Thanks to its striking tall spikes of attractive flowers, Delphinium is a popular plant in many backyards. It also produces stemmed flowers that can be cut for use in floral arrangements. These plants, however, can fall prey to disease and pests. Care must be taken to provide the correct growing conditions so that most of these common ailments can be avoided.
Delphiniums require fertile, moist, well-drained soil for optimal growth. If you have heavy clay-like soil, add mature compost and some gravel to the planting site to improve drainage. Waterlogging is a serious problem and can cause the roots to decay. Once root rot sets in, chances are the plant cannot be saved. It is, therefore, best to provide good drainage from the start. Never overwater the plant, while taking care to keep the soil moist at all times. In the winter, mulch the plant with an organic material, and also dig outlets, if required, to let out excess water or melted snow that sits in the root area. Keep mulch away from the crown, to increase aeration.
Black leaf spots are caused by small mites called cyclamen mites, which devour parts of the leaves and stems of the delphinium plant. Application of insecticidal soap twice a week is quite effective in removal of these pests. Distorted, curly leaves are caused by aphids or plant lice. These insects feed on the plants in groups, thereby causing severe harm. Insecticidal soap can again be effective in treatment. Slugs and snails can also attack delphiniums. These creatures are most active during the night time. They feed on the leaves and stems, leaving holes in the foliage. You can pick them out by hand when you see them in action, or you can use baits to trap and kill them. One of the most commonly used baits to trap slugs is beer. You can pour some beer in a plate and keep it next to the delphiniums. Slugs will climb in and drown in the beer.
Powdery mildew is caused by fungi that grow in moist, humid conditions. Wilting, patchy leaves are a common sign of mildew, and can be avoided to a great extent by providing good air circulation, and by keeping the foliage dry. Avoid wetting the foliage when watering the plant. You can treat powdery mildew by applying a fungicide with sufficient sulfur content. Apply regularly, according to instructions, as mildew can be a recurring problem if not treated effectively.
You can prevent most of the above ailments by providing good drainage and air supply. If the plant becomes infested by disease, remove and promptly dispose of the diseased parts. In case of root rot, the entire plant will most probably have to be disposed of as well. Avoid planting any seedlings or newer plants in the same spot, as the disease can spread easily through the soil. Also try to transplant close-by healthy plants to another location. Avoid using cuttings from diseased plants for propagation.