Annual Medicago: from a model crop challenged by a spectrum of necrotrophic pathogens to a model plant to explore the nature of disease resistance

Abstract

Background: Annual Medicago spp., including M. truncatula, play an important agronomic role in dryland farming regions of the world where they are often an integral component of cropping systems, particularly in regions with a Mediterranean or Mediterranean-type climate where they grow as winter annuals that provide both nitrogen and disease breaks for rotational crops. Necrotrophic foliar and soil-borne pathogens dominate these regions and challenge the productivity of annual Medicago and crop legume species.

Scope: This review outlines some of the major and/or widespread diseases these necrotrophic pathogens cause on Medicago spp. It then explores the potential for using the spectrum of necrotrophic pathogen-host interactions, with annual Medicago as the host plant, to better understand and model pathosystems within the diseases caused by nectrotrophic pathogens across forage and grain legume crops.

Conclusions: Host resistance clearly offers the best strategy for cost-effective, long-term control of necrotrophic foliar and soil-borne pathogens, particularly as useful resistance to a number of these diseases has been identified. Recently and initially, the annual M. truncatula has emerged as a more appropriate and agronomically relevant substitute to Arabidopsis thaliana as a model plant for legumes, and is proving an excellent model to understand the mechanisms of resistance both to individual pathogens and more generally to most forage and grain legume necrotrophic pathogens.

Fig. 1

Phoma blackstem on Medicago truncatula ‘Jemalong’.

Fig. 2

Phoma blackstem on Medicago polymorpha ‘Serena’.

Fig. 3

Pseudopeziza leaf spot on Medicago polymorpha ‘Circle Valley’.

Fig. 4

Stagonospora leaf spot on Medicago polymorpha ‘Serena’.

Fig. 5

Severe rotting of roots of Medicago polymorpha seedlings.

Fig. 6

Root disease on established Medicago polymorpha plants.

Fig. 7

Field swards of species of annual Medicago for identification of varietal resistances or susceptibilities to Phoma blackstem disease. Note yellowing and poor growth of the most susceptible varieties.