Differentiating Resistance Levels and Biochemical Responses of Soybean Cultivars Infected by Diverse Diaporthe Species Using Two Inoculation Methods

Diaporthe spp. are among the most serious pathogens of soybean. Many different Diaporthe species can infect soybean plants. The species differ in their aggressiveness or virulence and in the severity of the damage they cause. Resistance breeding in soybean has been performed for only two Diaporthe species, so far. It would be very advantageous to identify soybean cultivars with resistance against other Diaporthe species as well, both as sources of resistance for breeding and to inform farmers which cultivars should be planted when a given Diaporthe species shows high incidence. We performed greenhouse experiments to differentiate levels of resistance using the Stem Cut and Stem Wound methods for inoculation of the plants with Diaporthe. Symptom severity was rated visually, and at 5 dpi the level of lipid peroxidation (LP), activity of superoxide dismutase (SOD), total phenolics and total flavonoids were measured. Among the four Diaporthe species tested, D. caulivora was most aggressive, followed by D. longicolla. Of the cultivars evaluated, Magnolia exhibited the highest level of resistance with no significant differences observed among the other cultivars. Although biochemical responses could be observed, it was impossible to determine the specific response responsible for elevated resistance in Magnolia.