Cause Alternaria brassicae and Alternaria alternata can infect leaves, petioles, stems, flower parts, and seed pods of sugar beet. Alternaria brassicae is a common pathogen on crucifer crops and weeds. Alternaria alternata can be a greater problem in plants with mechanical injuries or growing under water or nutrient deficiencies. These fungi survive on residues of infected, susceptible crop plants and weeds. Infected plant debris that is on the soil surface can produce asexual spores (conidia) when conditions are conducive. Conidia can be spread by wind and splashing water droplets. Cool, humid conditions can lead to a build-up in disease levels but conidia are dispersed during warm, dry periods. Alternaria brassicae and A. alternata can also be seedborne.
Symptoms Older leaves and older plants are more susceptible. Small, dark or yellow leaf spots first develop, enlarging to circular areas that are brown to gray in color with or without concentric rings and possibly with black or purple borders and/or surrounded by yellow halos. Sometimes the leaf spots are limited by leaf veins so the spots are angular in appearance rather than circular. The centers of leaf spots may be coated with sooty black spore masses, and can crack or may drop out, producing shot holes. Leaf spot size ranges from areas barely visible (0.1 inch or smaller diameter) to more than 2 inches in diameter. Leaf lesions can lead to premature defoliation. Brownish lesions develop on collar portions and may be apparent on infected stecklings. Circular black spots develop on storage roots, and may appear post-harvest.
Cultural control
- Rotate out of sugar beets for at least three years.
- Control susceptible weeds and volunteers in between crops.
- Use pathogen-free seed.