The Influence of Genetic Variation in Disease Susceptibility On the Photosynthetic Competence of Soybean Rust Infected Leaves.

The impact of soybean rust (SBR) on leaf photosynthesis has been shown to be greater than can be accounted for by the impact of the lesions on radiation absorption.  Therefore, the impact of SBR on photosynthesis should be incorporated into the development of a yield loss model. One method of quantifying the effect on leaf photosynthesis is through the calculation of a ‘virtual’ lesion: an area of diseased leaf where photosynthesis is zero.  The derived function relates net photosynthetic rate (Px) of a diseased leaf to that of a healthy leaf (Po) by:  Px = Po (1-x)^β  where x is the proportion of the leaf area covered by visible lesions and β is defined as the ratio between the sizes of the `virtual' and the `visual' lesions. The value of β indicates whether the effect of disease on photosynthesis is higher (β >1), lower (β <1) or equal (β=1) to that due to the visual lesion area. Despite the importance of this disease, no β value for SBR has been reported for either sensitive or resistant soybean lines.  Quantifying the effects of SBR on resistant and susceptible soybean lines and inclusion of its effect on photosynthesis in yield loss model is of great importance for a more complete understanding of yield response to diseases and might increase the accuracy of yield loss models. The objective of the current study was to determine the role of genetic variation in disease susceptibility on leaf photosynthesis in field and control environment studies. The specific objectives are to quantify the β value for SBR-infected leaves as influenced by growing conditions and genetic variation in disease susceptibility. Our results suggest that effect of the lesions on relative photosynthesis was greater than that accounted for by visible symptoms (β>1) in all genotypes included in our studies.