Role of Host-plant Resistance and Disease Development Stage on Leaf Photosynthetic Competence of Soybean Rust Infected Leaves

Host-plant resistance is known to reduce fungal growth of Phakospora pachyrhizi Syd. & P. Syd., the causal agent of soybean rust (SBR) in soybean (Glycine max L. Merr.). This disease has been shown to reduce soybean leaf photosynthesis in susceptible soybean plants. Since resistant lines have reduced fungal growth, the resistance genes may protect these plants against injury to leaf photosynthesis. The objectives of this study were to determine the impact of a host-plant resistance gene and disease developmental stage on leaf photosynthesis. Two controlled-environment studies and a field experiment were conducted using genotypes resistant and susceptible to SBR. Photosynthesis was measured at the pre- and the post-sporulation disease developmental stage and its quantitative impact was calculated for the genotypes. The susceptible genotypes formed tan, sporulating lesions, and the resistant genotype formed reddish-brown (RB), nonsporulating lesions. The resistant genotype reduced disease severity (measured as relative lesion area). The negative impact of SBR on leaf photosynthesis was the same for resistant and susceptible genotypes (at equivalent disease severity levels), and the pre- and postsporulation disease development stages. Since the resistant genotype formed significantly lower lesion area, the reduced disease severity and the lack of sporulation in the resistant genotype will likely minimize the impact of the disease on canopy photosynthesis and yield.