The Epidemiology of the Soybean/Diaporthe Phaseolorum Var. Caulivora Pathosystem in Louisiana.

Field and greenhouse experiments were conducted during 1988-1991 to investigate the epidemiology of the soybean/Diaporthe phaseolorum var. caulivora (Dpc) pathosystem. The following points were addressed: (i) the effect of inoculation timing on soybean yield and disease severity, (ii) the efficacy of Dpc inoculum during the growing season, (iii) the possibility of alternative hosts for Dpc, and (iv) the relationship between threecornered alfalfa hopper injury and stem canker severity. Inoculation timing had a significant effect on the yield of Dpc susceptible or moderately susceptible soybean cultivars. Soybean yield was reduced most (83 to 93%) and disease severity was greatest (84 to 99%) when cultivars were inoculated at the Vc or V3 growth stage. Yield reduction was not as severe when cultivars were inoculated at the late vegetative or early reproductive growth stages. Significant positive correlations were recorded between rainfall and stem canker severity and relative humidity and stem canker severity. The ability of Dpc to infect soybean during the growing season was investigated. Stem canker incidence was greatest 4-7 weeks after planting. While infection of soybean by Dpc decreased 7 weeks after planting, Dpc inoculum was still infecting soybean 11 weeks after planting. Results from Dpc host range experiments indicated weeds commonly found in south Louisiana soybean fields can serve as alternative hosts for Dpc. Dpc colonized and reproduced in several morning glory species, several leguminous weeds, and wild poinsettia. Threecornered alfalfa hopper injury resulted in increased stem canker severity compared to soybean not injured by this insect. Stem diameter and length and seed and pod yields were reduced compared to noninjured plants infected with Dpc