Plant and Soil 133: 131-140, 1991. © 1991 Kluwer Academic Publishers. Printed in the Netherlands. PLSO 8844 Bacterial antagonists of Sunflower (Helianthus annuus L.) fungal pathogens P. HEBBAR I, O. BERGE ~, T. HEULIN 1 and S.P. SINGH 2 ICentre de POdologie Biologique, UPR 6831 du CNRS, associ~e h l'Universitd Nancy 1, BP 5, F-54501, Vandoeuvre-les-Nancy Cedex, France and 2Biological Control Centre, National Centre for Integrated Pest Management, ICAR, PB 2491, Bellary road, Bangalore-560024, India Received 15 August 1990. Revised December 1990 Key words: antagonistic bacteria, biological control, Helianthus annuus, Pseudomonas spp,, Sclerotium rolfsii Abstract Bacteria isolated on nutrient agar and King's medium B from sunflower leaves, crown and roots inhibited in vitro growth of the leaf spot and wilt pathogens Alternaria helianthi, and Sclerotium rolfsii, respectively, and also the root rot pathogens Rhizoctonia solani and Macrophomina phaseolina. Antagonistic bacteria from leaves were mainly actinomycetes and pigmented Gram-positive bacteria, while those from roots and crowns were identified as Pseudomonas fluorescens-putida, P. maltophilia, P. cepacia, Flavobacterium odoratum and Bacillus sp. In soil bioassays, when used as seed inoculum in the presence of S. rolfsii, P. cepacia strain N24 increased significantly the percentage of seedling emergence. Bacterial strains which exhibited broad spectrum in vitro antagonistic activity were tested for colonisa- tion of sunflower roots, when used as a seed inoculum. Good colonisers (104 to 106 bacteria/g root) were consistent in their ability to reduce disease and fungal wilt. A seedling having a primary root length <5 cm with fewer lateral roots, necrosed cotyledons or crown and a wilted shoot indicated its diseased status. On an average, only 30% of seedlings were diseased when treated with the antagonistic strains, in the presence of the pathogen, while 60% of the seedlings were diseased in the presence of the pathogen alone. In microplots treated with strain N24, only 1 to 3% of the seedlings were wilted, while 14% of the seedlings were wilted in the presence of the pathogen alone. The results obtained show that bacterial antagonists of sclerotial fungi can be used as seed inocula to improve plant growth through disease suppression Introduction Fungal diseases cause extensive damage to both foliage and roots of sunflower. The economically important diseases of sunflower in India are leaf spots caused by Alternaria helianthi, wilts caused by Sclerotium rolfsii and rarely root rots caused by Rhizoctonia solani and Macrophomina phaseolina. Environmental factors and physical conditions of the plants and pathogen density play a major role in the severity of the disease (Davet, 1987; Nelson et al., 1989). Soils suppressive to disease development have been recognised for many years and either en- vironmental or biological factors were found re- sponsible for the disease suppression. There is a growing body of evidence that in some plant populations plant-microbial associations occur, wherein the microorganisms have the ability to protect the plant from the pathogens. For exam- ple, in wheat monocropping systems certain fluorescent pseudomonads have a role in the spontaneous decline and the severity of the take° all disease caused by Gaeumannomyces graminis (Cook and Baker, 1983; Cook and Weller, 1987). The populations of antagonistic microor-