The association of necrotrophic fungal pathogens and plant parasitic nematodes with the loss of productivity of annual medic-based pastures in Australia and options for their management

Abstract

Annual medics (annual Medicago spp.) are often an integral component of dry-land farming and cropping systems worldwide, including significant areas of the winter rainfall regions of southern Australia. Necrotrophic fungal pathogens frequently dominate such areas because of the ease of survival of these trash-and soilborne pathogens on infested residues over the relatively dry summer period. Necrotrophic fungal pathogens, in general, are favoured by the nutrient impoverished soils across many parts of these regions, which provide little microbial buffering against these pathogens. This review first, outlines the major and most widespread diseases caused by necrotrophic fungal pathogens and plant parasitic nematodes; second, defines the association of these pathogens with the loss of productivity of annual medic-based pastures; and finally, investigates the spectrum of control options for their management. Important necrotrophic foliar fungal pathogens include Phoma medicaginis, Colletotrichum trifolii, Leptosphaerulina trifolii, Pseudopeziza medicaginis, Stemphylium botryosum, S. vesicarium and Stagonospora meliloti. Necrotrophic foliar fungal pathogens, for example Phoma medicaginis, are also known to stimulate production of phyto-oestrogenic compounds to high levels that can adversely affect ovulation rates in sheep. There are also necrotrophic fungal root pathogens in Australia that have been associated with significant decline in productivity and pose such a serious threat to annual medic pastures such that reseeding is required. In particular, Rhizoctonia solani and various Fusarium species such as F. avenaceum, F. acuminatum, F. culmorum, F. graminearum and F. lateritium, and various Pythium species, in particular P. irregulare, P. ultimum and P. spinosum, are of concern. Another important soilborne and root-attacking necrotrophic pathogen is Phytophthora clandestina. The association of Fusarium spp. with annual medic roots, crowns and burrs in Australia is cause for additional concern as some of them are responsible for the production of deleterious mycotoxins. Plant parasitic nematodes are important as a potential yield limiting factor in annual medics. Approaches to disease management include strategies that have been utilised to varying degrees for the control of necrotrophic foliar and root pathogens in annual medic pastures. This paper reviews the current literature on the topic and provides an assessment of options available for their management. In particular, host resistance offers the most cost-effective, longterm control, particularly as some useful resistance to several of these pathogens has been identified. One or more cultural control strategies, including grazing, fertiliser application, rotations and seed health, offer further opportunities for restricting losses in annual medics from diseases caused by necrotrophic fungi and/or plant parasitic nematodes, especially if applied as an integrated management strategy with only minor reliance on fungicides. As many of these foliar and soilborne fungal pathogens and parasitic nematodes can also threaten crops grown in rotation, these pathogens pose a wider threat to the farming systems employed across southern Australia and reduce the potential for providing a ‘disease break’. In addition, the role played by these pathogens is likely far wider and of greater adverse impact than previously considered as a consequence of their potential for causing mycotoxin and/or phyto-oestrogen production. The full array of losses, not just herbage and seed yields, needs to be considered for the more important fungal and nematode pathogens. Genetic resistance to individual pathogens provides the best option for management. The success and outcome with sourcing resistance in other annual pasture legumes such as Trifolium spp. highlights the value of seeking out new sources of host resistance from the Mediterranean centre of origin, even if the particular diseases of interest do not occur there, in the same way that has been shown for herbicide resistances and for Kabatiella on subterranean clover. pasture legumes such as Trifolium spp. highlights the value of seeking out new sources of host resistance from the Mediterranean centre of origin, even if the particular diseases of interest do not occur there, in the same way that has been shown for herbicide resistances and for Kabatiella on subterranean clover.