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

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