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