Cause Fusarium oxysporum f. sp. delphinii has been reported on delphinium from Washington. The fungus usually infects the roots or stems through wounds. Macroconidia and microconidia are formed in sporodochia on stems and can be disseminated by windblown and/or splashed rain. Warm temperatures, wet conditions favor disease severity, and pH has little effect. While the resting structures (chlamydospores) formed in plant debris can survive 1 year, the fungus can grow and reproduce in the soil without a host, so the pathogen can live many years in the same site.
Symptoms Sunken, water-soaked lesions appear on petioles and stems. Lesions expand inward and rapidly lengthwise and may coalesce. These lesions darken yet retain a water-soaked appearance. The fungus is visible as numerous salmon-color pustules of spores (sporodochia) in the cankered area, which becomes sunken and cracked. The pith cavity below the canker is filled with white mycelia. In cross section, the vascular bundles of stems and petiols are discolored. Older leaves turn yellow and brown. The fungus may infect roots and invade vascular tissues, resulting in poorly developed root systems, wilting, and killing the plant. Before death, the inflorescences droop, and leaves yellow, droop, and curl up.
Cultural control
- Use disease free transplants or seeds.
- Remove affected stems as soon as they are detected.
- Applying nitrate forms of nitrogen (versus ammonium nitrogen) helps reduce disease severity but will not eliminate the disease.
- Do not plant delphinium in a bed where the disease has been a problem. Rotate to other areas for several years.
- Infested soil can be removed and clean soil used to refill a bed area.
- Preplant soil solarization has been helpful in reducing populations of certain soilborne pathogens and weeds in western Oregon. Place clear plastic (preferably anti-condensation film) directly on smooth, rototilled ground, which has been irrigated to field capacity and then allowed to drain for 1-2 days. Bury the edges of the plastic to trap the heat. Solarize for 4-6 weeks (or longer) during the hottest part of the summer, beginning in early- to mid-July.
Chemical control Not effective as a sole treatment but must be integrated with cultural controls. Use these materials preventively only at seeding or transplanting. Do not use to salvage the crop, as treatments are generally ineffective and only help increase the risk of developing resistant fungi.
- Cleary's 3336 EG at 8 to 16 oz/100 gal water as a drench. Group 1 fungicide. 12-hr reentry.
- Medallion WDG at 2 oz/100 gal water if not phytotoxic. Use with oils or adjuvants may damage plant. Group 12 fungicide. 12-hr reentry.
- Terraguard SC at 4 to 8 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
Reference Kondo, T., Chu, E., Kageyama, K., and Sugiyama, S. 2013. Stem canker and wilt of delphinium caused by Fusarium oxysporum f. sp. delphinii in Japan. Journal of General Plant Pathology 79:370-373.