Quantification of sugar beet resistance to fungal Cercospora infestation using Magnetic Resonance Imaging and quantitative real-time PCR

Sugar beets Cercospora leaf spot (CLS) is the most destructive foliar disease caused by the fungal pathogen Cercospora beticola resulting in up to 40 % crop loss. Therefore, introduction of resistant sugar beets into breeding programs is a crucial objective. Generally, plants can defend against pathogens at different stages of infestation from the initial contact, during invasion, all the way to the final stage where disease affects shoot and root. After penetrating through the stomata, the onset and intercellular propagation of hyphal growth can be limited by specifically expressed pathogenesis-related (PR) proteins of the plant. At this point, quantitative real-time PCR analysis of the fungal calmodulin gene showed reduced pathogen propagation in leaf tissue of a lowly susceptible (LS) genotype. In general, following the intracellular propagation, necrotic leaf spots appear due to fungal toxin production resulting in leaf damage. We scored visually a lower disease severity of the LS genotype compared with a highly susceptible (HS) one and could quantify leaf damage by spectrally resolved imaging. Finally, the below-ground taproot system was also affected in development. This effect on taproot morphology was measured non-invasively with Magnetic Resonance Imaging (MRI) revealing a delay in taproot growth of the LS genotype. Comparing the genotypes, the HS genotype showed stronger disease progression and fungal propagation on infected leaves, however it displayed stronger taproot growth. This phenotyping study revealed differences between plant resistance levels on leaf and root scale already 14 days after inoculation before considerable disease severity was reached. Hence, the combination of these measurements could be used to characterize existing diversity for pre-breeding programs and inform the selection of candidate genotypes and traits linked to CLS resistance