Resistance in annual Medicago species to Phoma medicaginis and Leptosphaerulina trifolii under field conditions

Abstract

Advanced breeding lines and potential new cultivars of annual Medicago species were screened for resistance to phoma leaf and stem disease (caused by Phoma medicaginis) and pepper spot (caused by Leptosphaerulina trifolii) in the field, and their performances were compared with appropriate control cultivars. Medicago murex had the best overall disease resistance of the Medicago species tested. There were large differences between varieties in the incidence of both diseases. Based on end-of-season scores, the most resistant to phoma leaf disease included M. murex cv. Zodiac and SAR 3490, M. tornata SA10012, and M. truncatula cv. Parabinga, with scores 52.8 on a 0-10 scale (0, nil disease; 10, >90% leaves affected). On the basis of area under disease progress curve (AUDPC), the most resistant to phoma leaf disease, with AUDPC values of <157, included all M. murex varieties except 87FB2.30, and M. truncatula cv. Caliph. Based on end-of-season scores, the most resistant to phoma stem disease, all with scores c4, included M. murex cv. Zodiac, SAR 3490, and 87F01.48; M. sphaerocarpos cv. Orion (SEP 29.1), SEP 29.2, and SEP 30.3; and M. tornata cv. Tornafield and SA10012. Resistance to phoma stem disease in these lines was confirmed by AUDPC values, especially for M. murex cv. Zodiac, SAR3490, and 87FO1.48, and for M. tornata SA10012, with values <154. Based on end-of-season scores, the most resistant to pepper spot leaf disease included M. polymorpha cvv. Serena and Circle Valley, with scores 52.8. On the basis of AUDPC, all M. murex varieties except SAR3490 and 87FO1.5 could be included among those most resistant to pepper spot leaf disease, with AUDPC values <275. Coumestrol levels in stems ranged from 117 to 1420 mg/kg DW, with lowest levels (5330 mg/kg) in M. murex cv. Zodiac, 87FO 1.48, and GRC 87; M. sphaerocarpos cv. Orion (SEP 29.1), SEP 29.2, and SEP 26.2.1; M. tornata cv. Tornafield; and M. truncatula Z601. There was positive correlation between the severity of phoma disease on stems and end-of-season (September-November) coumestrol concentrations.