Field Screening of Commercial and Experimental Soybean Cultivars for Their Reaction to Macrophomina phaseolina

From 1992 to 1994, 24 soybean cultivars, ranging in maturity from early group III to late group IV, were assessed in the field for resistance and susceptibility to Macrophomina phaseolina. Cultivars were placed into four relative maturity classes-3.2 to 3.4; 3.5 to 3.8; 3.9 to 4.0; and 4.3 to 4.9-with six cultivars per class. Twelve cultivars were resistant and 12 were susceptible to Heterodera glycines, the soybean cyst nematode. Cultivars were planted on two dates (early May and early June) to assess the effect of environment on yield and lower stem and taproot colonization by the fungus at growth stage R7. Based on seed yields and the levels of lower stem and taproot colonization by M. phaseolina, four cultivars-Asgrow 4715, DeltaPineland 3478, Hamilton, and Jackson II-were rated moderately resistant to M. phaseolina. Seed yields and M. phaseolina levels in host tissues were affected by planting date, relative maturity class, H. glycines response, and the environmental conditions that prevailed over the 3 years of the study. However, in general, the environmental, agronomic, and host genetic factors present in these experiments did not affect the individual cultivar/pathogen interaction. Quantification of microsclerotial propagules in lower stem and taproot tissues at growth stage R7 was a reliable, but time-consuming, method to measure the degree of host compatibility between soybean cultivars and M. phaseolina.