Top 10 tree diseases

Features - Tree Care

Be on the lookout for these diseases that are creeping up.

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1. Anthracnose

Anthracnose is an umbrella term for multiple fungi that attack leaves, twigs, flowers and fruits of many trees and shrubs. It can be found on many species throughout most of North America. Anthracnose on most species is cosmetic, which means generally not a cause for concern. Some leaf necrosis followed by defoliation will occur, but the plant typically recovers. There are two species that anthracnose can cause a twig blight in addition to the defoliation, and those are sycamore and flowering dogwood. Because of this, sycamore and flowering dogwood can decline significantly due to this disease and are therefore recommended for management.

All images courtesy of The Davey Institute at The Davey Tree Expert Co.

For management of sycamore anthracnose, begin foliar applications of fungicides two weeks before bud break. Alternatively, trunk inject with a systemic fungicide. In the case of dogwood anthracnose, foliar applications must begin at bud break and continue all through the growing season to protect the tree. Good sanitation practices can also help reduce disease inoculum.

2. Apple scab

Apple scab is an early-season fungal leaf disease that affects crabapples in the urban landscape. Some crabapple cultivars are more tolerant to the disease than others; however, currently there is no fully resistant cultivar available. Symptoms of apple scab are velvety brown lesions on the leaves that are quickly followed by chlorotic leaves and defoliation.

While mostly aesthetic, homeowners may find apple scab objectionable. It can be managed with fungicide applications beginning at budbreak.

3. Cedar rusts

Cedar rusts are common fungal diseases of rosaceous plants, such as hawthorn and crabapple. As with apple scab, some cultivars and species are more susceptible than others. Rust diseases require junipers as an alternate host; however, removal of this host to break the life cycle is not recommended as rust spores can travel from far distances. Symptoms of rust on key host plants are orange or rust-colored leaf spots that form on hawthorn and crabapples in the spring. Particularly in hawthorn, twig cankers and galls can develop and cause dieback. On the juniper species spore-producing structures form on the branches but are typically not harmful to the health of the plant.

Due to this, disease management is typically only recommended on the crabapple and hawthorn species. This disease can be managed with fungicide applications on these hosts beginning at budbreak or when orange spore masses develop on junipers. For the juniper host, pruning out rust galls when noted can help with aesthetic value of the plant.

4. Diplodia tip blight

Diplodia tip blight is a very common fungal disease of Austrian and other pines. This disease infects when candles and needles begin to expand in the spring. Key symptoms include dead shoot tips with needles half elongated, the presence of resin and witches brooming. Black fruiting structures are present at the base of needles and on cones.

Management includes pruning to increase airflow and applying fungicides beginning at bud swell.

5. Dothistroma needle cast of pines

Dothistroma needle cast is a fungal disease that causes a premature defoliation in mature trees and stunting and death of young trees. It can be found in more than 35 pine species and hybrids throughout North America. This disease is specifically prevalent in non-native species of pines.

Symptoms of this disease are yellow or red bands on needles from the previous years’ growth, followed by necrosis. Even though the symptoms of this disease appear on older needles, it does affect during budbreak.

Therefore, for management, begin applications of a fungicide at budbreak. Pruning out severely infected branches can also help in managing the disease.

6. Lethal Bronzing of palms

Lethal Bronzing of palms is a disease caused by an unculturable bacterium that has no cell wall called a phytoplasma. This organism is spread by a vector. The vectors are planthoppers which pierce and suck plant tissue and transmit the pathogen from plant to plant. Primarily found in the state of Florida and Texas, this pathogen affects mostly Phoenix palms; however, it can infect others as well. (16 confirmed species).

The pathogen infects via the phloem of the host and cannot survive outside a plant or insect; therefore, it cannot be mechanically transmitted by pruning tools or infected roots touching new roots. This organism causes problems with vascular flow. Symptoms from LB are variable: if fruit is present, the first symptom is generally premature fruit drop. If fruit has not set, inflorescences will become necrotic. The spear leaf (newest, unfolded frond) will collapse, then the middle ring of fronds will display a reddish-bronze color (hence the name). Length of time between infection and symptoms is approximately four to five months. Symptom progression occurs at varying rates in different palm species.

