Cause The fungus Calophoma aquilegiicola (formerly Diplodina delphinii) has been found by the OSU Plant Clinic. The fungus overwinters on host debris and in the crown of living plants. The disease is favored by warm wet weather, high humidity, and excessive fertilization. Injury is not needed for the fungus to gain entry into various plant parts. In the Eastern United States, the disease was found to be a problem on Delphinium elatum and several commercial cultivars but not on D. grandiflorum or D. cardinale. The fungus may also be a problem on clematis.
Many other fungi can cause leaf spots on delphinium but only a few have been reported in the PNW including Ramularia crassiuscula (formerly Ramularia delphinii) and Stagonosporopsis delphinii (formerly Phyllosticta delphinii).
Symptoms Leaf spots, stem cankers and crown rots are associated with this fungus. Leaf spots are similar to those produced by bacterial leaf spot. Tarry black lesions on the upper leaf surface and light brown lesions on the lower leaf surface. Petioles develop brown water-soaked areas especially near the base of succulent plants. Less vigorous plants show black local lesions on the petiole. Inflorescences and seed pods develop a blackish decay.
Lower stems show localized, brown to black necrotic lesions that are variable in size and shape. Small back fruiting bodies (pycnidia) may be seen in a concentric arrangement in the diseased area. Crowns and root develop black or dark brown vascular systems. The entire crown may rot along with the sudden wilting and death of the entire plant. Generally, crown rot is found on plants 2 or more years old. Young tender basal shoots arising from the crown develop a brown to black water soft rot similar to petioles.
Cultural control
- Avoid overcrowding plants. Space plants for good air circulation and quick drying.
- Avoid overhead irrigation or any practice that keeps plants wet for extended periods of time.
- Remove and destroy dead and dying plant debris at the end of the season.
- Rotate to other areas for at least 2 years before planting delphinium again.
Chemical control Few if any tests have been done but these materials may be helpful before severe symptoms develop.
- Armada 50 WDG at 3 to 9 oz/100 gal water. Do not use a silicone-based surfactant. Not for nursery or greenhouse use. Group 3 + 11 fungicide. 12-hr reentry.
- Banner MAXX at 5 to 8 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
- Compass 50 WDG at 1 to 2 oz/100 gal water. Do not use organosilicate additives. Group 11 fungicide. 12-hr reentry.
- Eagle 20 EW at 6 to 12 fl oz/100 gal water. Group 3 fungicide. 24-hr reentry.
- Mancozeb-based products can be used. Group M3 fungicides. 24-hr reentry.
- Fore 80 WP at 1.5 lb/100 gal water plus a spreader-sticker.
- Protect DF at 1 to 2 lb/100 gal water plus 2 to 4 oz spreader-sticker.
- Myclobutanil 20 EW T&O at 6 to 12 fl oz/100 gal water plus spreading agent. May observe a PGR effect. Group 3 fungicide. 24-hr reentry.
- Spectracide Immunox Multi-Purpose Fungicide Spray Concentrate for Gardens at 1 fl oz/gal water. Group 3 fungicide. H
- Terraguard SC at 4 to 8 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
- Ziram 76 DF at 1.5 to 2 lb/100 gal water. Group M3 fungicide. 48-hr reentry.
Reference Laskaris, T. 1950. The diplodina disease of delphinium. Phytopathology 40:615-626.