Butterfly bushes (Buddleia species) are susceptible to fungal infection that is often easily avoided through proper care and culture. As the name suggests, this aesthetically pleasing shrub attracts butterflies to the garden. Butterfly bush disease not only impacts the visual interest of your garden but decreases the availability of nectar to butterflies.
Appropriate care results in vigorous plants that are more likely to resist and fight off disease than weakened or stressed plants. For healthy butterfly bushes, plant in locations that offer full sunlight; full sun exposure promotes flowering. Butterfly bushes thrive in extremely well-drained soil with a pH of 6.0 to 7.0. Waterlogged soil can lead to fungal infection. Plant in USDA Hardiness Zones 5 to 9.
Downy mildew is a fungal infection caused by a variety of fungal pathogens that often use butterfly bushes as host plants. Due to the diverse nature of this infection, similar problems occur on many different plants as a result of different fungi. The fungus Peronospora hariotii is the most common culprit of downy mildew on butterfly bushes, notes Clemson University Extension.
Symptoms and Damage
The cultivar Buddleja x Weyeriana is most susceptible to downy mildew disease, though all butterfly bushes are vulnerable. During periods of cool temperatures and high moisture, fungal pathogens spread sporadically through infested water. Spores germinate in environments that remain wet for long periods of time; when leaves remain wet, the potential for infection is high, according to Purdue University Extension. Look for lesions and grayish spots on leaf surfaces that take on an angular appearance, as well as falling leaves. The name "downy mildew" suggests the common symptom of a gray mildew-like growth on plant surfaces. Damage ranges from mild to severe.
For natural control, sanitize pruning tools between each cut and from one plant to the next to prevent the unintentional spread of fungal disease. Remove and destroy infected plant parts to inhibit further disease development. Because the main cause of infection is wet leaves, make sure your butterfly bushes are well spaced to allow sun exposure to all leaves and avoid overhead irrigation that creates excessively wet conditions, advises Purdue University Extension.
Chemical control is effective as a preventive option. Application of fungicides after infection has taken place will not cure your butterfly bush of downy mildew. For chemical management of butterfly bush disease, the Purdue University Extension explains that the necessary chemicals are not suitable for home application. Contact a professional to determine the proper chemicals and for safe application. When in doubt, contact a local county extension agent for advice. Fungicides must be applied every week to 10 days, so set up a control program before you see signs of downy mildew.