Efficacy of Chemical Inducers of Resistance for Controlling Rhizoctonia Damping- Off and Root-Rot Diseases in Sugar Beet Crop

Abstract

Rhizoctonia solani was isolated from sugar beet plants showing damping-off and root-rot symptoms, collected from various fields located in several governorates of Egypt. All the obtained isolates were able to attack sugar beet plants causing damping-off and root-rot diseases. In the pathogenicity test, R. solani isolate No.3 isolated from Malawi was the most virulent. The efficacy of six different chemical inducers of resistance was evaluated in vitro and in vivo for reducing damping-off and root-rot diseases in sugar beet. All the tested resistance inducers significantly reduced damping-off and root-rot severity and increased survived plants under greenhouse conditions. The reduction in damping-off and root-rot severity was increased with increasing chemical inducer concentration, with 400 ppm was the most effective in reducing damping-off and root-rot severity. The most effective treatments were salicylic acid and coumarin at 400 ppm conc., being the most effective in reducing damping-off and root-rot incidence compared to the control treatment. Meanwhile, xanthan was the least effective treatment. Salicylic acid, coumarin and benzoic acid were the most effective treatments on the macerating enzymes produced by the pathogen i.e., total pectinase, pectin lyase and cellulase and reducing R. solani growth and dry weight. In physiological studies, the activity of oxidative enzymes such as peroxidase (PO) and polyphenol oxidase (PPO), as well as phenolic content were higher in plants inoculated with R. solani and treated with chemical inducers than in untreated plants. Salicylic acid had the highest level of total phenols content and activity of the tested oxidative enzymes, followed by coumarin. Whereas benzoic acid showed the least enzyme activity. Under field conditions, during 2020/2021 and 2021/2022 growing seasons, all tested resistance inducers significantly reduced damping-off, root-rot and disease severity in addition to increasing the percentage of survived plants as well as improved sugar beet plant growth parameters (weight of sugar beet, total soluble solid (T.S.S), sucrose content and yield) compared to control. Meanwhile, xanthan recorded the lowest effects in both growing seasons. These findings suggested that these chemicals may play an important role in the control of sugar beet dampingoff and root-rot diseases by inducing systemic resistance in sugar beet plants, in addition to their effect on reducing the pathogen growth and suppressing production of macerating enzymes in vitro and in vivo, consequently affect the compatibilities of the pathogen and its ability to colonize the host.