Effects of Cercospora leaf spot disease on sugar beet genotypes with contrasting disease susceptibility

The fungal pathogen Cercospora beticola causes the most destructive foliar disease of sugar beet, namely Cercospora leaf spot (CLS), which results in economically important yield losses. Current breeding efforts aim at developing sugar beet lines with lower fungal susceptibility as well as high productivity to ensure reduced fungicide applications in the context of integrated pest management. However, the main challenge remains to select sugar beet genotypes that produce the required yield quality and quantity, and to quantify their defense ability. Well-established visual disease scoring can be supported and supplemented by new techniques that enable earlier disease detection and genotype resistance classification. In this thesis, visual disease scoring was combined with novel invasive and non-invasive techniques to analyze shoot and root disease responses to Cercospora infection. The fungal growth within the sugar beet leaf tissue was quantified using molecular analysis of the fungal calmodulin gene. This allows for the pre-selection of resistant genotypes before disease symptoms were visible. It could be shown that plants with high susceptibility (HS) allowed a stronger fungal colonization in the leaf tissue than ones with low susceptibility (LS). These results correlated with the respective disease severity. The HS genotypes consistently displayed more severe disease symptoms than LS plants. In particular, the moderately susceptible (MS) genotype seemed to be a promising candidate according to its variable response indicating a more adaptable reaction to changing environmental conditions. Therefore, this MS genotype might produce more yield under low-to-moderate disease pressure compared with HS plants, which would be comparatively more strongly infected and deliver reduced yield. The cuticle forms a barrier between the plant and the environment and therefore provides resistance against pathogens. Cuticular wax may affect host-pathogen recognition and conidia adhesion on the leaf surface. In this study, LS plants were found to contain larger amounts of cuticular waxes. Mature leaves, which showed reduced fungal colonization, also had higher wax levels than immature leaves. Further experiments are needed to investigate the precise role of sugar beet cuticular waxes on C. beticola infections. Field evaluations are essential for the characterization and selection of sugar beets with increased resistance, because controlled greenhouse experiments differ from agricultural growth conditions. In this regard, spectral sensors are the interface between field and greenhouse. They can be used to monitor disease progression non-invasively before and after symptom development, and to quantify leaf damage with the use of vegetation indices in a breeding scenario. In a field experiment of this study, the leaf signatures of infected sugar beet genotypes were quantified at a progressed disease stage. It could be shown that vegetation indices for quantifying canopy greenness (NDVI – Normalized Difference Vegetation), leaf water content (LWI) and Cercospora leaf spot disease (CLSI) correlated significantly with plant susceptibility. Laboratory measurements of leaf transmittance using the new Abstract HyperART-system, which is developed in a complementary PhD project at IBG-2, enables leaf absorbance quantification and provides insight into the signature changes at the first signs of CLS. Detailed time-courses of foliar disease effects on taproot development were monitored and quantified by Magnetic Resonance Imaging (MRI). An early growth reduction within two weeks after inoculation was investigated during a low foliar symptom expression phase according to initial measurements of cell damage caused by fungal colonization and ramification in the leaf tissue. Plants displayed an increased growth of cambial rings that were connected to new, non-inoculated leaves. This response partly compensated for the reduced growth of cambial rings that were associated with inoculated leaves. The sugar beet genotypes showed different taproot growth after CLS infestation. These plant responses on root performance could be associated with susceptibility. For resistance breeding, these might be potential traits to select candidate genotypes and to elucidate the physiologically underlying mechanisms to ensure both yield quantity and quality. Using a combination of invasive and non-invasive methods, this study revealed plant responses above and below the ground during foliar infection at different disease stages. Moreover, a full picture of sugar beet responses to Cercospora leaf spot was obtained. In addition to visual disease scoring, plant resistance can be verified with novel detection systems such as MRI, molecular analysis and spectral imaging, which provide a more detailed view of disease progression. Therefore, the new techniques can assist breeders to identify sugar beet varieties with increased fungal resistance. In particular, the early detection of reduced taproot growth with MRI has the potential to facilitate the discovery of traits associated with enhanced resistance.