Effect of different mixtures of some bioagents and Rhizobium phaseoli on bean damping-off under field condition

Bean is considered a very common vegetable crop in many countries worldwide. Beans are used as green pods or as dry beans. Egypt cultivates beans either for local consumption or for exportation especially during winter and spring seasons. Root rot and damping-off during early stages of bean growth, cause great losses for growers. Growers usually use highly toxic chemical fungicides to protect their investment and get rid of damping-off problem. Using these toxic chemicals led to rejection of exported shipment. The present work offers biological control as a substitute for the toxic chemicals. In this work three different biocontrol mixtures were used. These mixtures consist of Trichoderma spp. isolates (T1+T2) or mixture of Bacillus subtilis isolates (B1+B2) or combination between T. harzianum isolate plus B. subtilis isolate (B2+T2) to illustrate their effect on plant protection against damping-off and also their side effect on beneficial bacterium Rhizobium phaseoli. Data obtained showed that the mixture of the different bioagents either formulated as powder or suspension were varied in their effect on controlling damping-off disease in beans under field condition. A mixture consists of B.subtilis+ T.harzianum (B2+T2) either in form of powder or suspension showed the highest effect in disease control compared with control treatment and the other two mixtures (T1+T2) and (B1+B2). Using of the mixture (B2+T2) as powder or suspension led to increase in fresh and dry weight of shoot and root system of treated plants compared with all other treatments. Using of the same mixture (B2+T2) also led to an increase in the number of pods/plant, yield and percentage of dry weight/ 100(g) pods. Regarding effect of biological treatment on chemical components of the treated plants, it was clear that using the mixture of (B2+T2) led to clear increase in free phenols, total phenols, reduced sugars, amount of chlorophyll in addition to percentage of protein compared with control treatment and the other used mixtures(T1+T2) or (B1+B2). When Rh. phaseoli was considered in plant protection process, obtained data indicated that Rh.phaseoli alone has very slight effect against incidence of damping-off disease. This slight effect increased in presence of any used biological mixtures. The highest synergistic effect was recorded when Rh. phaseoli was in combination with the mixture (B2+T2). Obtained results also revealed that the presence of Rh. phaseoli with biocontrol mixtures led to changes in some chemical components in the treated plants compared with the control and the other mixtures and Rh.phaseoli alone. The highest increase in these components (free phenoltotal phenolreduced sugarschlorophyllprotein) was noticed when Rh.phaseoli was used in presence of (B2+T2) mixture. Effects of adding Rh.phaseoli with biocontrol mixtures on yield were also studied under field conditions. Data obtained indicated that all mixtures of biocontrol agents in combination with Rh.phaseoli led to increase number of rhizobium nodules, fresh and dry weight of roots and shoots, in addition to increase in number of pods/plant, yield/plant and weight of 100 pods. Key words— Bacillus subtilis, bean, Biocontrol agents, damping-off, Rhizobium phaseoli, Trichoderma harzianum. ————————————————————