Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/18/2011
Publication Date: 8/17/2011
Citation: Mengistu, A., Ray, J.D., Bellaloui, N., Smith, J.R. 2011. Seasonal progress of charcoal rot and its impact on soybean productivity. Plant Disease. 95:1159-1166. 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. In this study, a field experiment was conducted in 2004 through 2005 to determine the effects of charcoal rot on yield of soybean in irrigated and non-irrigated environments. The population of the fungus gradually increased after the plants had five leaves and then rapidly from the visible bean in pod to mature pod (R7) growth stages for all genotypes. The optimum time for disease assessment under irrigation was the R7 soybean growth stage when the fungal population accounted for yield loss better than at other growth stages. Under irrigation, yield differences between resistant and susceptible soybean increased with increases in fungal population levels for most comparisons. In eight irrigated paired comparisons, resistant genotypes significantly out-yielded susceptible genotypes. The extent of yield loss due to charcoal rot in the non-irrigated environment was inconsistent. In this environment, resistant genotypes significantly out-yielded the susceptible in only three of eight comparisons. This is the first study that provides evidence that M. phaseolina causes significant yield loss in infested fields and is the first that demonstrates that yield loss can be better measured in irrigated than in non-irrigated environments. Soybean breeders may use irrigated infested environments to select for resistance to charcoal rot and yield.
Technical Abstract: The effects of charcoal rot caused by Macrophomina phaseolina on yield of soybean were evaluated in the field using two genotypes each in maturity groups III and IV. Four separate experiments were established in an area of a field fumigated with methyl bromide. The experiments were: 1) artificially infested with M. phaseolina and irrigated, 2) non-inoculated with M. phaseolina and irrigated, 3) artificially infested and not irrigated, and 4) non-inoculated and not irrigated. The objective of these experiments was to determine the effect of charcoal rot on soybean genotypes and yield in non-infested and artificially infested soil, and in irrigated and non-irrigated environments. The population density of M. phaseolina in root and lower stem tissue, expressed as colony forming units per gm of tissue (CFU/g) at the V5, R1, R3, R5, R6 and R7 growth stages, was compared among four genotypes. Maturity group III susceptible DK-3964 and MG III moderately resistant AG-3905 were compared and MG IV susceptible Egyptian and MG IV moderately resistant DT97-4290 were compared. The CFU increased slowly from the V5 to R6 growth stages and then rapidly from the R6 to R7 growth stages for all genotypes. Under irrigation, the CFU at growth stage R7 accounted for more yield loss variation (42%) than the area under the disease progress curve (36 %), and R7 was the optimum sample time for a single point assessment of CFU. In eight irrigated paired comparisons, resistant genotypes significantly out-yielded susceptible genotypes. Further, the regression between CFU and yield difference in irrigated environment was significant. The extent of yield loss due to charcoal rot in the non-irrigated environment however, was inconsistent. In this environment, resistant genotypes significantly out-yielded the susceptible in only three out of the eight comparisons. This is the first study that provides evidence that M. phaseolina causes significant yield loss in infested fields, and the first that demonstrates that yield loss can be better measured in irrigated than in non-irrigated environments.