Endophytic activity of Beauveria bassiana in tomato and its efficacy against red spider mites (Tetranychus evansi)

Abstract

The tomato red spider mite, Tetranychus evansi Baker and Pritchard, is an important pest of tomatoes in Kenya. Acaricides commonly used in excessive amounts by farmers have led to cases of resistance as well as widespread residues in the food chain and the larger environment. There is need to seek alternative control measures such as the use of endophytes. In this study, laboratory and greenhouse experiments were carried out to screen for Beauveria bassiana isolates that could establish as endophytes in tomato varieties and control red spider mites. Five Beauveria bassiana isolates (ICIPE 10, ICIPE 35, ICIPE 273, ICIPE 279 and ICIPE 283) were evaluated for their ability to endophytically colonize three tomato varieties namely, Cal J, Kilele and Anna. B. bassiana isolates were cultured on Sabouraud Dextrose Agar (SDA) for three weeks and harvested in 10 ml sterile distilled water in 20-ml universal bottle containing 0.01% Tween-20 and glass beads. A concentration of 1×109 conidia ml-1 was prepared from the stock concentration and seeds sterilized and soaked for two hours. Control seeds were soaked in sterile distilled water containing 0.01% Tween-20 for two hours and planted in sterile soil in the growth chamber for four weeks and in the screenhouse for six weeks. Out of the five isolates, only ICIPE 35 was able to colonize up to 20% of leaves, stems and root area of the three tomato varieties and persisted for 4 and 6 weeks in the growth chamber and screen-house, respectively. In the growth chamber, ICIPE 35 was able to increase growth parameters i.e. plant height, fresh shoot and root weight by 50% in the tested tomato varieties compared to the control. When endophytically colonized tomato plants were infested with the red spider mite, Tetranychus evansi, the mite density was up to 4 times lower in endophyte treated plants than that of non- inoculated plants. In addition, assessment of plant damage associated with red spider mites attack in B. bassiana enhanced plants revealed less damage compared to the controls. The mechanism of control for the endophytic B. bassiana was analyzed by determining the production of lipases and protease from the three tomato varieties that produced clear zone of inhibition and also formed lipid crystals which lacked in the control plates. . The specific lipases and protease were however not analyzed as part of this study. The results of this study suggests that B. bassiana has the potential to establish as an endophyte in tomato, enhances plant growth and reduces mite infestation. Therefore, use of B. bassiana as an endophyte could complement existing control measures for the management of red spider mites in tomatoes.