Role of root disease in the poor establishment of Medicago pastures after cereal cropping in South Australia

Abstract

Field experiments were conducted in 1991, 1992 and 1994 at three locations in South Australia to investigate the involvement of root pathogenic fungi in the poor establishment and growth of annual medics (Medicago spp.) in naturally regenerating pastures. The severity of root disease across the three sites, rated on a 0-4 scale, was 2.4 (1991), 2.7 (1992) and 1.7 (1994). Medicago emergence and/or herbage production increased in the presence of potassium phosphonate applied as a soil drench or as a postemergence spray at 1.0 kg to 3.0 kg a.i. ha-1 in every year and with metalaxyl applied as a postemergence spray (0.83 kg a.i. ha-1) in 1992 and as a soil drench (5 kg a.i. ha-1) in 1994. The severity of root disease was reduced in the presence of both fungicities. Metalaxyl specifically reduced root infection of Medicago by Pythium spp. whilst potassium phosphonate showed a broader spectrum activity, reducing infection by Fusarium spp., Phoma spp., Pythium spp. and Rhizoctonia solani. Fungal isolates showing pathogenicity towards Medicago littoralis cv. Harbinger in growth cabinet experiments included Fusarium acuminatum, F. avenaceum, F. culmorum, F. crookwellense, F. equiseti, F. graminearum, Phoma medicaginis, Pythium irregulare and Rhizoctonia solani.