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

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 na turally regenerating pastures. The severity of root disease across the thre e sites, rated on a 0-4 scale, was 2.4 (1991), 2.7 (1992) and 1.7 (1994). M edicago emergence and/or herbage production increased in the presence of po tassium 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 pr esence of both fungicides. Metalaxyl specifically reduced root infection of Medicago by Pythium spp. whilst potassium phosphonate showed a broader spe ctrum activity, reducing infection by Fusarium spp., Phoma spp., Pythium sp p. and Rhizoctonia solani. Fungal isolates showing pathogenicity towards Me dicago littoralis cv. Harbinger in growth cabinet experiments included Fusa rium acuminatum, F. avenaceum, F. culmorum, F. crookwellense, F. equiseti, F. graminearum, Phoma medicaginis, Pythium irregulare and Rhizoctonia solan i.