Butterfly Bush Diseases

Butterfly Bush is a popular ornamental shrub that blooms nonstop from spring to late summer. Not only do these lovely shrubs attract large numbers of butterflies, but their nectar also draws hummingbirds, moths, and many types of native bees and honeybees. For the most part, few diseases infect Butterfly Bush, and they are a low-maintenance plant. However, some fungal infections are more likely to strike, including Botrytis, Downy Mildew, Leaf Spot, Phytophthora Root Rot, and Rust. All of these fungi live in the soil naturally and only become an issue when plants are weak and the atmospheric conditions are right for them to easily spread. Early diagnosis and treatment for any of these diseases will ensure that your Butterfly Bush recovers and remains a vigorous grower in your garden for many years.

Downy Mildew

Downy Mildew often occurs when plants do not have enough air circulation either because their canopy is too dense or plants are planted too close together. Transmission is most common during humid and mild weather. Plants that have wet foliage for long periods of time are much more likely to develop this fungal disease. Watering early in the day and using drip irrigation or soaker hoses will help to avoid this problem. Many edible and ornamental plants can be affected, and the fungus can move between plants easily by overhead watering or using infected pruners or other tools without sterilizing them between uses. 

Identifying Downy Mildew

Grey, fuzzy patches of the mold grow on the undersides of the leaves, while the tops may have purple or dark brown spots. The fungus infects any green plant material, and affected areas will die if left untreated. 

Treating Downy Mildew

Cut out any infected leaves or stems. Improve air circulation by thinning any overly dense growth or moving plants farther apart. Avoid watering late in the day during humid weather. Watering early gives the plants time to dry fully during the daytime. Fungicides can be used if they are labeled specifically for use on Butterfly Bushes. Use as directed by the manufacturer, keeping in mind that sprays only protect uninfected areas of the plant and do not cure parts that have already been infected.  

Fungal Leaf Spot

Fungal Leaf Spot occurs on many different varieties of ornamental and edible plants. Most often plants that are weakened in some way, such as being under or overwatered, will be infected first. The fungus spores can overwinter on the shrubs or in any infected leaf litter not removed at the end of the growing season. When temperatures get warm enough and the humidity rises, the fungi produce spores, which can be washed to other parts of the plant or nearby plants by spring rains or overhead watering. Many generations of microspores can be produced if the conditions remain favorable during the summer months. 

Identifying Fungal Leaf Spot

Yellow, brown, or black spots appear on the bottom and top of affected leaves. Sometimes new shoots at the end of twigs will be infected, giving them a contorted look. Eventually, affected foliage will die and drop from the plant. Mature shrubs usually recover better than smaller, newly planted shrubs. Eventually, the growth may slow, leading to a complete decline of the plant. 

Treating Fungal Leaf Spot

Prune the affected portions of Butterfly Bush to slow or stop the spread of the spores from one part of the plant to another. Dispose of infected plant material in the trash. Clear away any fallen plant material so that spores cannot overwinter near the shrubs. Fungicides labeled for use specifically on Butterfly Bush may help protect unaffected portions of the shrub through the rest of the growing season. Always follow the manufacturer's instructions.  

Rust

Rust occurs when days are humid and warm, but not hot. The fungi are host specific, but can travel long distances by the wind. This infection stunts and eventually arrests plant growth. Flowering may not be affected immediately, but if left untreated, the whole plant will suffer and need to be removed. Weak and unhealthy plants growing in the wrong conditions will be more susceptible to infection. Plant Butterfly Bushes in a location with full sun and well-draining soil to keep the plants growing vigorously. 

Identifying Rust

Rust looks exactly as it sounds. Small, dusty, reddish-yellow or orange spots will appear on the lower portions of the plant and leaves. Eventually, the leaves will start to yellow and the rust spots will expand and grow together. 

Treating Rust

Rust spreads by rain or overhead watering and can be washed into the surrounding soil. Only water Butterfly Bush with drip irrigation or soaker hoses to avoid wetting the leaves. Affected plant material should be pruned as quickly as possible after diagnosis to keep the spores from spreading across the shrub. Proper air circulation is essential to keep Rust fungus from occurring. Do not feed Butterfly Bush with high-nitrogen fertilizers. Nitrogen encourages dense, but weak foliage that impedes air circulation in the shrub.

Botrytis

Botrytis blight is also commonly called gray mold and is easily spread during mild, humid weather. The mold spores attack injured portions of leaves, flowers, or non-woody stems and spread into healthy portions of the plant if left untreated. Botrytis is more common in potted or greenhouse plants that do not receive adequate air circulation around plants. Plants with Botrytis can make a full recovery and will have no permanent damage. Overhead watering and wet foliage make it easier for Botrytis to move from plant to plant. Using a drip irrigation or soaker hose system ensures that foliage remains dry while the roots have time to absorb more moisture.

Identifying Botrytis

Botrytis presents as spots on leaves that look like they have been soaked in water and are yellowing or brownish. Eventually, those spots grow grayish-brown mold spores and have a fuzzy look. The plant will begin to decline because of an impaired ability to photosynthesize.  

Treating Botrytis

The most basic way to treat Botrytis blight is by promptly pruning out affected foliage, stems, or flowers to prevent the spread of spores to healthy parts of the plant. Make sure to sterilize any pruners after use on affected plants. Wiping the pruners with a cloth dipped in alcohol is generally sufficient. For future prevention, prune back damaged wood so that fungal diseases such as Botrytis do not have a chance to infect your shrubs. Do not compost any infected plant material in your home compost. Homemade compost rarely reaches high enough temperatures to completely kill off the spores of Botrytis. 

Phytophthora Root Rot

Several fungi and bacteria that naturally live in the soil can cause root rot. Phytophthora root rot is one of the most common fungi to attack Butterfly Bushes. Plants growing in very wet or waterlogged soils are most susceptible to infection. Overwatering container-grown Butterfly Bush can also lead to rotting of the root system, although the rot is more likely to be caused by cultural issues than an actual fungus. The fungi can spread by moving infected soil or an infected plant from one location to another. Most often, the plants show few signs of infection until the root system is so heavily diseased that recovery is unlikely. 

Identifying Phytophthora Root Rot

The top of the shrub will begin to slow in growth and turn yellow or eventually brown as it dies. The roots will start to turn from a healthy creamy white color to cinnamon brown or even black as they die off. Digging up plants that are growing in overly damp conditions and inspecting the roots is the only way to diagnose the condition. Lower stems may also have dark brown or black streaking of the tissue.

Treating Phytophthora Root Rot

The best treatment is avoiding the situation altogether by ensuring that the location of your Butterfly Bush has proper drainage. Avoid growing plants that are not resistant to Phytophthora. Do not move an infected plant to another part of the garden. Digging up and disposing of the infected plant may be the best solution to prevent further infections.

Butterfly Bush Disease Chart

Sources: "Butterfly Bush (Buddleja sp.)-Diseases." Pacific Northwest Extension. pnwhandbooks.org

Author Robbin Small - Published 5-30-2023