Occurrence of diseases and insect pests in select soybean (Glycine max (L.) Merr.) and sorghum (Sorghum bicolor (L.) Moench) rotations in Mississippi
Pichardo, Sergio Tomas
AdvisorPitre, N. Henry
Baird, E. Richard
CommitteeCollison, H. Clarence
Field and greenhouse studies were conducted during 2004 through 2006 at the Rodney R. Foil Plant Science Research Center, Starkville, MS. Six sorghum and soybean rotation treatments were tested to determine their effect on plant pathogen, insect, and nematode diversity and density levels. Treatments included 1) continuous sorghum, 2) continuous soybean, 3) sorghum-soybean-sorghum rotation, 4) soybean-sorghum-soybean rotation, 5) sorghum-soybean-soybean rotation, and 6) soybean-sorghum-sorghum rotation. Several nematode and insect species were identified during the study, but were always below economic thresholds. Six insect species were identified on soybean during each growing season and used as the indicator species for this study. The most prevalent were threecornered alfalfa hopper (Spissistilus festinus (Say) and bean leaf beetle (Cerotoma trifurcata (Forester). Sorghum webworm (Nola sorghiella Riley) and corn earworm (Helicoverpa zea (Boddie) were the most common insects on sorghum panicles. Rotations did not affect the diversity or density levels of the indicator soybean or sorghum insects during the three year study. Plant disease levels during the investigation showed variable results. Three foliar fungal pathogens including Diaporthe phaseolorum (Cooke & Ellis) Sacc. var. meridionalis, Septoria glycines Hemmi, and Cercospora sojina Hara on soybean, and Gloeocercospora sorghi D. Brain & Edgerton ex Deighton on sorghum were observed. The only virus disease on soybean was bean pod mottle, but levels were not affected by the rotations during the study. Zonate spot caused by G. sorghi was the most prevalent foliar sorghum disease, but was not affected by the rotations. Six frequently isolated fungal pathogens from either soybean or sorghum roots included Macrophomina phaseolina (Tassi) G. Goidanich, Rhizoctonia solani Kühn, D. phaseolorum, Aspergillus spp., Trichoderma spp and Fusarium spp. Aflatoxin contamination of sorghum seed was low (<20 ppb) the first two years of the study, but was high (790 ppb) in 2006. Significantly greater soybean and sorghum yields were obtained from rotated systems compared to monoculture systems in 2005. In a greenhouse test, M. phaseolina infection of soybean led to significantly greater root disease ratings, lower plant height and dry weight than the untreated control. Sorghum plant growth was not affected by M. phaseolina and R. solani.