MSc

The present study on ‘‘Bioefficacy of endophytic actinomycetes on plant growth promotion and management of bacterial wilt in tomato” was undertaken in the Department of Plant Pathology, College of Horticulture, Vellanikkara during 2009-11.The main objectives were to isolate endophytic actinomycetes from healthy tomato plants collected from five different locations, to study the antagonistic effect of those endophytic actinomycetes against bacterial wilt pathogen under in vitro conditions and their evaluation against bacterial wilt pathogen under pot culture experiment.

Endophytic actinomycetes were isolated from tomato plants in Vellanikkara, Cherumkuzhy, Elanad (Thrissur district), Ozhalapathy and Eruthenpathy (Palakkad district). Only a single type of isolate was obtained from the different locations. The population was maximum in the roots of sample collected from Cherumkuzhy (EACK) (8x101 cfu/g) followed by Eruthenpathy (EAET) (3x101 cfu/g). The least count of endophytic actinomycetes was obtained from Vellanikkara (EAVK), Elanad (EAEN) and Ozhalapathy (EAOP) (1 x101 cfu/g). The pathogen causing bacterial wilt in tomato was circular, smooth, convex, slimy, fluidal, creamy white colonies with light pink centre indicating its identity as Ralstonia solanacearum.

The efficacy of endophytic actinomycetes against R.solanacearum was evaluated under in vitro conditions. Out of the five isolates, maximum inhibition of the pathogen was observed with the Vellanikkara isolate (EAVK) (29.25%) which was on par with Ozhalapathy isolate (EAOP) (22.59%). The efficacy of culture filtrate of endophytic actinomycetes against R.solanacearum was also evaluated under in vitro conditions. Among the isolates, maximum inhibition of the pathogen was recorded with the Cherumkuzhy isolate (EACK) (44.63%) which was on par with Ozhalapathy isolate (EAOP) (42.59%). Based on the dual culture method and the effect of culture filtrate of antagonists, EAOP was found to be the best isolate under in vitro conditions. The morphological and cultural characters of endophytic actinomycetes were studied based on standard keys and the isolates were tentatively identified as Streptomyces.

Among the isolates, EAOP produced the maximum concentration of IAA (73.1 µg/ml) followed by EACK (41.8 µg/ml) and the least was produced by EAVK (3.2 µg/ml). The mechanism of action of endophytic actinomycetes was studied based on the production of siderophores, HCN, ammonia and non-volatile metabolites by the isolates. The siderophore production was maximum in EAOP isolate followed by EAVK isolate and the least was produced by EACK isolate. None of the isolates were able to produce hydrogen cyanide. Maximum ammonia production was shown by EAVK, EACK and EAET isolates. All the isolates were able to produce non- volatile metabolites.

Evaluation of endophytic actinomycetes against bacterial wilt pathogen was carried out under pot culture conditions. Minimum per cent wilt incidence was shown by plants in pots treated with urea (44 g m-2) and lime (500 g m-2) (T6) (29.63%) and it differed significantly from the other treatments. Among the endophytes, the plants treated with EAOP (T4) isolate showed minimum per cent wilt incidence (37.03%). The minimum per cent wilt index was also recorded with urea and lime (25.9%), followed by EAOP isolate (36.3%). Among the endophytic actinomycetes, EAOP was the most promising isolate for the management of bacterial wilt in disease.

The efficiency of endophytic actinomycetes in plant growth promotion was assessed under pot culture experiment. The plant height was maximum in plants treated with EA OP isolate (77.31 cm). The plants treated with EAEN isolate took the minimum number of days for flowering (34.83 DAP) where as the plants treated with EAVK isolate took the minimum number of days to first harvest (81.66 DAP). Plants treated with EAET isolate produced the maximum number of fruits per plant (17.57) and the plants treated with EAOP isolate produced fruits with maximum weight (20.30 g). Highest yield was recorded for the plants treated with EAET isolate (332.02 g/plant) and the lowest yield was for the control plants. It was observed from the study that EAOP and EAET were the promising isolates in plant growth promotion.

The five isolates were identified by National Center for Fungal Taxonomy (NCFT), New Delhi, based on their morphological and cultural characters as Streptomyces glaucescens (EAVK), Streptomyces griseoruber (EACK), Streptomyces griseous (EAEN), Streptomyces thermodiastaticus (EAOP) and Streptomyces griseolus (EAET). The identity of three efficient isolates (EAOP, EAET and EACK) obtained under pot culture experiment were further confirmed by 16S rRNA sequence analysis using PCR.

The present studies indicated that the Ozhalapathy isolate (EAOP) (Streptomyces thermodiastaticus) was the most efficient among the five isolates of endophytic actinomycetes in plant growth promotion as well as in the management of bacterial wilt in tomato.