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Fifteen isolates of binucleate Rhizoctonia fungi (BNR) were studied as potential biocontrol agents for protection of potato from Rhizoctonia canker in artificially infested greenhouse soil and potato fields naturally infested with Rhizoctonia solani (AG‐3). Eight of the BNR reduced incidence and severity of Rhizoctonia stem canker in greenhouse experiments by an average of 78 and 85%, respectively. In a field naturally infested with R. solani, selected isolates of BNR and the fungicide Tops 2.5D (thiophanate‐methyl) were equally protective of potato from Rhizoctonia stem canker. BNR isolates gave protection of potato from Rhizoctonia stolon canker similar to PCNB and superior to Tops 2.5D. Cultivars Atlantic, Irish Cobbler, Kennebec, Norchip, Russet Burbank, and Superior were protected equally from Rhizoctonia stem canker by selected isolates of BNR under field conditions. Isolates of BNR show potential as biocontrol agents for protection of potato from Rhizoctonia canker.

Root systems of tobacco dipped in suspensions containing 2 × 109 colony forming units (CFU)/ml of avirulent bacteriocin‐producing strains (ABPS) of Pseudomonas solanacearum and assayed immediately after planting in steam‐sterilized soil had 8 × 106 CFU/root system of ABPS. The bacterial population declined to an average of 5·3 × 105 CFU/root system after 30 days. Roots of seedlings dipped in bacterial suspensions of ABPS were more effectively protected against wilt caused by P. solanacearum than those dipped in suspensions of an avirulent nonbacteriocin‐producing strain (ANBPS). Lipopolysaccharide (LPS) isolated from one ABPS (121) inhibited the attachment of bacteria on roots by 70% but had no effect on the reduction of wilt, whereas bacterial cells significantly reduced the disease severity as compared to LPS or water treatment. In steam‐sterilized soil containing a 1:1 mixture (5 × 105 CFU/g of oven‐dried soil) of ABPS 121 or 237 and the virulent strain K‐60, ABPS 121 reduced multiplication of the virulent strain in soil and in the rhizosphere of seedlings. When roots of seedlings were dipped in a suspension of 2 × 109 CFU/ml of ABPS before planting, root colonization by the virulent strain added to steam‐sterilized soil at 2 × 106 CFU/g of oven‐dried soil was significantly reduced. When roots were dipped in a suspension of ABPS and assayed 20 days after planting, 98% of the bacterial population was found in the original zone of inoculation and only 2% was detected in new growths of the root system. Plants which were grown in soil infested with ABPS 121 or K‐60 had both strains present at variable populations along all sections of roots.

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