Response to leaf inoculations with Macrophomina phaseolina in white clover

USA, environmental stresses include saturated soils in the winter and hot, drought conditions in the summer. Summers in the southeastern USA produce a harsh environment Though environmental and other factors can reduce for survival of white clover (Trifolium repens L.) stolons. Long periods of drought and hot temperatures are interspersed with rain showers white clover growth and stands at any time of the year, that create ideal conditions for fungal pathogenesis. Previous studies complete death of the stand usually occurs in the sumindicated that Macrophomina phaseolina (Tassi) Goidanich may be mer (Gibson and Hollowell, 1966). an important pathogen that limits survival of white clover stolons in Fungal disease pressure plays an integral role in the the summer. The objective of this study was to determine the range survival of white clover stolons in the summer. The in response of 20 white clover cultivars, germplasms, and breeding normal summer environment of the southeastern USA and naturalized populations for resistance to M. phaseolina using a is ideal for fungal pathogenesis. Fungicides have been leaf tissue assay. Discs were cut from leaves excised from 50 plants shown to improve white clover yield and persistence of each entry and inoculated with an agar plug cut from the margin (James et al., 1980; Pederson et al., 1991). Fungicide of a M. phaseolina colony. Leaf discs were scored according to the rate of necrosis induced by the pathogen. The experiment was conducted as treatment during the summer months improves white a randomized complete block with four replicates and was repeated clover stolon density, growing point density, and stolon with 50 additional plants from each entry. Differences in responses length (Pederson and Pratt, 1995). Germplasm sources of entries to inoculation with M. phaseolina were observed in each differ in their response to fungicide treatment; Brown run of the experiment. Brown Loam Syn. 2 germplasm and North Loam Syn. 2 germplasm showed less response to fungiGA population had the least disease and the greatest number of plants cide treatment than ‘Regal’ and ‘Louisiana S-1’ white selected as resistant to M. phaseolina. Large-leaf plants selected for clover (Pederson and Pratt, 1995). These data suggest resistance gave highly consistent responses when retested, with 35% that this germplasm may have some resistance or tolerof the plants having no leaf necrosis following inoculation with M. phaseolina. The leaf tissue assay was not as reliable for selecting ance to summer fungal diseases. Naturalized populaconsistent resistant phenotypes among small-leaf white clover entries, tions that have survived in pastures for a number of as 37% of the plants selected as resistant were rated as susceptible years have also been suggested as sources of resistance upon retesting. Resistance to M. phaseolina was observed in adapted or tolerance to summer stolon diseases (Cope, 1978). white clover germplasm, and development of new cultivars with this Numerous fungi are frequently isolated from white resistance should improve white clover summer survival. clover stolons including Colletotrichum spp., Curvularia trifolii (Kauff.) Boedijn, Fusarium spp., Mycoleptodiscus terrestris (Gerdemann) Ostazeski, M. phaseolina, tolonsare the means by which individual white clo- Rhizoctonia spp., Sclerotium rolfsii Sacc., and others ver genotypes persist in the field. The seedling tap- (Halpin et al., 1963; Latch and Skipp, 1987; McGlohon, root system only survives for the first year or two of 1959; Pederson and Pratt, 1995). The relative imporgrowth. When the taproot, primary stem, and basal por- tance of individual fungal species for stolon survival has tion of the stolons die, plant survival is entirely depen- rarely been addressed. Some fungi appear to play no dent on new stolon growth from the terminal ends. Indi- active role in stolon necrosis, arriving only as secondary vidual stolons from the same plant are genetically pathogens when death and decay have already begun. identical, but may no longer be connected. New growth Other fungi may be quite pathogenic. Following a previand branching continues at the stolon tips while the ous study of white clover stolon growth with and without older basal portion of the stolon dies. The genotype will fungicide treatment (Pederson and Pratt, 1995), Pratt survive as long as new stolon growth at the tip exceeds (1998, unpublished data) evaluated the pathogenicity of the progression of necrosis from the basal end.