Anthracnose

Disease Profile and Management

Disease Profile

What is Anthracnose?

Anthracnose is a leaf blight disease caused by a fungus that can cause leaf loss, branch dieback, and even death of trees in the worst cases. It spreads during cool, wet weather, remains dormant in hot, dry periods, and overwinters on plant material on and under trees.

Treatment recommendations range from simple plant maintenance to be done by homeowners to systemic chemical treatments including trunk injection by a pesticide applicator.

What types of trees are susceptible?

The most susceptible trees are sycamore, white oak, and dogwood. But ash, maple, elm, hickory, walnut, birch, and linden are also susceptible [3]. In the intermountain west, the most susceptible are maple, ash, sycamore, and oak [1].

Consequences and Severity

In most years, anthracnose is insignificant [1]. It can be tolerated in isolated incidences when trees are otherwise healthy. However, during years when extended cool, wet spring conditions occur, it can be very damaging [1]. Consecutive years with these spring conditions can further weaken trees and cause tree death if untreated [1].

Considerable defoliation, and sometimes complete leaf loss, can occur by late spring or early summer [2][3].
Dieback and wilting of small branches, and formation of cankers and witches brooms can occur [1][2].
Repeated infections can lead to major limb loss, crown dieback, and susceptibility to pests [3].

Disease life cycle

In cool, wet conditions, such as springtime, fungal spores spread disease, and are dispersed by wind and rain [1]. Spreading occurs the most when it’s raining and the temperature is between 50-68 degrees, and this can happen in spring or anytime during the growing season [2].

When spores land on new leaves, they germinate and create lesions. Spore-bearing structures, or acervuli, are produced on these areas of dead leaf tissue [1], and continue to spread the infection to other leaves. The disease can also spread from the leaves back through the twigs and cause branch dieback [3]. It survives during hot, dry weather by remaining dormant, and overwinters in fallen leaves, petioles, buds, fruit, twigs, and branches [1][2].

Diagnostic Clues

Symptoms first appear on leaves as small, water-soaked lesions along main leaf veins, between veins[1], and on margins [2]. These leaf spots enlarge and become dead tissue. They can be tan, reddish-brown, or black depending on the species of tree [1]. Dogwoods display tan lesions with red edges that often join together [3]. Symptoms on twigs begin as discolored, depressed areas in the bark, then splitting bark, and acervuli are visible as small, black dots on twigs [1].

On dogwoods, leaves may drop in spring, but be retained in fall [3], giving false hope of improvement. Repeated infection causes epicormic sprouting (water sprouts) on the trunk [3].

Sycamore leaves can be infected as they emerge from buds, resulting in blight of the entire leaf [1].

Management

Because Anthracnose is difficult to eradicate, we focus on management strategies to prevent or suppress the disease. There are different types of control and the most effective tactic is a combination of strategies [1].

Cultural and Mechanical Control

This is the starting point for managing Anthracnose, and for mild cases, may be enough to suppress the disease. Rake and destroy fallen leaves, prune infected branches, and maintain tree vigor with watering and fertilization [1]. We also can’t speak highly enough of mulching. Avoid using high nitrogen fertilizers [3], especially quick-releasing turf fertilizers within the root zone of susceptible trees. Plant resistant trees. Resistant sycamore cultivars include: Bloodgood, Columbia, and Liberty [1]. London Plane is a sycamore hybrid that is somewhat resistant [3].

Chemical Control

Systemic fungicides

Systemic treatments can be applied by trunk injection, trunk/basal bark spray, soil drench, and even foliar spray. They are absorbed and transported throughout the entire tree by its vascular system, and remain effective for 1-3 years. They are generally much safer for the environment and the applicator.

Treating high-value trees by systemic injection is recommended to prevent death [1]. Systemic injection treatments in late summer or fall provide the best disease suppression, but can also be done in spring with some effectiveness [3]. The two products we use are PHOSPHO-jet and Propizol, developed by Arborjet.

PHOSPHO-jet inhibits fungal cells and increases a tree’s natural defense system which includes stronger plant cells, root development, and quicker recovery time [3]. Propizol is more direct and aggressive, and best for areas with high spring moisture conditions, and chronic or severe infections [3].

Propizol should only be applied in the fall, when trees are in full leaf and actively growing, to prevent or slow the spread of infection in the following spring [3], but PHOSPHO-jet may be applied in fall after leaf coloration, or in spring prior to twig infection [3]. Combining PHOSPHO-jet (trunk injection) + Propizol (bark spray) as a fall treatment provides the best protection [3]. After treatment, trees may still defoliate, but are more likely to recover when treated with PHOSPHO-jet [3].

Contact fungicides

Contact fungicides are less expensive and more readily available, but much more limited than systemics. Even though some products have a systemic component, they are really only effective at suppressing disease when the chemical makes direct contact. On large trees this is impractical. On small trees, timing is crucial [1], and they need to be sprayed until the point of runoff, increasing the hazard to applicator and environment. A second or third treatment is needed if cool, wet weather persists, every 10-14 days [2][4].

Contact fungicides available for anthracnose include Daconil (containing chlorothalonil) and Spectracide Immunox(containing myclobutanil) [4], among others.

Biorational fungicides

Arborjet ECO-1 FRUIT & VEGETABLE SPRAY is a botanical pesticide that prevents fungal attack of plant tissues by disrupting the cell wall of spores and hyphae and is OMRI Listed® for organic use [3].

Arber Bio Fungicide is a microbial-based fungicide. It uses beneficial bacteria to create “a protective shield over the surface of leaves and roots”, and it is also OMRI Listed® for organic use [5].

References

1. Anthracnose of Ornamental Trees. https://extension.usu.edu/pests/ipm/notes_orn/list-treeshrubs/anthracnose

2. Anthracnose–Apiognomonia spp.; Discula spp.; Kabatiella spp.

https://extension.usu.edu/pests/ipm/ornamental-pest-guide/diseases/anthracnose

3. Arborjet. https://arborjet.com/problems_solutions/anthracnose/

4. Dormant Oil, Sycamore Scale and Anthracnose. https://pestadvisories.usu.edu/2020/03/17/dormant-oil/

5. Arber Bio Fungicide. https://growarber.com/products/bio-fungicide