Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 21, 2007
Publication Date: November 7, 2007
Citation: Mengistu, A., Ray, J.D., Smith, J.R., Paris, R.L. 2007. Charcoal Rot Disease Assessment of Soybean Genotypes Using a Colony Forming Unit Index. Crop Science. 47:2453-2461. Interpretive Summary: Charcoal rot is a disease of soybean caused by a fungus (mold) called Macrophomina phaseolina that causes significant economic yield losses in the United States and around the world. Efforts to manage charcoal rot in soybean through cultural practices, fungicide applications and biological control had limited effects on disease severity. Progress in developing resistant soybean genotypes (varieties) is hampered because methods currently used for assessing this disease have produced erratic results. In this study, five disease assessment methods were compared in evaluating 24 soybean genotypes for two years. Four genotypes were classified as moderately resistant to M. phaseolina based on measurement of number of fungal particles in soybean stem and root tissue. Fungal particles in soybean stem and root tissue provided a consistent and objective method for classifying soybean genotypes across years and environments. Disease assessment using the intensity of internal root and stem discoloration is a quicker alternative to measuring fungal particles. This method allows researchers and plant breeders to evaluate large numbers of breeding lines to develop resistance against M. phaseolina.
Technical Abstract: Charcoal rot [Macrophomina phaseolina (Tassi) Goid] of soybean [Glycine max (L.) Merr.] is a disease of economic significance in the United States and around the world. Yield losses will remain high until resistant genotypes are developed. Progress in developing resistant genotypes is hampered because the methods for assessment of soybean genotype reaction to M. phaseolina have produced erratic results that were inconsistent over years and locations. Researchers need a standard method that is fast, less expensive, and easy to classify soybean genotypes for their reaction to M. phaseolina We propose a new system based on colony forming units (CFU) to classify genotype interaction to M. phaseolina at four levels: resistant, moderately resistant, moderately susceptible, and susceptible. The new system, the colony forming unit index (CFUI), is derived by dividing the CFU of a genotype by the CFU of a susceptible standard. In addition, four other methods were compared and correlated to the CFUI. The four methods were: percent height of stem discoloration due to charcoal rot measured at R7 (PHSD), foliar symptoms due to charcoal rot taken at R7 (FS), area under the disease progress curve calculated based on FS data collected four times during the growing season up to R7 (AUDPC), and the intensity of internal root and stem discoloration due to charcoal rot taken at R7 (RSS). Twenty-four soybean genotypes in Maturity Groups III-V were evaluated in 2002 and 2003 in naturally infested fields that had additional inoculum added at planting time. Based on the CFUI, four genotypes were classified as moderately resistant to M. phaseolina. Of the other four disease assessment methods, RSS gave moderately comparable results to CFUI (r = 0.71in 2002 and r = 0.69 in 2003). The other disease assessment methods were either not as well correlated to CFUI or were inconsistent across years. The CFUI provided a good measure of disease resistance across environments, but it is still time consuming. RSS provided a less accurate, but quicker alternative that may be suitable for high-through-put breeding programs.