Management includes removal of infected palms and preventative injection of remaining viable palms in the area with an antibiotic. These treatments will need to be administered every three to four months for the life of the palm. If a palm looks healthy but has received a positive test through tissue sampling, it is considered lost and should be removed immediately.

7. Oak wilt

Oak wilt is a fungal pathogen that causes vascular problems for oak trees. The fungus can be transmitted two ways: vectored by a sap beetle or through root grafting. The tannins released from open wounds attract the sap beetles. Therefore, pruning oaks should be avoided during the spring and summer. Fungal mats form under the bark of infected trees and the sap beetles are also attracted to those, which is where they pick up spores for transmission. It is important to note that 90% of infections occur from root grafting and only 10% vector. Trees can transfer the fungus through the vascular system of one tree to another if their roots fuse together.

Oak wilt, Bretziella fagacearum, causes disruption of the xylem tissue, causing symptoms such as leaf discoloration, leaf necrosis, tip dieback, flagging, vascular discoloration and, ultimately, death.

Red oaks and live oaks have no curative treatment options once infected; however, white oaks can be treated if less than 20% of their canopy is showing symptoms. All oaks can be treated preventatively with injections of propiconazole. If an oak tree is infected and there are other oaks surrounding it, you can create a root graft barrier and inject remaining trees with the proper fungicide to prevent further spread.

8. Powdery mildew

Powdery mildew is a fungal disease caused by different species of fungi that grow on the outer, upper leaf surface of many trees and shrubs. This disease does not directly penetrate the foliar tissue; however, it can cause leaf distortion, premature defoliation and decrease in photosynthesis production. The fungus can be easy to identify due to the white mycelium growing directly on the upper surface of the leaves. It is mostly an aesthetic problem and will not cause the tree too much stress.

There are fungal foliar sprays if infection is heavy, but they are not always necessary. Another option to decrease disease pressure is to slightly prune dense canopies. This can increase airflow, which creates a less desirable environment for the fungus to grow.

9. Armillaria root rot

Armillaria root rot is a group of fungi that cause decay in infected trees. These species will infect when trees are already stressed and susceptible to secondary problems. Armillaria is a leading contributor in oak decline, a group of issues coming together to cause oak trees to die off.

Some Armillaria species can produce three different fungal signs- Rhizomorphs, mycelial mats and clusters of mushrooms. These signs are all helpful identification tools.

These fungi have two ways to cause infection- via spores and/or through roots via rhizomorphs.

It is argued that Armillaria can be curatively treated with phosphonates and/or growth regulators. However, the efficacy of these treatments is not clear. Preventative treatment can be as simple as general plant health care and avoiding wounding of trees.

10. Black knot

Prunus trees, such as plums and cherries, are coveted in our landscapes for their flowers and their fruit. Unfortunately, they are susceptible to a destructive fungus called black knot, Apiosporina morbosa. Black knot of prunus causes a unique canker that can block vascular movement through cankers/galls.

Black knot cankers/galls release spores in the spring and are either blown or splashed to a new host site. New growth on trees and shrubs is particularly susceptible to infection. Once infected, the twigs start to swell and turn an olive-green color. Due to the initial canker color lacking contrast with the natural color of tree twigs, most black knot infections are not noticed until the second year of infection. In the second season, the swollen cankers turn a dark black color, which is also when they start to release spores.

If black knot is noticed before the cankers are out of control, pruning out of the fungus is possible and effective. When pruning out cankers, it is important to prune at least six inches below the swelling, but 12 inches is ideal. It is important to sanitize your tools between each cut, so you don’t actively spread the fungus. When pruning is complete, make sure to remove all debris. If black knot is detected too late, meaning there are too many cankers to prune out, then there are no treatment options.

The authors are technical advisors for The Davey Institute at The Davey Tree Expert Co.