Severity of charcoal rot disease in soybean genotypes inoculated with Macrophomina phaseolina isolates differs among growth environments

Authors: Mengistu, Alemu; Read, Quentin; LITTLE, CHRISTOPHER - Kansas State University; KELLY, HEATHER - University Of Tennessee; Henry, Peter; Bellaloui, Nacer

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/6/2025
Publication Date: N/A
Citation: N/A

Interpretive Summary: Charcoal rot of soybean is a major disease of economic significance around the world. The objective of this test is to evaluate and identify new method(s) that can identify resistant and susceptible genotypes when inoculated with the fungus that causes charcoal rot in non-field environments. Four independent experiments were performed to determine the variability in disease severity when soybean genotypes are inoculated with isolates up to a total of 100 different variants of the charcoal rot fungus in laboratory, greenhouse, and growth chamber tests. The results showed that as the number of isolates increased, differences in charcoal rot resistance varied due to differences in isolates, environment, soybean varieties and methods (or all the above). The tests in the non-field environments did not consistently identify the same soybean lines as being susceptible or moderately resistant to charcoal rot as were identified in field testing. Stakeholders will continue to benefit from the use of field assessment method to identify resistant breeding lines in naturally infested fields. The moderately resistant genotypes could be reliable sources for developing charcoal rot resistance and benefit the growers.

Technical Abstract: One hundred Macrophomina phaseolina (Mp) isolates, and 16 soybean genotypes were used to measure the area under the disease progress curve (AUDPC) using modified cut-tip and stem-wound lesion length measurements. Four independent experiments were conducted using Charcoal rot (CR) susceptible and resistant soybean genotypes inoculated with varying numbers of Mp isolates. Linear mixed models were fit to AUDPC values from the four experiments and model predictions of disease progress were used to determine the best method to correctly classify MR (moderately resistant) and S (susceptible) genotypes. In a growth chamber study (Experiment 1), both Mp isolates tested effectively differentiated MR and S. In Experiment 2, stems of sixteen genotypes grown in field and stem-wound inoculated with one isolate, classified the MR genotypes correctly but not all S genotypes. Correct classification of MR genotypes dramatically increased with plant age, approaching 100% correct at 120 days after planting. Experiment 3, in which field-grown soybeans were inoculated with 20 Mp isolates using stem wounding, identified MR genotypes relatively accurately, with more than half the isolates having >75% accuracy in detecting MR genotypes. In Experiment 4, where greenhouse-grown soybeans were stem wound-inoculated with 100 isolates, classification was somewhat less accurate than samples grown in the field, with median correct classification rate of MR genotypes under 60%. The severity relationship between Mp isolates inoculated with field and greenhouse-grown soybeans was modest, with a Spearman rank correlation of 0.39. This indicated that the non-field inoculation methods tested under non-field environments do not consistently differentiate MR and S genotypes as the current field assessment method.