Abstract:
Maize lethal necrosis (MLN) is a virus disease complex that was recently reported in eastern Africa. It causes huge yield losses, hence threatening food security in the region. It is caused by a co-infection of Maize chlorotic mottle virus (MCMV) and Sugarcane mosaic virus (SCMV) in maize and other cereal crops. Thrips and beetles are known vectors of MCMV while SCMV is vectored by aphids. Since MLN is new to Africa, there are no known effective management strategies against the disease. This study examined the potential of 10 selected fungal isolates to colonize maize plants and induce resistance against SCMV and MCMV, and the effects of fungal biopesticides on incidence and severity of MLN and its insect vectors. Sterilized maize seeds were soaked in fungal inocula, sown in pots and evaluated for endophytic colonization. Endophytically-colonized maize plants were challenge-inoculated with SCMV and/or MCMV, to assess their effects on incidence, severity and virus titers over time. To test the performance of biopesticides,Metarhizium anisopliae and Trichoderma asperellum, and an insecticide Nurelle*D 50/500EC against MLN incidence, severity and its insect vectors, seven treatment plots were adopted as follows: 1) untreated plot; 2) soil applications of T. asperellum with foliar sprays of Nurelle*D; 3) soil applications of T. asperellum with foliar spray of M. anisopliae ; 4) soil applications of T. asperellum with foliar spray of M. anisopliae alternated with Nurelle*D; 5) foliar spray of Nurelle*D alone; 6) foliar sprays of M. anisopliae alone; 7)soil application of T. asperellum alone. Isolates of Trichoderma harzianum, Trichoderma atroviride and Hypocrea lixii colonized different sections of maize plant. Plants colonized by T. harzianum and M. anisopliae resulted in up to 1.4- and 2.7-folds reduced SCMV severity and titer levels, respectively over the controls. Single and combined Nurelle*D application with M. anisopliae and T. asperellum reduced population densities of various thrips species up to 5.1 folds, and reduced incidence and severity of MLN up to 1.9 and 1.4 folds, respectively, compared to controls. The present study demonstrates that seed treatment with fungal entomopathogens can induce resistance in maize plants against SCMV infection. Combined use of fungal biopesticides and an insecticide were effective in management of MLN vectors and consequently MLN itself. These approaches can therefore be deployed in integrated management of MLN viruses and vectors. However, further screening for fungal isolates that could be effective against MCMV is needed.