Potential Weed Hosts forDiaporthe phaseolorumvar.caulivora,Causal Agent for Soybean Stem Canker

Abstract

Greenhouse and outdoor studies evaluated weed species common to Louisiana soybean (Glycine max) fields as potential hosts for Diaporrhe phaseolorum var. caulivora. Species tested were: barnyardgrass (Echinochloa crus-galli), black nightshade (Solanum nigrum), curly dock (Rumex crispus), entireleaf morning-glory (Ipomoea hederacea var. integriuscula), hairy indigo (Indigofera hirsuta), hemp sesbania (Sesbania exaltata), ivy-leaf morning-glory (Ipomoea hederacea), johnsongrass (Sorghum halepense), northern joint-vetch (Aeschynonsene virginica), pitted morning-glory (Ipomoea lacunosa), prickly sida (Sida spinosa), redweed (Melochia corchorifolia), sicklepod (Cassia obtusifolia), smallflower morning-glory (Jacquemontia tamnifolia), spiny amaranth (Amaranthus spinosus), tall morning-glory (Ipomoea purpurea), and wild poinsettia (Euphorbia heterophylla). Soybean cultivars susceptible to stem canker were included as controls. Plants were inoculated twice in each experiment using ascospores in water. Host status of each species was confirmed by recovering the pathogen from weeds using selective medium or by observing production of perithecia. All weed species tested except johnsongrass, barnyardgrass, and curly dock were hosts for D. p. var. caulivora. Lesions 1 to 2 cm long and ≤0.5 cm wide were common on soybean, hemp sesbania, and hairy indigo. These consistently yielded mycelium of D. p. var. caulivora. All other weed hosts were asymptomatic.