Divergent selection for resistance to fusarium root rot in birdsfoot trefoil

Persistence of birdsfoot trefoil (Lotus corniculatus L.) is limited by the interaction of several factors including root and crown diseases caused by Fusarium oxysporum Schlecht. (Snyd. and Hans.). A greenhouse evaluation method was developed to screen and characterize birdsfoot trefoil germplasm for reaction to fusarium root rot. Plants were grown in 104-cell styrofoam seed starter trays. Roots were allowed to grow through the bottom of each cell into the soil in boxes below. Twelve weeks after seeding, roots were cut 6 cm below the crown and inoculated with a composite of F. oxysporum fungal isolates by spreading a layer of sand and inoculum across the box surface. Ten weeks later, plants were scored for percentage of internal rot (IR) in a transverse root section and length of vertical discoloration (VD) from the inoculation site. One cycle of bidirectional selection for reaction to F. oxysporum was conducted within the adapted cultivar San Gabriel. Plants scoring IR <5% and IR >30% were selected and intercrossed to produce resistant aud susceptible Cycle 1 populations. The parental source population, resistant and susceptible Cycle 1 populations, and five Uruguayan and North American germplasms were characterized for fusarium root rot reaction. Mean disease severity varied among birdsfoot trefoil entries (IR range: 5.7-18.7%, VD range: 1.2-3.8 cm). The resistant Cycle 1 population had lower IR and VD scores than the parental population, San Gabriel. Phenotypic mass selection was effective in changing the frequency of root rot reaction, indicating that breeding for resistance to fusarium root rot has the potential to increase the persistence of birdsfoot trefoil in the field.