Cercospora leaf spot of sugarbeets

Cercospora beticola

Spores are needle-shaped (2-3 x 36-107 µm), colorless, and have several cross-walls (septations). Conidial morphology varies greatly with environmental conditions. 
Infection and lesion formation initially occur on older leaves before progressing to newer ones. Lesions at maturity are 1/8 inch in diameter and appear light gray-colored to dark tan with a brown to purple border. Severely affected leaves wither and die from coalescing lesions. A diagnostic feature is the presence of tiny black dots (pseudostromata) that form in leaf substomatal cavities within the grayish-tan lesions. The pseudostromata produce conidiophores borne in clusters that serve as conidia-bearing structures. Pseudostromata are visible with a hand lens, and after exposure of leaves to high humidity, entire lesions appear fuzzy due to the presence of numerous conidia.