Management of Bean Rust by some Bioagents and Essential Plant Oils

Antagonistic bacteria and fungi occurring on bean phylloplane were isolated and evaluated for their activity as bioagents against Uromyces appendiculatus (Pers. ex Pers.) Unger., the causativeof bean rust. Isolates of Bacillus spp. and Trichoderma spp. were selected. Four isolates of Bacillus spp. were purified and identified as Bacillus megaterium, B. pumilus, B. subtilis and B. thuringiensis. Also,four fungal isolates of Trichoderma spp. were purified and identified as Trichoderma album, T. hamatum, T. harzianum and T. viride. The inhibitory effect of the isolated bioagents as well as the essential plant oils of lemongrass, neem and thyme was assessed in vitro on the germination of the urediospore of the causal fungus. The inhibitory effect of Bacillus spp. ranged between 37.7-53.9%, Trichoderma spp. between 39.3-58.8% and the essential plant oils between 34.9 - 52.2%. Among Bacillus spp., B. thuringiensis recorded the highest inhibitory effect on urediospore germination of the causal fungus followed by B. subtilis then B. megaterium and P. pumilus. Among Trichoderma spp., T. viride gave the highest inhibition followed by T. harzianum then T. hamatum and T. album. Furthermore, among the essential oils, thyme oil resulted in the highest inhibitory effect followed by lemongrass oil then neem oil. Under greenhouse conditions, spraying bean plants withthe tested bioagents and the essential plant oils, five days before or after inoculation with U. appendiculatus urediospore significantly reduced the severity of the disease with significant increase in the produced green pods yield compared to the control. B. thuringiensis, T.viride and thyme oil were the most efficient treatments in reducing the severity of the infection with the disease and increasing the number of the produced pods and the weight/plant compared with control treatment. Therefore, they were used in another trial in different alternations in comparison with the fungicide Topas for management the disease. In addition, using of these treatments each alone was of low efficiency and produced low green pod yield. Meanwhile, the alternation among any of B. thuringiensis and T. viride with thyme oil showed the highest efficiency in reducing the disease and increasing the produced green pod yield nearby the efficiency of the fungicide Topas. In addition, the three oxidative-reductive enzymes, i.e.phenylalanine ammonia-lyase (PAL),peroxidase (PO) and polyphenoloxidase (PPO) were greatly increased in the leaves of all treated plants compared with control treatment. Moreover, plants sprayed with thyme recorded the highest activity of the three enzymes followed by those sprayed with B. thuringiensis then T. viride and the fungicide Topas.