Techniques are described which make possible the large.scale isolation and screening of potential antagonists to fungal root pathogens. Sixty selected soils from five States have been sampled. Of more than 3500 isolates tested, about 40% inhibited one or more of nine pathogens on agar, and about 4% were effective in soil; a lower percentage might be effective under commercial conditions. Such large. scale methods are essential to success.The ability of pathogens to grow through a soil sample in the laboratory, and to produce damping· off in glasshouse tests, generally agreed with the performance of the fungi in the same soil in the field. Some of the organisms found to be effective in inhibiting a pathogen on agar media also did so in soil, but those ineffective on agar were also ineffective in soil.The effect of antagonists on pathogens is a continuum, ranging from stimulation, through no inhibition, to inhibition of one or several pathogens.The antagonists isolated were mostly BacillU8 subtiliB, B. megaterium, and Streptomyce8 spp., with occasional B. cereU8, B. pumilU8, B. polymyxa, B. badiu8, Pseudomonas putida, and P. jluore8cens.Numbers of surviving Pseudomonas spp. declined rapidly in soil treated above 40°C, as did Streptomyce8 spp. above 60-70°C, but BacillU8 spp. survived 80°C for 10 min.Antagonists added to soil steamed at 100°C for 30 min multiplied there, and could be re·isolated; damping. off of seedlings grown in the soil was prevented by some. They did not increase, and failed to increase the protection against damping.off, in soil treated at 60°C for 30 min with aerated steam; this presumably depended on the balance between the quantity of surviving resident flora and the amount of antagonist inoculum added. Some Bacillus isolates stimulated growth of pepper, snapdragon, and tomato seedlings in tests in nutrient-deficient, but not in fertile, soil. Some isolates of Bacillus, Streptomyce8, and P8eudomona8 inhibited seedling germination or reduced seedling growth. Still others were without noticeable effect. Thus, there is a continuum from stimulation of the host, through no effect, to inhibition of s'eedling growth, produced by the soil microflora.Actinomycetes were more inhibitory to Phytophthora than to other pathogens tested on agar. The percentage of effective antagonists among actinomycetes were higher than among Bacillu8. Pythium ultimum and Sclerotium rolfsii were least inhibited, of the fungi tested, by Bacillus and Streptomyces on agar.There are three possible approaches to biological control of soil-borne plant pathogens: (1) Treat soil at 60°0 for 30 min to eliminate plant pathogens and leave an effective antagonistic microflora to suppress later chance contamination.(2) Add suitable organic amendments to the soil to stimulate development of an antagonistic microflora. (3) Inoculate soil which has been steamed at 100°0 for 30 min with selected antagonists. Microbiological evidence supporting (1) and evidence for the feasibility of (3) are supplied in this paper. Both (1) and (...

Suppression of root rot in avocados caused by Phytophthora cinnamomi was demonstrated in soil from a grove at Tamborine Mt., Queensland. The addition of P. cinnamomi inoculum in amounts sufficient to cause severe root rot of plants in other soils, untreated or steam-air treated at 60°C for 30 min, produced little or no damage in the suppressive soil. Suppressive soil was found to have higher populations of bacteria and actinomycetes than soils conducive to root rot. Few sporangia were formed by P. cinnamomi and P. citrophthora in suppressive soil or soil leachate. The suppression of sporangium formation was found to be microbial and not related to the nutrient level of the soil leachate. Mycelium of P. cinnamomi grew through untreated conducive soils, but developed poorly in untreated suppressive soils. The fungus grew readily through all soils steam-air treated at 49, 60 and 100�C for 30 min. After 6 weeks the isolation frequency of P. cinnamomi had declined in the suppressive soil treated at 49 or 60°C for 30 min. Exchangeable calcium and magnesium, nitrogen, and organic matter were higher in soils suppressive to root rot than in conducive soils. Rain-forest soil, where the pathogen is not damaging, was comparable in this respect to the suppressive soil.

Biological control of Rhizoctonia solani on wheat by seed inoculation with microorganisms was investigated. Initially inoculants were selected from bacteria and actinomycetes isolated from two soils and screened for antagonism to R. solani on agar. Of 148 isolates tested on agar, 96 were antagonistic to R. solani and of these 42 were added to pasteurized soil sown with peppers and inoculated with the pathogen. Seven isolates which controlled disease were screened further as seed inoculants of wheat in pasteurized and field soils inoculated with R. solani. Four inoculants, viz. Streptomyces griseus 2-A24 and three Bacillus subtilis isolates 1-B80, 1-B77 and 1-B68, significantly reduced symptoms caused by R. solani and also increased grain yield and dry matter of wheat. S. griseus 2-A24 and B. subtilis 1-B80 increased grain yield by 30% over controls. The magnitude of these differences suggests that biological control and growth stimulation are involved in yield increases.

Leachates from different soils extracted at similar moisture potentials varied in their ability to induce sporangium formation, zoospore release and sporangium breakdown in Phytophthora spp. Sporangium production by P. cinnamomi did not occur in soil leachates heated at 40–50°C for 10 min or in soils treated with steam-air at 60° for 30 min. No evidence was obtained that Pseudomonas or Chromobacterium spp. were involved in the induction of sporangium formation. Chemotaxis of bacteria to sporangia occurred at zoospore release. Where sporangial breakdown occurred, the chemotaxis persisted for a longer period as the cytoplasm was withdrawn from the sporangial wall to be released eventually, without zoospore formation, through the papilla. The bacteria were oriented at right angles to the sporangial wall. Electron micrographs showed that in the vicinity of a bacterium, the outer, thin, electron-dense layer of the sporangial wall had disappeared and the cytoplasm of the sporangium had withdrawn from the cell wall.