraphyllus var. pungens exhibited antixenosis for oviposition to Earias vittella. Larval growth index and total development growth index was low when E. vittella reared on A. tetraphyllus shoots. Earias vittella prefer shoots having long hairs for oviposition. Length and density of trichomes on the fruits had insignificant correlation with oviposition preference as well as with infestation in fruits. But the rigidity and sharpness of trichome offer resistance to fruit borer. Presence of high phenol content, tannin content and gossypol content in the shoots of A. tetraphyllus also impart resistance to E. vittella.

Anatomical and histochemical studies revealed that resistant shoots of A. tetraphyllus were characterized by cutinized epidermal layer, larger epidermal cells, thick hypodermal and collenchyma layers compactly arranged ground tissues and vascular bundles. Similarly, the factors imparting resistance to fruit borer in A. tuberculatus were rigid and sharp trichomes, compactly arranged hypodermal and mesocarp cells with wider and larger vascular bundles.