THE EFFECT OF SALICYLIC ACID AND Bacillus subtilis ON CUCUMBER ROOT AND STEM ROT, CAUSED BY Fusarium oxysporum F.SP. radicis cucumerinum

In this study, the effects of exogenous application of salicylic acid (SA) and Bacillus subtilis on cucumber plants against, Fusarium oxysporum f.sp. radicis cucumerinum (Forc) were investigated. In vitro, the effect of different levels of SA (2-8 mM) on growth of B. subtilis was negative, while concentrations more than 5 mM SA completely inhibited the mycelial development of Forc. Applications of SA (3,5 and 7 mM) and B. subtilis post or / and prior inoculation with the pathogen with regard to evaluation of disease severity, root and shoot fresh weight and length of shoot were conducted under greenhouse conditions. The results indicated that cucumber plants treated by combination of 7 mM SA (foliar spray) and B. subtilis (soil drench), prior to fungal infection, exhibited reduction of fungal infection and increased plant growth as compared to control and other treatments. Evaluation of total phenol content and polyphenol oxidase activities showed that the combined application of SA and B. subtilis significantly increased the above plant defense compounds compared to SA and B. subtilis alone and control. The peak levels of them were observed in 7 and 5 days after elicitors application, respectively. According to these results, SA as a chemical elicitor and B. subtilis as biocontrol agent and plant growth promoter can be integrated for effective protection of cucumber plants against Forc infection.