Intra-Leaf Variability of Incubation Period Sheds New Light on the Lifestyle of Cercospora beticola in Sugar Beets

The length of incubation period, i.e., the time between first contact of host and pathogen and the appearance of symptoms, varies among diseases and depends on environmental conditions. <i>Cercospora beticola</i> is the most important fungal pathogen in sugar beet production worldwide, as Cercospora leaf spot (CLS) reduces the leaf area contributing to yield formation. Using sugar beet cultivars differing in CLS resistance, a single infection period of <i>C. beticola</i> resulted in minor differences in the incubation period among host genotypes and among individual plants of cultivars, greater differences among leaves within plants, and substantial variation within individual leaves. Under greenhouse conditions not suitable for secondary infections, the first CLS lesions appeared 10 days after inoculation; however, the number of leaf spots and CLS severity further increased significantly for another 7 to 17 days. A geographic information system approach enabled the tracking of colony appearance and growth of all CLSs on inoculated leaves for up to 27 days. Asymptomatic colonization of leaves was associated with thick hyphae which switched to thin hyphae or melanization after lesion appearance. The lifestyle of <i>C. beticola</i>-intercellular tissue colonization, triggering of necrotic host reaction-is discussed considering the experimental results as well as literature resources.