RELATIONSHIP BETWEEN PHOMA BLACK STEM SEVERITY AND HERBAGE AND SEED YIELD AND COUMESTROL CONTENT IN 3 MEDICAGO-POLYMORPHA VAR BREVISPINA CULTIVARS

A field trial was conducted to determine the relationship between Phoma black stem disease (Phoma medicaginis) severity, herbage and seed yields and coumestrol content in Medicago polymorpha var. brevispina medic cultivars Circle Valley, Santiago and Serena. There was a significant negative correlation between Phoma black stem disease severity with herbage yields in early spring (mid to late September) (r = -0.65, P < 0.01) and mid spring (mid October) (r = -0.81, P < 0.01), seed yields at the end of the growing season in late spring (r = -0.83, P < 0.01) and 100 seed weight (r = -0.83, P < 0.001). In addition, there was significant positive correlation at the end of the growing season between Phoma black stem disease severity and coumestrol content of dry stems (r = 0.72, P < 0.001) and pods (r = 0.77, P < 0.001). Meaned over all cultivars, severe Phoma black stem disease resulted in overall reductions in mid spring herbage yields of up to 17% and in end of season seed yields of up to 36% and increases in coumestrol content of up to 75 and 321% for stems and pods respectively. The highest level of coumestrol measured in stems was 550 ppm for cv. Serena and in pods was 525 ppm in cv. Circle Valley. Of the three culitvars, cv. Santiago was the least susceptible to Phoma black stem disease.