Management of root diseases of annual pasture legumes in Mediterranean ecosystems - a case study of subterranean clover root diseases in the south-west of Western Australia

Abstract

Subterranean clover (Trifolium subterraneum) is an important component of Mediterranean dryland pasture ecosystems, such as in the south-west of Western Australia, where it is utilised as a winter annual pasture that provides nitrogen as well as disease breaks for rotational crops. Necrotrophic soil-borne fungal pathogens dominate Mediterranean ecosystems because of the ease of survival of these pathogens on infested residues over the dry summer period, and because of low levels of microbial competition in the impoverished and nutrient-defi cient soils characteristic of these regions that predisposes plants to root diseases. In addition to herbage and seed yield losses from soil-borne fungal and nematode pathogens, changes in botanical composition, in the number of regenerating plants, their persistence, and factors affecting feed quality are signifi cantly affected. Further, where the causal organisms of the diseases on subterranean clover are also common on other rotational crops, the impact of these soilborne pathogens appears far wider in Mediterranean ecosystems than previously considered. Under these conditions, soil-borne pathogens pose a serious threat to the productivity of this self-seeding pasture legume, to the extent that reseeding may become necessary. Pathogens such as Phytophthora clandestina, various Pythium species particularly Pythium irregulare, Aphanomyces sp., Rhizoctonia solani, one or more Fusarium species, Phoma medicaginis and Cylindrocarpon didymium are of concern, as are the nematode parasites from the genera Meloidogyne, Heterodera, Pratylenchus, Trichodorus and Radopholus. In this ecosystem, root pathogens operate together as disease complexes and the challenge therefore has been to source host genotypes with resistance to multiple pathogens. In addition to plant nutrition, environmental factors, in particular rainfall (soil moisture) and soil temperature, have a marked effect on both the disease severity caused by individual pathogens and on the interactions that occur between the different root pathogens. Approaches to disease control in this region include a range of management strategies. Cultural control strategies, including manipulation of grazing and rotations, offer some benefi ts. Manipulation of soil fertility also offers scope as this can enhance root physiology related to host resistance, overall plant growth and vigour, and also to improve the effective biological buffering against the pathogens. Fungicide treatments and manipulation of management practices may have a place in an integrated control system incorporating cultivars with useful resistance to root diseases. Clearly, host resistance offers the most cost-effective, long-term control, especially as resistance to several of these soil-borne pathogens has been identifi ed. The Mediterranean Basin, which is the centre of origin of this pasture legume, has proved to be a productive source of resistance to soil-borne necrotrophic pathogens and is likely to be a source of new subterranean clover cultivars.