Gnotobiotic evaluation of Dalbergia sissoo genotypes for resistance against Fusarium solani via dual culture set up

BACKGROUND: Dalbergia sissoo (shisham), an important multipurpose tree native to the Indian subcontinent and also planted in other countries, has been afflicted with large scale mortality in all age groups due to wilt disease, causing huge economic losses. Fusarium solani f. sp. dalbergiae (Fsd) has been identified as one of the causal organisms for wilt disease in D. sissoo. One of the approaches of disease resistance studies involves co-cultivation of trees and pathogens under controlled conditions to screen resistant tree genotypes. A gnotobiotic condition, where the pathogen is known, enables accurate screening of disease-resistant genotypes. In the present study, ten genotypes of D. sissoo were cloned in vitro and evaluated against two strains of Fsd in a dual culture setup under gnotobiotic conditions with an objective to identify resistant genotypes of D. sissoo against Fsd. RESULTS: Callus and plantlets of ten genotypes of host plant multiplied in vitro were inoculated with conidial suspension of two strains of Fsd at three concentrations; 1 × 10(1), 1 × 10(3), and 1 × 10(5) conidia/ml. Gnotobiotic evaluation of dual culture setup shows variations among genotypes in their response towards in vitro Fsd infection; and two genotypes (14 and 66) exhibited resistance against Fsd strains. Callus of genotypes 14 and 66 significantly restricted the fungal mycelium growth whereas callus of remaining genotypes was completely infested by Fsd mycelium within 9 days. Similarly, plantlets of genotype 14 and 66 had lesser disease severity and remained green and had fewer necrotic lesions in roots whereas plantlets of the remaining eight genotypes died within 15 days. CONCLUSION: Gnotobiotic evaluation of callus and plantlets of ten genotypes of D. sissoo against Fsd strains has reduced time and space otherwise required for field trials. Genetic variations amongst the genotypes resulted in varying responses towards virulent Fsd strains and only two out of ten genotypes showed promising resistant characteristics. In dual culture setup, both callus and plantlets of the same genotypes responded similarly against Fsd strains, which signify that in vitro screening can be used as an indirect selection method for disease resistance.