Verticillium Wilt

Verticillium wilt can reduce sunflower yields on lighter soils by up to 50 per cent.

Figure 1

Host Crops

Sunflowers, potatoes, alfalfa, sweet clover

Biology

The fungus is seed and soil borne. It survives as very small, black, resting bodies (Microsclerotia) in diseased sunflower debris and broadleaved weeds. The root tips of sunflower plants are invaded and, eventually, all parts of the plant become affected. When the plant dies, the fungus produces the black fungal bodies that return to the soil with the plant debris. The fungus is able to survive in the soil for several years.

Symptoms Of Damage

The symptoms may appear early in the season but are most common near the time of flowering. Diseased plants usually occur in groups of a few to many plants. Symptoms develop first on the lower leaves and gradually appear on leaves farther up the stem. The leaf tissue between the veins yellows and becomes necrotic, while the areas near the veins remain green. (Figure 1) This produces a mottling effect.

Cross-sections of the stem reveal a brown discolouration of the vascular tissue (visible as a brown ring). Severely-diseased plants die early, before seed development. They exhibit brown, withered leaves and black streaks at the base of the stem. With light infestations, the diseased plants also die prematurely but the leaves may only be yellow and not exhibit the typical mottling.

Scouting Techniques

At or near flowering stage in single or groups of plants, look for yellowing between leaf veins. These areas turn brown and die. Severely-infected plants will turn brown, starting at the bottom leaves, and die prematurely.

Economic Thresholds

None available.

Control Tips

Plant only resistant varieties. Many oilseed and some confectionary hybrids show resistance to verticillium wilt. Rotations of at least four years between sunflower crops, two of which must be cereals, are recommended to help prevent this disease.

For further information, contact your GO representative.