Plant growth stage and <i>Phoma medicaginis</i> inoculum concentration together determine severity of Phoma black stem and leaf spot and consequent phytoestrogen production in annual <i>Medicago</i> spp

Phoma black stem and leaf spot disease (Phoma medicaginis) not only destroys annual Medicago spp. forage seed yield but also reduces herbage quality by consequent phytoestrogen production that ovulation of grazing animals. Two controlled environment studies evaluated the effects plant developmental stage in rugosa ‘Paraponto’ M. scutellata ‘Sava’ different inoculum concentrations P. medicaginis littoralis ‘Harbinger’ polymorpha ‘Serena’ on development coumestrol production. Disease incidence severity were dependent stage, cultivar level (all p ≤ 0.001). was least 4-week-old plants; highest inoculated 10-week-old Sava (1353 mg/kg) uninoculated Paraponto (87 mg/kg); there a positive correlation incidence/severity factors with across cultivars growth stages (p < levels determined concentration (both Highest Serena 107 conidia/mL (265 lowest Harbinger (6 significant These emphasize both opportunity for farmers to better use stands reproducing animals early growing season when are lowest, need heightened farmer vigilance at later greater risk