Anthracnose—Various fungi, including Apiognomonia, Colletotrichum, Discula, Glomerella, Gnomonia, Marssonina, and Stegophora spp.
Anthracnose is a group of diseases resulting from infection by various fungi. Often called bud, leaf, shoot, or twig blight, damage is most severe when prolonged spring rains occur during or shortly after new plant growth develops.
Identification
Anthracnose symptoms vary with the host's growth stage and species, weather, and time of year. Small black, brown, or tan spots appear on infected leaves of some hosts, including elm and oak. Dead leaf areas may be more irregular on hosts such as ash.
Chinese elm anthracnose, Stegophora ulmea, also called black leaf spot, is a serious disease in coastal areas of California. Fungal infection causes foliage to develop irregular, black, tarlike spots and leaves drop prematurely. Chinese elm anthracnose produces especially large cankers that can weaken, girdle, or kill limbs and trunks.
Dogwood anthracnose, Discula destructiva, causes leaves and flower bracts to develop large brownish lesions with well-defined, dark, discolored margins that may be grayish or purple. Infected leaves may drop prematurely or dead gray leaves may remain on twigs in the canopy overwinter. Dogwood anthracnose can severely canker branches, especially in the lower canopy, causing branch dieback and sometimes death of the entire tree.
Sycamore anthracnose, Apiognomonia veneta =Discula platani, kills sycamore shoots and leaves. Anthracnose lesions commonly develop first around the major leaf veins, but scattered shoot terminals and entire leaves and groups of leaves are often killed.
Anthracnose fungi overwinter primarily in cankers and lesions in infected twigs. On evergreen species such as Chinese elm and live oaks, the fungi can occur year-round on leaves as well as woody parts. Spores are produced when infected tissue is wet. Spores spread by splashing and windborne rain to infect new twigs and foliage. Spore production and infection continue as long as new growth and wet conditions occur together. Once foliage matures and weather becomes warm and dry, disease development slows or stops.
Damage
Anthracnose does not seriously harm most hosts. Exceptions are when defoliation occurs repeatedly, and certain species at some locations where branch dieback and cankering are extensive.
Anthracnose fungi discolor leaves or kill portions of leaves, entire leaves, small groups of leaves, and twigs scattered throughout the canopy. Small branches can be cankered, girdled, and killed on more susceptible hosts, including some species of ash, dogwood, elm, oak, and sycamore. The resulting regrowth from lateral buds can give trees a gnarled or crooked appearance.
Solutions
Where prolonged spring rains or foggy conditions are common, avoid planting especially susceptible species. For example, the Bloodgood, Columbia, and Liberty cultivars of London plane are less susceptible or resistant to sycamore anthracnose, but Yarwood is commonly damaged.
Prune and dispose of infected twigs during the fall or winter where practical on small trees so overwintering infections are pruned off. Rake up and dispose of fallen leaves and do not mulch around hosts using fresh or uncomposted leaves or flowers from plants susceptible to that same anthracnose. Increase sunlight and air circulation (for example, by pruning nearby plants to reduce shade and spacing new plants far enough apart).
Prevent sprinkler wetting of foliage. Provide good cultural care and otherwise minimize stress to improve plants' ability to tolerate anthracnose damage. See Pest Notes: Anthracnose for more information including specific fungicide recommendations. |
Ash anthracnose leaf dieback
Sycamore leaf and shoot dieback
Chinese elm black leaf spot
Oak anthracnose midvein necrosis
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