Effect ofAlternaria cassiae, Pseudocercospora nigricans, and Soybean (Glycine max) Planting Density on the Biological Control of Sicklepod (Senna obtusifolia)

Abstract

The interactions of two fungal biocontrol agents,Alternaria cassiaeandPseudocercospora nigricans, and soybean planting density on sicklepod mortality and dry weight were studied in the field over 2 yr. The experimental field was divided into three equal areas: one without soybean and two where the soybean was sown in densities of 20 and 36 seeds per meter row with a 0.95-m row spacing. The fungi were sprayed alone or in a mixture at three growth stages of sicklepod plants grown at three levels of crop interference resulting from the three soybean planting densities. The fungal treatments were: an untreated control,A. cassiae(105spores/m2),P. nigricans(3.3 g mycelium/m2), and the mixture of these two fungi. Sicklepod was at the cotyledonary leaf, two-leaf, and four-leaf stages when treated.Alternaria cassiaewas most effective in reducing both sicklepod survival and dry weight. The mixture ofP. nigricansandA. cassiaewas generally comparable to but not better thanA. cassiaealone in killing the weed (mortality) and reducing its growth (dry weight). Soybean density did not have significant effects on the mortality or the dry weight of sicklepod. Thus, there is no advantage to combining the highly effective biocontrol agentA. cassiaewith the less effectiveP. nigricansor with soybean interference to control sicklepod. However, the results validate the efficacy ofA. cassiaeby itself as a bioherbicide.