Development of a simple hydroponic assay to study vertical and horizontal resistance of soybean and pathotypes of phytophthora sojae

Genetic resistance can be a very useful tool to manage plant diseases, as using resistant plant varieties for crop production can reduce the reliance on chemical pesticides used to protect these crops against plant pathogens. This study aimed to develop a simple assay to study different types of genetic resistance (partial and complete) of cultivated soybean to a microbial agent, Phytophthora sojae, that causes disease symptoms of seed rot, seedling damping-off and root rot on the various growth stages of soybean. Although methods for evaluating the genetic resistance of soybean to this pathogen have previously been developed, several different assays are required to evaluate the two different types of genetic resistance of soybean to this pathogen, and a single assay allowing the evaluation of both types of resistance had not been previously reported. Soybean plants were grown in a recirculating hydroponic system in the greenhouse and infected with the pathogen. This system allowed the direct observation of the live roots, since plants were grown in a clear water-based nutrient solution rather than in opaque potting mix or soil. The appearance and growth patterns of the aerial parts (stems and leaves) and the roots of the soybean plants correlated with the previously known complete resistance profile in a set of nine soybean varieties, with resistant plants showing more development and a healthier appearance of aerial parts and roots, and higher plant dry weight, when infected with two different isolates of the pathogen. The assay was also used to assess complete resistance in 10 commercial soybean varieties in development for which the researchers did not initially know the resistance profile (blind test for), and the results were later confirmed to match expectations for these varieties. The interaction between soybean plants of known complete resistance profile and the pathogen can also be used to study the genetic profile for virulence of the pathogen, in this assay and in others previously developed. Two isolates of the pathogen were tested in this fashion using this new hydroponic assay on nine soybean varieties of known complete resistance profile, and the results matched expectations based on the previously tested virulence profile of these isolates using an older method. This new assay should be a useful tool to evaluate the resistance of soybean lines to this pathogen and also the pathogen’s genetic profile and thus disease-causing potential.