Pathogens
Four species of Ophiognomonia (O. alni-viridis, O. hiawathae, O. intermedia and O. michiganensis), Cryptocline betularum, Marssonina betulae, and Colletotrichum are known to occur on birch (Betula) in the United States (Sinclair and Lyon 2005, Walker et al. 2012).
Hosts
Based on samples submitted to the UMass Plant Diagnostic Lab, river birch (B. nigra), paper birch (B. papyrifera), gray birch (B. populifolia), and European white birch (B. pendula) are the most common hosts of birch anthracnose in managed landscapes. The severity of the disease on each host is dependent upon which anthracnose pathogen is present.
Symptoms & Signs
Birch anthracnose is not considered a serious disease but can be damaging in certain years. Symptoms typically appear as dark-colored, circular leaf spots that coalesce over time to create large necrotic blotches. Leaf yellowing and premature leaf shedding early in the growing season occur when environmental conditions are ideal the disease are ideal. At times, significant portions of the canopy can be defoliated in late spring and early summer. Because several fungal species across four genera (Colletotrichum, Cryptocline, Marssonina and Ophiognomonia) are responsible for the disease, symptoms vary depending on the specific host–pathogen combination. Infections are often restricted to the foliage on samples from landscape ornamentals. However, Marssonina and Colletotrichum can infect woody tissues. A study on the pathogenicity of M. betulae found that the fungus could cause sunken twig cankers and canopy dieback on European white birch (Green and MacAskill 2007). By mid-summer, when conditions are warmer and drier, many anthracnose fungi are dormant but can reemerge during the autumn season when conditions become cooler and wetter. Birch exhibit indeterminate growth rates and can produce new shoots and foliage if environmental conditions are conducive. When anthracnose outbreaks lead to premature leaf shedding, the hope is that trees will continue to produce new foliage to compensate. Many otherwise healthy trees often have the reserves to flush new growth following an anthracnose outbreak and may be largely unaffected. However, leaf shedding due to anthracnose early in the growing season combined with mid-summer drought stress can result in significant stress for landscape birch.
Management
Anthracnose fungi can be challenging to control because outbreaks are difficult to predict. The disease is most severe when mild and persistently wet weather coincides with new leaf development in the spring. Outbreaks are often locally abundant but scattered across the landscape. Because anthracnose fungi overwinter on discarded leaves from the previous season, thoroughly collect and destroy all fallen leaves to reduce inoculum at the site. Pruning to increase light penetration in the canopy and improve air flow can also reduce disease severity. Chemical management is not usually warranted, but foliar sprays can sometimes be effective when performed early in the season just before bud break and on regular intervals when new leaves are developing. Fungicides labeled for use against anthracnose include: azoxystrobin, copper salts of fatty and rosin acids, copper hydroxide, copper sulfate, mancozeb, metconazole and thiophanate-methyl + mancozeb. In general, anthracnose fungi don't present a major threat to established, healthy trees. However, in conjunction with other stresses, it can play an important role in tree death.
Citations
Green S., and MacAskill, G.A. 2007. Pathogenicity of Marssonina betulae and other fungi on birch. Plant Pathology 56, 242-250. https://doi.org/10.1111/j.1365-3059.2006.01554.x
Sinclair W.A. and Lyon, H.H. 2005. Diseases of Trees and Shrubs, 2nd edn. Cornell University Press, Ithaca, NY.
Walker, D.M., Castlebury, L.A., Rossman, A.Y. et al. 2012. Phylogeny and taxonomy of Ophiognomonia (Gnomoniaceae, Diaporthales), including twenty-five new species in this highly diverse genus. Fungal Diversity 57: 85–147. https://doi.org/10.1007/s13225-012-0200-y