EFFICACY OF RESISTANCE INDUCERS, FREE-RADICAL SCAVENGERS AND AN ANTAGONISTIC STRAIN OF PSEUDOMONAS-FLUORESCENS FOR CONTROL OF RHIZOCTONIA-SOLANI AG-4 IN BEAN AND CUCUMBER

Sixteen chemicals from different groups of known resistance inducers w ere tested as a soil drench in a humosoil:sand mix infested with an is olate of Rhizoctonia solani AG-4. Among these, 5-nitrosalicylic acid ( 2 mM aqueous solution), o-acetylsalicylic acid (2 mM), 2,6-dichloroiso nicotinic acid (0.25 mM), 2-aminoisobutyric acid (2 mM) and lichenin ( 2 mM) controlled pre-emergence damping-off and post-emergence seedling mortality of bean cv. Dufrix, but gave little or weak and variable di sease control in cucumber cv. Delikatess. Two-component mixtures of th ese five chemicals controlled the disease effectively in bean, mixture s containing 2-aminoisobutyric acid being the most effective, with see dling stands of up to 94% in infested soil. As none of these five chem icals (less than or equal to 2 mM) reduced mycelial growth of R. solan i on PDA plates, induced resistance in seedling tissues is implied in their disease control. Control was augmented in both hosts when any on e of the five inducers was applied in a mixture with a cell suspension (1 x 10(9) cells mL(-1)) of an antagonistic fluorescent strain of Pse udomonas fluorescens; these mixtures showed additive effects and provi ded much better disease control in bean than in cucumber. The free-rad ical scavengers (antioxidants) ascorbic acid, benzoic acid, gluconic a cid lactone and thiourea inhibited R. solani growth in vitro and effic iently controlled both pre-emergence damping-off and post-emergence se edling mortality of bean; ascorbic acid and benzoic acid also offered significant protection in cucumber. However, disease control values we re reduced substantially when inducers were applied with antioxidants, because of strong antagonistic interactions in most of these mixtures . Probable mechanisms of disease control by 5-nitrosalicylic acid, o-a cetylsalicylic acid, 2,6-dichloroisonicotinic acid, 2-aminoisobutyric acid and lichenin and their interactions with P. fluorescens and antio xidants are discussed.