Characterization Of Fusarium Oxysporum And Phoma Sclerotioides, Pathogens Of Birdsfoot Trefoil And Alfalfa

Abstract

Soil-borne pathogens causing root and crown rots and vascular wilts are important causes of stand decline of perennial forage legumes. Fusarium wilt of birdsfoot trefoil (Lotus corniculatus), caused by an aggressive strain of Fusarium oxysporum, is associated with severe vascular discoloration, wilting and chlorosis, and rapid plant death of birdsfoot trefoil. Phoma sclerotioides, causal agent of brown root rot (BRR), causes root and crown lesions as alfalfa (Medicago sativa) and other perennial legumes emerge from winter dormancy, leading to increased plant mortality, reduced spring regrowth, and diminished yields. Fusarium wilt of birdsfoot trefoil has been a serious problem for trefoil production in parts of New York and Vermont since the 1970s. Analysis of F. oxysporum isolates causing this disease indicated that the pathogen has a unique host range relative to previously designated F. oxysporum formae speciales. Vegetative compatibility analysis and phylogenetic analysis of multilocus sequence data suggest that the pathogen is monophyletic. We propose designating the fungus F. oxysporum f. sp. loti. P. sclerotioides has long been known to be an important constraint to alfalfa production in Alaska and central and western Canada. In eastern North America, it has been reported only in Nova Scotia. Surveys of alfalfa production fields in Maine, New Hampshire, New York, and Ontario in 2004, 2005, and 2007 indicate that the pathogen is widespread in eastern North America, often occurring at high incidence levels. Patterns of infected alfalfa within fields suggest that the pathogen was not recently introduced. Surveys of alfalfa production fields in Colorado, New Mexico, and Pennsylvania conducted in 2006 and 2007 suggest that central Pennsylvania and high mountain valleys at the latitude of northern New Mexico may delineate the southern extent of the distribution of P. sclerotioides within alfalfa production regions of eastern and western North America, respectively. Phylogenetic analyses of multilocus sequence data places North American isolates of P. sclerotioides into multiple strongly supported clades, and morphological differences among isolates correspond to genetic differences. On the basis of genetic and morphological differences, we established seven infraspecific varieties within P. sclerotioides: P. sclerotioides var. sclerotioides, champlainii, viridis, obscurus, steubenii, macrospora, and saskatchewanii.