Relative resistance, associated yield losses and phyto-oestrogen production from fungal foliar diseases in new and old annual Medicago cultivars

Abstract

The relative resistance, associated herbage and seed yield losses and phyto-oestrogen production from fungal foliar diseases in two new annual Medicago cultivars, viz. M. sphaerocarpos cv. Orion and M. truncatula cv. Caliph, were compared with those of four old cultivars, viz. M. murex cv. Zodiac, M. polymorpha var. brevispina cw. Santiago and Circle Valley, and M. truncatula cv. Cyprus. Orion was much more resistant to Phoma stem disease than Circle Valley and Santiago, but was more susceptible than Zodiac. However, Orion was the most susceptible cultivar to Phoma leaf disease. Orion and Caliph were susceptible to Leptosphaerulina stem disease, resembling Santiago, but much more susceptible than Circle Valley and Zodiac. Caliph was comparable with Cyprus with moderate susceptibility to both Phoma and Leptosphaerulina on stems. Both Caliph and Cyprus were resistant to Pseudopeziza. In mid-October and early November, foliar diseases reduced overall herbage yield of the six varieties by 16.2 and 20.3% respectively, compared with the fungicide-treated plots of the same cultivars. Foliar diseases reduced seed yield by 37.3%, and mean seed weight by 13- 7% compared with fungicide treated plots. Foliar disease in the disease block increased the content of the phyto-oestrogen coumestrol in stems from 230 to 500 ppm and in pods from 30 to 130 ppm, compared with the fungicide block. There was a positive correlation between the severity of Phoma on stems and leaves and the level of coumestrol in stems and pods. Orion produced large amounts of coumestrol in stems and pods in response to foliar diseases (e.g. 470 ppm in stems) but less than Zodiac. Caliph also produced large amounts of coumestrol in stems and pods in response to foliar diseases (e.g. 230 ppm in stems), and was similar to Cyprus but had less coumestrol than all other cultivars tested (stems) or Zodiac, Circle Valley and Santiago (pods). Orion carried the highest level of P. medicaginis seed contamination (28%), followed by Santiago (15%), Cyprus (11%), Circle Valley (l0%), Zodiac (10%) and Caliph (6%).