The Δ CTB2 strains fail to cause disease symptoms on sugar beet leaves. A Enlarged sections of the surface of sugar beet leaves reveal that exposure to the wild type Ahlburg strain led to the characteristic leaf spot symptoms 21 days after inoculation. Ahlburg ctb2 disruption strain 2-1 failed to cause typical leaf spot symptoms. Ahlburg ectopic strain produced wild type like leaf spots. Treatment with water served as the negative control. B Ferrara wild type caused leaf spot symptoms, although significantly smaller compared to Ahlburg wild type. Ferrara cbt2 disruption strain 2-1 failed to cause leaf spot symptoms in contrast to Ferrara ectopic mutant. Experiments were repeated twice with ten plants for each fungal strain. Both disruptants of each wild type isolate gave similar results (data not shown).