Effects of Heterodera glycines population densities on Fusarium solani f. sp. glycines colonization of soybean roots and development of sudden death syndrome

Sudden death syndrome (SDS), caused by Fusarium solani f. sp. glycines (Fsg), is an important disease of soybean in the U.S. Soybean cyst nematode, Heterodera glycines, has been identified in every U.S. region where soybean is grown and exacerbated SDS symptoms in previous greenhouse experiments, but the interaction has not been defined in the field. Experiments were conducted in six field environments and two greenhouse experiments with four Essex x Forrest near-isogenic soybean lines (NILs) with four combinations of resistance and susceptibility to both pathogens to investigate the effects of H. glycines on SDS development and root colonization by the fungus. Fsg colonization was estimated in colony-forming units (CFU) with a traditional plating bioassay on a semi-selective medium. High variability in the field data made it difficult to draw conclusions from these experiments. Substantial disease only developed in one environment on the SSDSRSCN NIL. Variability in the data was less from the greenhouse experiments, but still high for CFU. Area under the SDS progress curve did not differ between NILs. However, disease severity increased linearly with increasing nematode densities at planting across NILs that were infected with a low level of Fsg. The results from these experiments suggest that H. glycines worsens SDS symptoms under some conditions. Cultivars with resistance to both pathogens should be planted in fields with a history of both pathogens.; Estimation of root colonization by Fsg with the plating bioassay was difficult and the results were often unsatisfactory in these experiments. Therefore, an alternative method was explored, relative quantification with real-time quantitative PCR (QPCR), to estimate fungal colonization of soybean roots. QPCR was more sensitive than the plating bioassays at detecting the fungus: Fsg DNA was detected in 371 out of 372 root samples with QPCR, while CFU were only detected in 14.5% of the samples with the plating bioassay. The plating bioassay required at least 10 days longer to complete and the results had higher variability than those obtained with QPCR. Overall, QPCR had many advantages over the plating bioassay and has the potential for several more practical applications.