Severity of Charcoal Rot Disease in Soybean Genotypes Inoculated with <i>Macrophomina phaseolina</i> Isolates Differs Among Growth Environments

Charcoal rot of soybean caused by <i>Macrophomina phaseolina</i> 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 (S) genotypes when inoculated with the fungus that causes charcoal rot in nonfield 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. Linear mixed models were fit to area under the disease progress curve values from the four experiments and model predictions of disease progress were used to determine the best method to classify moderately resistant (MR) and S genotypes. In a growth chamber study using a modified cut-tip inoculation method, 28 of the 32 <i>M. phaseolina</i> isolates tested differentiated MR and S with >87% accuracy. In a study where 16 field-grown soybean genotypes were stem-wound inoculated with one isolate, the MR genotypes were correctly classified, but not all S genotypes were. Correct classification of MR genotypes dramatically increased with plant age, approaching 100% accuracy at 120 days after planting. In a study of stem-wound inoculation of field-grown soybean genotypes with 20 <i>M. phaseolina</i> isolates, MR were identified with more than half the isolates having >75% accuracy in detecting MR genotypes. In a study of greenhouse-grown soybeans stem-wound-inoculated with 100 isolates, classification was less accurate than samples grown in the field, with median correct classification <60% for MR genotypes. The severity of the same isolate tested across different studies was inconsistent; the relationship between 20 isolates in the field and greenhouse studies was modest, with a Spearman rank correlation of 0.39 (<i>P</i> = 0.0006), and the rank correlations with the growth chamber study were weak. The results showed that, except for the growth chamber study, the nonfield environments did not consistently identify the same soybean lines as being S or MR to charcoal rot as were identified as in naturally infested field testing because of differences in isolates, environment, soybean varieties, and methods (or all the above). Stakeholders will benefit more from the use of the field assessment method in naturally infested soil to identify reliable sources of resistance than from the nonfield methods.