Seed-borne diseases in pasture grasses and legumes: state of the art and gaps in knowledge

Abstract

Information on fungal seed-borne diseases on main pasture grasses and legumes from the literature was reviewed. These diseases reduce biomass production, quality of forage, and persistence due to progressive plant mortality. The main fungal pathogens associated with forage seeds belong to the orders Hypocreales, Pleosporales, and Helotiales in the phylum Ascomycota. Hypocreales includes the genus Fusarium, which reduces seedling establishment, and contaminates plant tissues with mycotoxins. Pleosporales includes many genera associated with seeds of legumes (Leptosphaerulina and Ascochyta), grasses (Bipolaris, Pyrenophora, Curvularia, Drechslera, Alternaria, Exserohilum, and Phoma), and both (Stemphylium). Some fungal genera within this order induce the accumulation of coumestans (leafspot-producing fungi) or produce secondary metabolites that contaminate tissues (Alternaria). Within Helotiales, the main genera are Sclerotinia (affecting mainly legumes), Clarireedia and Gloeotinia (affecting grasses). Pyricularia (order Magnaporthales), Colletotrichum (order Glomerellales), and Cercospora (order Mycosphaerellales) also include seed-borne fungi that provoke diseases on forage species as well as Rhizoctonia (order Cantharellales) and Ustilago (order Ustilaginales) which belong to the phylum Basidiomycota. These pathogens affect pastures by (i) compromising seedling establishment at early stages and (ii) constraining growth by reducing yield and seed quality at later stages. Future research should address (i) generation of reliable data on forage yield loss due to seed-borne diseases, (ii) assessment of the interaction between seed-borne pathogens and other biotic and/or abiotic stresses, (iii) delve into the study of the role of wild and/or cultivated forage species as inoculum reservoirs of pathogens, and (vi) shed light on the contamination issue due to mycotoxins generation.