Bottlebrush Diseases

Shrubs that are labeled low maintenance are great for busy gardeners who might not have all of the time they would like to devote to their garden. Low maintenance doesn't only refer to routine care of plants, but also to a higher amount of pest and disease resistance for a particular variety or cultivar of ornamental plant. Both evergreen and deciduous Bottlebrush shrubs are virtually free of most common garden diseases. Callistemon is an evergreen shrub native to Australia that is tough as nails and a reliable bloomer when planted in USDA zones 9-11. Bottlebrush Buckeye is a deciduous shrub native to damp woodland areas of the southeastern United States and grows well in USDA zones 5-8. Although both types of Bottlebrush shrubs require different growing conditions, they are susceptible to some of the most common garden diseases.

bottlebrush-bloom.jpg

Anthracnose Fungal Leaf Spot

Anthracnose is caused by a fungus that attacks the soft tissue of the foliage and stem tips. Dark spots will appear and look as if they are expanding all across the leaf. The newest growth is typically attacked first, with the spores spreading from the ground upward into the canopy of the shrub. The most mature leaves may never show signs of disease and may remain on the shrub. Premature leaf drop often occurs in deciduous shrubs that are affected. When the damaged foliage falls, it can infect other healthy growth. Often shrubs that are heavily infected with Anthracnose will die completely due to the lack of leaves and inability to photosynthesize.

Identifying Anthracnose Fungal Leaf Spot

The leaves of shrubs affected by Anthracnose Fungal Leaf Spot will begin to rapidly turn yellow or tan with dark spots of spores appearing. The leaves will dry out and turn brown, eventually falling from the shrub. The microscopic spores overwinter in any leaf litter left at the base of the shrubs and reinfect plant tissue as the weather warms and watering of the shrubs resumes in the spring.

Treating Anthracnose Fungal Leaf Spot

Chemical controls for Anthracnose are not reliable and need to be reapplied continuously for the life of the shrub. Cultural practices are a much better way of dealing with an Anthracnose outbreak. Always choose plants that are resistant to leaf spot diseases and are healthy to begin with. Half-dead plants from the bargain bin may seem like a good challenge to bring back to life, but often remain weakened and sick, making them a target for diseases and pests. Fungi typically need damp conditions to be mobile and move from plant to plant. Overhead watering or overwatering will make shrubs much more likely to be infected. Using soaker hoses or drip irrigation methods protects foliage from fungal outbreaks.

Powdery Mildew

Powdery Mildew favors conditions that are hot and dry during the day and cool at night, the opposite of what most other fungal diseases favor. Shrubs with large, flat leaves such as the Bottlebrush Buckeye, are particularly at risk. The telltale white or silvery dusting starts on only one or two leaves and can quickly spread to the rest of the plant within days. Powdery Mildew fungus is often not plant specific and can be spread between plant families in a garden. Young growth can be heavily infected, leading to disfigured leaves and stems. The mildew keeps the plant from properly photosynthesizing and eventually will weaken it.

Identifying Powdery Mildew

The white or silvery fungi cover the tops and bottoms of leaves in a thin, powdery layer and is easily brushed off by raindrops or overhead watering to infect more parts of the same plant or surrounding plants. Infection can spread quite rapidly.

Treating Powdery Mildew

Powdery Mildew needs to be treated rapidly to prevent it from spreading quickly. Remove any affected foliage that is within reach. Large shrubs will be more difficult to treat this way. Fungicides can be used, although they only protect portions of the plant that are not affected by the fungus and must be reapplied on a regular schedule through the growing season. Over-fertilizing with nitrogen-rich formulations can encourage dense foliage that is an easy target for Powdery Mildew. Allow plenty of space between shrubs to encourage good air circulation. Remove any fallen leaves from around the shrub and discard them in the trash or burn them. Composting is not recommended in home composting. 

Fungal Rust Spot

Less common on Bottlebrush shrubs is a fungal Rust Spot. This disease is very easily identified by small orangey-yellow spots that start on older leaves and spread to the new growth. Rusts often occur after a period of warm and damp weather and are more likely to be seen on shrubs later in the summer. Fungal Rust Spot is plant specific and rarely infects other plant families that are nearby. Even though the spores may stay in the garden for a very long time, with good garden hygiene, Bottlebrush shrubs can recover enough to survive for many years. 

