Field evaluation of one isolate of the fungus Beauveria bassiana selected by resistance to the ultraviolet light

Abstract

The solar radiation, specially the UV type, is one of the most limiting factors for the use of the entomopathogenic fungi in the field in an integrated management program of the coffe berry borer (cbb), Hypothenemus hampei. In the following study, a laboratory selection of isolates of the fungus Beauveria bassiana resistant to the UV light was made and the unit of the variable forming colonies per ml (UFC/ ml), diametral colony growth, enzyme activity and pathogenicity to the cbb were evaluated. The selected isolate was Bb 9218 resistant tú 10, 30 and 60 minutes of exposure to UV light, a control in which the same non-irradiated isolate was included, a commercial formu­lation with UV light filters and absolute control. The fieldwork was carried out in the Experimental Station "La Catalina", Depart­ment of Risaralda, in a zone of Colombian coffee variety plants with an age of 30 months, a planting distance of 2x1 m; and plantain shadow. The evaluated variables were the mortality percentage of cbb, the viability of the fungus in the field (UFC/ml), and the reduction ratio of the viability of the fungus at 30 days of evaluation. with respect to the initial time (1 hour). The exogen variables temperature, relative humidity, and global solar radiation were registered throughout the time of the study. The experimental unit comprised plots of sixteen trees in which the central tree was randomly taken as an effective plot; four treatments, each one with nine replicates were evaluated. The effect of the fungus was evaluated by means of an aleatorized analysis model and the 5% Tukey test was used tú com­pare treatments means. The statistical analysis showed the effect of the treatments which favored the Bb 9218 RLUV 60 min treatment with the highest percentage of mortality throughout the evaluation time (2,6, 14.3 and 19,8%, respectively) and showed significant ve differences with the other treatment, except at 30 days of evaluation, when this treatment was statistically equal to the non-irradiated one. The commercial formulación presented the Lowest mortality percentage throughout the evaluation times, except, the at 15 clays, which can be explained by the presence of the fungus as a product of previous sprayings in the ex­perimental plot. The effect of the fungus Bb on the cbb population was relevant at 30 days. when the mortality percentages were from 9 to 20.6%. The statistical analysis of the viability variable showed the effect of the treatments. the best treatment among the evaluation time was Bb 9218 RLUV 60 minutes. As regarding the viability reduction ratio from the initial time to the last time of evaluation, all the treatments presented a net reduction ratio upper than 90% and they were statistically equal; however, Bb 9218 RLUV 60 min showed the lowest reduction of the UFC/ml in the final time of evaluation. These results showed the cbb con­trol by the effect of one field spray with an isolate of this fungus, previously selected in the laboratory by resistance to the UV light, which showed the highest viability percentages through the time. This study confirms the importance of spraying improved strains with higher survival in the field and higher control against broca populations within integrated pest management programs.