Charcoal rot disease assessment of soybean genotypes using a colony-forming unit index

Abstract

Charcoal rot [Macrophomina phaseolina (Tassi) Goid] of soybean [Glycine max (L.) Merr.] is a disease of economic significance throughout the world. Progress in developing resistant genotypes has been hampered because of a lack of reliable and efficient methods for assessment of soybean genotypes. Researchers need a common system for classifying soybean genotypes for their reaction to M. phaseolina that is consistent across environments. We propose a classification system based on a colony-forming unit index (CFUI), derived by dividing the colony-forming unit (CFU) value of a given genotype by the CFU value of a susceptible standard. Four other assessment methods were compared and correlated to CFUI: percent height of stem discoloration measured at R7, foliar symptoms taken at R7, area under the disease progress curve based on foliar symptom data collected four times during the growing season up to R7, and the intensity of internal root and stem discoloration taken at R7 (root and stem severity [RSS]). Twenty-four soybean genotypes in Maturity Groups III through V were evaluated in 2002 and 2003 in naturally and artificially infested fields. Based on the CFUI, four genotypes were classified as moderately resistant to M. phaseolina. Among the disease assessment methods, RSS had the highest correlation with CFUI (r = 0.71 in 2002 and r = 0.69 in 2003). The CFUI provided a good measure of disease resistance across environments but was still time consuming. The RSS provided a less-accurate but more-rapid alternative that may be suitable for some breeding strategies. © Crop Science Society of America.