Forage legumes are essential for an efficient animal-based agriculture. They provide high quality feed for livestock and are a key component for sustainability of crop-pasture rotations. Birdsfoot trefoil (Lotus corniculatus L.) is the most important forage legume in Uruguay, where it is grown for pasture, hay and/or seed production. The major constraint for the use of birdsfoot trefoil is its poor persistence, attributed to the interaction of several biotic and abiotic factors that produce a cumulative stress load (8). Among these factors, crown and root diseases play a major role (1,3,4,7,8). Fusarium species are the main fungi associated with diseased crowns and roots of birdsfoot trefoil (3,4,6,7), F. oxysporum being the most prevalent (1,2,9). An understanding of the ecology and epidemiology of the birdsfoot trefoil-Fusarium pathosystem is essential for establishing efficient and durable means of disease management. Available information is very limited (5,8). My research objectives were to determine incidence and severity of crown and root rot in diverse ecological regions in Uruguay, to characterize diversity of the F. oxysporum population associated with diseased plants, and to assess the effectiveness of one cycle of phenotypic recurrent selection for resistance to Fusarium root rot using a greenhouse evaluation method.
Birdsfoot trefoil fields were selected to compose a 9-site-matrix of three locations and three stand ages. A stratified sample design was employed with 12 permanent quadrats per site; sample size was 25 plants per quadrat. Each plant was scored for disease severity following a five-class scale: 0 = no disease, 4 = plant dead. Disease incidence was calculated as percentage of diseased plants per quadrat. Sampling and stand counts were performed twice a year, during two successive years. Subsamples of diseased plants were used for fungal isolation. Crown and root rot occurred in all the regions surveyed, with F. oxysporum being the main pathogen involved, based on isolations. There was a large and significant effect of stand age on disease incidence and severity. Incidence varied from 2 to 100% and severity from 0.02 to 2.39. While incidence reached high levels (80%) in the seeding year, severity continued increasing with stand age. Stand counts significantly decreased with stand age, the summer being the critical season for plant survival. The average in March of the third year was 1 plant/m2, as compared to an average of 314 plants/m2 in September of the seeding year.
Vegetative compatibility was used as a measure of genetic relatedness of F. oxysporum isolates; nitrate nonutilizing (nit) mutants were recovered for each fungal isolate and paired in all combinations to perform complementation tests. A culture plate method was used to characterize isolate aggressiveness to seeds and seedlings of birdsfoot trefoil, which was rated following a five-class scale: 1 = healthy seedling, 5 = dead seed. No complementation was found among isolates, indicating the occurrence of a high degree of genetic diversity in the pathogen population. All the studied isolates of F. oxysporum incited a host reaction in birdsfoot trefoil, but varied in aggressiveness (range: 1.44-3.85). This variability in the pathogen population needs to be considered when selecting isolates for breeding purposes.
A greenhouse evaluation method was developed to screen and characterize birdsfoot trefoil germplasm for reaction to Fusarium root rot. The method resembled the soil-root mass-cut technique reported by other scientists (9,10). Plants were grown in 13 x 8-cell Styrofoam seed starter trays, each cell with a 0.5-cm-diameter hole in the bottom. Roots grew down through the hole into the soil in boxes placed below trays. Twelve weeks after seeding, roots were cut between the tray and the box, the box surface was inoculated by spreading a layer of sand and inoculum, and the tray was placed again over the box. Ten weeks after inoculation, plants were uprooted, washed and evaluated for root rot reaction, measured as percentage of internal rot (IR) in a transverse root section and as total length (cm) of vertical discoloration from the inoculation site. Plants of cv. San Gabriel, used as the source population, were inoculated with a composite of F. oxysporum isolates and root rot reaction was assessed. Plants scored 5% or less IR, and scored 30% or more IR were selected, separately intercrossed and seed from those plants was harvested to compose a resistant and a susceptible cycle-1 population (R1 and S1, respectively). Plants of the source and cycle one populations, plus two Uruguayan and three North American germplasms were characterized for root rot reaction. Mean disease severity varied significantly among birdsfoot trefoil entries (range for IR: 5.7-18.7%; range for VD: 1.2-3.8 cm). One cycle of phenotypic recurrent selection towards resistance or susceptibility significantly changed the stand's root rot reaction in both directions indicating that breeding for resistance has the potential to limit Fusarium root rot in Uruguay.
Correct and timely application of crop management practices should receive major consideration to help limit the rate of disease development and stand decline (8). The development of cultivars with increased resistance to Fusarium root rot could make a significant contribution towards increasing the production and persistence of birdsfoot trefoil (6,10). The results of this research project indicate that host plant resistance coupled with improved management practices should provide enhanced and sustainable performance of this forage legume.
REFERENCES
1. Altier, N. 1994. Current status of research on Lotus diseases in Uruguay. p.203-205. In P.R. Beuselinck and C.A. Roberts (ed.). Proc. First Intern. Lotus Symposium, 22-24 March, 1994, St. Louis, MO. Univ. Missouri, Columbia Pub.
2. Bergstrom, G.C., and Kalb, D.W. 1995. Fusarium oxysporum f.sp. loti: a specific wilt pathogen of birdsfoot trefoil in New York. Phytopathology 85:1555. (Abstr.).
3. Berkenkamp, B., Folkins, L., and Meeres, J. 1972. Crown and root rot of birdsfoot trefoil in Alberta. Can. Plant Dis. Surv. 52:1-3.
4. Beuselinck, P.R. 1988. Fungi associated with birdsfoot trefoil. Lotus Newsletter 19:11-14.
5. English, J.T. 1994. Diseases of Lotus spp. p.183-186. In P.R. Beuselinck and C.A. Roberts (ed.). Proc. First Intern. Lotus Symposium, 22-24 March, 1994, St. Louis, MO. Univ. Missouri, Columbia Pub.
6. Henson, P.R. 1962. Breeding for resistance to crown and root rots in birdsfoot trefoil, Lotus corniculatus L. Crop Sci. 2:429-432.
7. Kainski, J.M. 1960. Study of fungi involved in root rots and seedling diseases of birdsfoot trefoil. Cornell University, Agric. Exp. Sta. Memoir 369. 31p.
8. Leath, K.T. 1989. Diseases and forage stand persistence in the United States. p.465-478. In G.C. Marten et al. (ed.). Persistence of forage legumes. Proc. Trilateral Workshop, Hawaii, 1988. Am. Soc. of Agron. Madison, WI.
9. Richard, C., Michaud, R., Frève, A., and Gagnon, C. 1980. Selection for root and crown rot resistance in alfalfa. Crop Sci. 20:691-695.
10. Zeiders, K.E., and Hill, R.R., Jr. 1988. Measurement of resistance to Fusarium wilt/root and crown rot in birdsfoot trefoil populations. Crop Sci. 28:468-473.
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