How to Manage Pests
Pests in Gardens and Landscapes
Anthracnose is a term used to loosely describe a group of related fungal diseases that typically cause dark lesions on leaves. In severe cases it may also cause sunken lesions and cankers on twigs and stems. Anthracnose affects many deciduous and evergreen trees and shrubs and can also infect vegetables, flowers, fruit, and turfgrass in some regions in California.
Because the disease often starts on leaves and twigs of trees, it is sometimes called leaf, shoot, or twig blight. The list of fungi that cause anthracnose is extensive, with each species typically attacking one or only a few closely related host species. Some of the more commonly encountered species in California are listed in Table 1.
IDENTIFICATION AND DAMAGE
Anthracnose symptoms vary by plant host and due to weather conditions. On landscape trees, the fungi infect developing shoots and expanding leaves. Small beige, brown, black, or black spots later appear on infected twigs of hosts such as elm, oak, and sycamore. Dead areas on leaves can be more irregular on hosts such as ash, maple, and willow, while sycamore and oak anthracnose lesions typically develop along major leaf veins.
If leaves are very young when infected, they can become curled and distorted with only a portion of each leaf dying. Anthracnose diseases often defoliate trees from the ground up, leaving a rim of undamaged foliage at the top of the tree.
Mature leaves are generally resistant to infection, but when conditions are favorable, spotty lesions can occur. Heavily infected leaves fall prematurely throughout the growing season, and sometimes trees become completely defoliated. New leaf growth usually occurs after leaves drop prematurely.
In severe cases, notably on Chinese elm and some ash cultivars, anthracnose can sometimes attack and kill twigs and branches. Cankers—sunken, infected areas with swollen edges—may develop on twigs, branches, and the trunk, occasionally resulting in girdling (the destruction of the nutrient and water conducting tissues all the way around a branch or twig) and dieback. Regrowth from lateral buds can give branches a gnarled or crooked appearance. Anthracnose doesn’t seriously harm trees unless defoliation, branch dieback, or cankering occurs every year. In California, anthracnose rarely causes permanent damage to trees except for Chinese elm trees, which can develop large branch and trunk cankers, especially in areas with relatively higher humidity, in trees with particularly dense, compact canopies, or both.
On landscape plants, anthracnose fungi occur primarily on leaves and twigs. On deciduous trees, these fungi overwinter in infected twigs or dead leaf litter. In spring, the fungi produce numerous microscopic spores that spread via splashing rain or irrigation water to new growth where they germinate, entering leaves and newly expanded twigs. If moist conditions prevail, a successive generation of spores is produced from the infected parts of new leaves.
On irrigated evergreen species such as Chinese elm and coast live oak, the fungus can occur year-round on leaves and twigs, but on most deciduous trees the progress of the disease slows and becomes negligible during hot, dry weather.
With careful management, some cultivars of susceptible landscape plants can be aesthetically pleasing despite the presence of anthracnose. For new plantings, choose cultivars that are resistant to anthracnose-causing fungi, and space the plants far enough apart to maximize air circulation and increase sunlight.
Some pesticides are available to prevent anthracnose infections, but they don’t reliably control the disease. Preventative treatments have been shown to work best on Modesto ash; see the section on Chemical Control.
Once symptoms develop or become severe in a growing season, anthracnose can’t be effectively controlled.
Environmental factors also play an important role in managing anthracnose. Monitor current conditions such as rain and irrigation to determine if either favors anthracnose development. Dry spring weather could mean that disease management isn’t necessary. A wet spring or an irrigation system that wets the foliage could result in a disease outbreak that might warrant using control measures.
Avoid planting highly susceptible species including Modesto ash (Fraxinus velutina ‘Modesto’), American sycamore (Platanus occidentalis), and some cultivars of the London plane tree (Platanus x hispanica). California sycamore (Platanus racemosa) is very susceptible to anthracnose which disfigures the tree, giving it a twisted appearance. Trees usually survive these attacks but may lose apical control temporarily, resulting in increased lateral growth and decreased height. Among ash trees, the cultivars Moraine, Raywood, and the Evergreen Ash (Shamel) are more resistant to anthracnose than others. For Chinese elm, plant the more resistant Drake cultivar instead of True Green or Evergreen. Table 1 shows the relative susceptibility of some landscape trees to anthracnose.
