From our perspective as Delphinium plants, we are majestic perennials that bring striking vertical beauty to gardens. However, our tall flower spikes and lush foliage make us susceptible to various pests and diseases that can compromise our health and splendor. Understanding these threats is key to helping us thrive.
Several insects find our succulent growth and sap particularly appealing. The most frequent invaders are aphids. These small, soft-bodied insects cluster on our tender new shoots and the undersides of our leaves, sucking out our vital juices. This feeding weakens us, causing leaf curling and distortion, and they excrete a sticky substance called honeydew that encourages sooty mold. Another significant pest is the cyclamen mite. These microscopic mites infest our buds and young leaves, causing severe stunting, blackening, and distortion that prevents our flowers from opening properly. Slugs and snails are also a major nuisance, especially for our young seedlings and new growth in spring. They chew ragged holes in our leaves, often overnight, significantly damaging our photosynthetic capabilities.
Fungal and bacterial diseases pose a serious threat to our well-being, often thriving in the same cool, moist conditions we prefer. Powdery mildew is a common fungal issue that appears as a white, powdery coating on our leaves and stems. It is unsightly and inhibits our ability to photosynthesize effectively. More severe is Black Spot, caused by the bacterium Pseudomonas syringae pv. delphinii. This disease causes irregular black spots on our leaves, which can coalesce and lead to rapid wilting and collapse of entire stems from the base upward. Lastly, Crown and Root Rot, often caused by fungi in soggy, poorly-drained soil, is frequently fatal. It causes our crown tissue at the base to turn soft, brown, and mushy, leading to the sudden collapse and death of the plant.
To combat aphids, a strong spray of water can dislodge them, or you can introduce beneficial insects like ladybugs. For severe infestations, horticultural oil or insecticidal soap is effective. Controlling cyclamen mites is difficult; severely infested plant parts should be pruned out and destroyed immediately. Miticides may be necessary but should be used with caution. For slugs and snails, hand-picking at night, setting traps, or using iron phosphate-based baits can protect our foliage.
For powdery mildew, improve air circulation around us by spacing plants adequately and watering at the base to keep leaves dry. Fungicidal sprays containing sulfur or potassium bicarbonate can be used. There is no cure for bacterial Black Spot; prevention is paramount. Avoid overhead watering, space us properly for good air flow, and sterilize tools between plants. Immediately remove and destroy any infected plant material to prevent spread. Preventing Crown Rot involves ensuring we are planted in very well-draining soil and that our crowns are not buried too deeply. Avoid overwatering, especially in heavy clay soils.
