The role of variation in genetic susceptibility to soybean rust on the photosynthetic competence of infected leaves.

Three soybean rust infection types have been reported: 1) tan lesions indicate a compatible and susceptible reaction, 2) red- brown (RB) lesion type has been associated with few non-sporulating uredinia representing a resistant reaction, and 3) immune reaction with no visible evidence of infection. Differences among cultivars would imply genotypic differences in tolerance, which can be define as the ability of the host to endure the presence of the pathogen with reduced disease symptoms and/or damages. Soybean rust-induced yield loss has been associated with reduction in light interception due to premature leaf loss and the lesions on the remaining green leaves. Incorporation of the effect of the pathogen on photosynthetic efficiency of disease leaves from different soybean cultivars in model to predict production might increase the accuracy and precision of the estimates supply by this model. Bastiaans (1991) proposed the concept of a "virtual lesion" and developed a simple model Y= (1-x)? to fit empirical data and describe the relationship between the reduction in relative photosynthesis in a diseased leaf (Y), and the disease severity (x). The value of â indicates whether the effect of disease on photosynthesis is higher (? > 1), lower (? < l) or equal (? = 1) to that accounted for by the observed diseased area. Reliable estimate of disease effects on growth and yield depends on the ability to accurate quantify the parameter ?. The objective of the current study was to determine the role of host-plant variation in disease susceptibility on soybean leaf gas exchange in field and control environment studies. The specific objectives are to quantify the ? value for SBR-infected leaves as influenced by plant growing conditions and genetic variation in disease susceptibility.Edição do Proceedings of the National Soybean Rust Symposium, New Orleans, 2009