Identifying Fungal Rust Spot

Small, powdery, rust-colored spots appear on older leaves. The fungus spreads to other leaves, yellowing them first before the spores appear. Heavily infected leaves will begin to brown and dry up, sometimes falling from the shrubs. After some time, the shrub will decline in growth and may stop producing flowers and new foliage. 

Treating Fungal Rust Spot

Removal of infected plant material is essential in controlling Fungal Rust Spot outbreaks. Burn or throw away any prunings. Rake up and dispose of any fallen foliage around the shrub to keep the fungus from overwintering. Overhead watering spreads the disease quickly and should be avoided. Any affected branches should be pruned out of the shrub during the dormant season. Fungicides can be used to protect undamaged parts of the shrub. Only use a fungicide specifically listed for use on Bottlebrush and follow all of the manufacturer's directions for use.

Witches Broom

Witches Broom is also referred to as Twig Gall and is more likely to infect the evergreen Bottlebrush shrub, Callistemon. Witches broom is caused by a fungus that enters parts of the shrub, causing growth to be non-typical and sometimes faster than normal. Unusual growth can happen at the trunk, branches, and tips of flowers and is rarely a reason for major concern. As long as the growths are removed while small, the shrub will make a full recovery and continue to grow for many more years.

Identifying Witches Broom

The bundles of growth are easy to spot and will appear foreign to the shrub. Flowers with Witches Broom appear to have a second flower ready to emerge from the end or side. Branches and trunks of the shrub will look like knobbly stems with no foliage or buds for foliage. The ends of existing stems may have masses of large, woody nodules. 

Treating Witches Broom

Witches broom is very easily treated with regular pruning of infected plant material. Sanitize all pruning tools before and after cutting Witches Broom. A solution of 1 part bleach to 10 parts water is strong enough to kill off any fungi. Do not compost infected plant material in a home compost pile. The temperatures needed to kill off pathogens in compost are not typically reached when composting at home. 

Verticillium Wilt

The last disease generally only infects Callistemon spp and is a soil-borne fungus that is very plant specific. Verticillium Wilt enters roots that are weakened by overwatering and causes the plant's internal tissues to die off. This disease restricts how the plant functions, causing parts of the plant to die quickly while possibly allowing other parts to live for a time.  

Identifying Verticillium Wilt

Diagnosing Verticillium Wilt can be difficult, although there is one telltale sign. The fungus enters the plant and immediately travels upward. If the fungus enters the shrub through a damaged branch, then all of the dieback would be expected to be above the point of entry. When cutting into infected branches there will also be dark streaking of the vascular tissue. 

Treating Verticillium Wilt

The only treatment for Verticillium Wilt is complete removal of the infected shrub. Because this fungus is plant specific, do not replant the area with more Bottlebrush shrubs. Choose shrubs and trees that are proven resistant to Verticillium or cultivars that are certified to be resistant. Avoid over-watering and over-fertilizing, which can both severely weaken shrubs making them more susceptible to infection. 

Bottlebrush Shrub Disease Chart

Disease

Identifying

Treating

Anthracnose Fungal Leaf Spot

Yellowing leaves with dark spots

Remove and destroy infected plant material

Powdery Mildew

Silvery or white dust on the tops or bottoms of leaves

Remove and destroy affected plant material

Fungal Rust Spot

Rust-colored spots on older foliage

Remove and destroy affected plant material

Witches Broom

Unusual growth on the trunk, branches or ends of the flowers

Prune out affected plant material at least 6 inches below the growth

Verticillium Wilt

Portions of the shrub die off while other parts seem unaffected

Remove the shrub and do not replant with a plant in the same family

Sources

"Bottlebrush—Callistemon spp. Family Myrtaceae (Myrtle family)." University of California Integrated Pest Management. ipm.ucanr.edu

"Callistemon citrinus: Red Bottlebrush." University of Florida Environmental Horticulture. hort.ifas.ufl.edu 

Robbin Small Profile Pic

 Author Robbin Small - Published 5-15-2023