Cause Verticillium dahliae, a fungus that survives in soil as microsclerotia. Another species V. isaacii has also been isolated from wilted sunflower plants in the Columbia Basin, Washington. Survival structures germinate and developed into mycelium on root hairs, elongation zones, and caps of lateral roots. Within a week, the mycelia colonize vascular bundles of lateral roots and taproots. Over the next several weeks the fungus can be found in epidermal cells, cortical tissues, and vascular elements of stem, petiole, and leaf veins. It spreads through and plugs the xylem. Toxins are also produced, which along with xylem occlusion results in wilt symptoms. After 3 months the fungus may be found in seeds and pollen. The fungus can be seedborne or in plant debris, contaminating seed lots.
Symptoms Generally, lower leaves wilt first, followed progressively by more leaves up the stem. Tissue between the leaf veins becomes chlorotic, then brown, leaving it mottled. A reddish-brown discoloration develops in the vascular tissue inside infected stems and later the soft pith tissue may shrink and turn black. Severely infected plants may be stunted and die before flowering. Black elongated patches may develop on the outside of the lower parts of stems late in the season.
- Plant resistant cultivars.
- Rotate with nonhost plants.
- Plant pathogen-free seed.
Reference Ryley, M., Gulya, T., Mathew, F., Thompson, S., Block, C., Markell, S. and Harveson, R. 2021. Sunflower Wilt Diseases: Charcoal Rot, Phialophora Yellows, and Verticillium Wilt. Plant Health Progress, 22:75-85.
Zhang, Y.Y., Zhang, J., Gao, J., Zhang, G., Yu, Y., Zhou, H., Chen, W., and Zhao, J. 2018. The colonization process of sunflower by a green fluorescent protein-tagged isolate of Verticillium dahliae and its seed transmission. Plant Disease 102:1771-1778.