Black leaf spot causes irregular, shining, tar-like spots on the upperside of the leaves. The spots are brown on the undersides of the leaves. The petioles, stems and flowers may also be attacked. The lower leaves are attacked first. The disease spreads in cool wet weather. Remove infected leaves when noticed and cut off and discard old stems. The disease is caused by Pseudomonas delphinii and there is no chemical control listed.
Soft crown rot is also called delphinium black leg. It occurs during hot, humid weather and enters through wounds. The crown and roots decay causing rapid wilting and death of the plant. Decayed plant parts have a foul odor and a soft black discoloration is seen at the base of the stem. Avoid excessive watering. This problem is caused by Erwinia carotovora.
Diplodina crown rot causes a crown rot noticeable after the plants have grown for two or more seasons. Infected plants may winter kill or produce shoots which later rot at the base. The disease is worse when moisture is excessive and the tissues are succulent. No chemical controls are listed.
Crown rot caused by Sclerotium rolfsii causes sudden wilting and death of the plant. The lower leaves become wilted and badly infected plants are easily pulled up. Avoid sites known to be infested.
Stem canker begins as one or more small brown spots on the stems. The lower leaves turn yellow with the yellowing progressing upwards. When advanced, the flower spike droops and the leaves curl upwards. Remove infected plants. The causal organism is Fusarium oxysporum var delphinii. No chemical control is listed.
Powdery mildew causes a white powdery growth on the leaves. Spray with sulfur or benomyl to control this disease.
A number of leaf spots are found on Delphinium. Some of the most common are caused by fungi in the genera Cercospora and Septoria. No chemical control is listed.
Verticillium wilt attacks and kills Delphinium. Avoid infested soil. No chemical control is listed.
During periods of low temperatures with wet soil the leaves may be chlorotic. This is a physiological rather than a pest problem.