When planting London plane, also called plane tree, decide whether anthracnose or powdery mildew resistance is more needed based on prevailing environmental conditions at that site and the planned pruning method; see Cultural Practices. For instance, the Bloodgood London plane tree cultivar is resistant to the fungus that causes anthracnose but not to the fungus that causes powdery mildew. On the other hand, the anthracnose-susceptible cultivar Yarwood is resistant to powdery mildew.
Pruning. Prune and destroy or bury infected leaves, twigs, and branches during fall or winter. Severe pruning of larger diameter branches is not a good practice for most trees, because it triggers bushy watersprouts, which are poorly attached to the trunk and are susceptible to diseases such as powdery mildew. To stimulate vigorous growth of severely infected trees, fertilize after the leaves open and spring rains have stopped. Avoid irrigation systems that wet leaves.
Sanitation. Rake and dispose of fallen leaves and twigs during the growing season and in fall. Prune during winter to increase air circulation in the canopy and remove the previous season’s dead and diseased twigs and branches.
When planting, space the plants far enough apart to maximize air circulation and increase sunlight, both of which facilitate faster drying of leaf surfaces when trees are fully grown.
Sycamores are often pollarded. Pollarding is a practice where trees are severely pruned to remove all of the previous year’s growth. Pollarding isn’t recommended for most trees but can be useful for London plane trees to control anthracnose since the practice removes all pathogen-infected shoots. However, pollarding increases susceptibility of London plane trees to powdery mildew because it stimulates the growth of new shoots and foliage. If you wish to annually pollard London plane trees, the cultivar Yarwood is a good choice, because it is highly resistant to powdery mildew.
Several fungicides are available that provide varying degrees of anthracnose control on Modesto ash. Fungicides can protect only healthy tissue and don’t eradicate existing infections. Complete spray coverage and timing are crucial in preventing the disease. Thoroughly spray all new growth as buds begin to open in spring. Apply pesticides before rainy periods. If no rains are predicted, you can delay this application. If moist weather prevails, additional applications may be required at intervals of about 2 weeks to protect new growth.
The most effective fungicides for control are the protective fungicides containing chlorothalonil e.g., Daconil), copper sprays containing copper diammonia diacetate (e.g., Liquicop), propiconazole (e.g., Banner Maxx II), and the systemic fungicide thiophanate-methyl (e.g., Cleary’s 3336, for professional use only). Chlorothalonil and thiophanate-methyl offer the greatest control in Modesto ash. Always follow label directions.
Complete coverage of large, tall trees is difficult to achieve; spraying is not very efficient and might not be justified or feasible. While control might occur in some situations, anthracnose can return annually and warrant a continued, preventative spray program. Fungicide applications for anthracnose control generally work well on ash but their efficacy has not been proven on other species in California.
Notes about Table 1:
Dreistadt SH, Clark JK, Martin TL, Flint ML. 2016. Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide. 3rd Ed. UCANR Publication 3359. Oakland, CA.
McCain AH. 1983. Sycamore Anthracnose. UC ANR Publication 2618. Oakland, CA.
Nameth S, Chatfield J. 1996. Anthracnose Leaf Blight of Shade Trees . Ohio State Univ. Ext. Factsheet HYG-3048-96. Columbus, OH. (Accessed April 15, 2020).
Pataky NR. 1997. Anthracnose Disease of Shade Trees . Univ. Illinois. Ext. Rep. on Plant Dis. N. 621. Urbana-Champaign, IL. (Accessed April 15, 2020).
Sabaloni J, Hesketh K, McCain AH. 1982. Chinese Elm Anthracnose. UC ANR Publication 21322. Oakland, CA.
Sinclair WA, Lyon HH, Johnson WT. 2005. Diseases of Trees and Shrubs, 2nd Ed. Cornell University Press. Ithaca, NY.
Svihra P, McCain AH. 1994. Ash Anthracnose. UC Cooperative Extension, Marin Co. HortScript No. 2.
Pest Notes: Anthracnose
TECHNICAL EDITOR: K Windbiel-Rojas
Produced by University of California Statewide IPM Program
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