Pathogen
Oak anthracnose is caused by the fungal pathogen Apiognomonia errabunda (previously known as Discula quercina) (Sogonov et al. 2007, Boroń et al. 2019).
Hosts
Many species of oak (Quercus) are infected, and regionally important species include: white (Q. alba), swamp white (Q. bicolor), scarlet (Q. coccinea), pin (Q. palustris), northern red (Q. rubra), and black (Q. velutina). White oak is particularly susceptible to the disease and generally suffers higher levels of damage compared to other oaks. However, at times, members of the red oak group can also suffer from serious anthracnose outbreaks. Apiognomonia errabunda can also be regularly found on beech (Fagus) and linden/ basswood (Tilia). Occasionally, it can also be found on maple (Acer), chestnut (Castanea), ash (Fraxinus), poplar/ cottonwood (Populus), sumac (Rhus), and mountain-ash (Sorbus) (Sogonov et al. 2007, Boroń et al. 2019).
Symptoms & Disease Cycle
Most often, oak anthracnose causes only minor damage to landscape oaks. However, after prolonged periods of wet weather early in the growing season, especially on members of the white oak group, damage can be severe. Symptoms first appear as water-soaked, blighted leaf margins or as blotches along primary veins as new foliage develops. Over time, lesions become dry, papery and gray-colored. Pubescent leaves can become distorted or shriveled and may be shed prematurely from the canopy. Mature leaves are more resistant to infection, having developed their thick, waxy cuticle and lesions on these leaves are often smaller in size. Apiognomonia produces the majority of its spores asexually from pads of fungal tissue that rupture through the surface of the leaf and petiole. The pads (known as acervuli) can be found on the upper or lower surface of the leaves, along the veins or midribs, while on twigs, they appear orange-brown and darken over time (Sinclair and Lyon 2005). Spores are spread mostly by wind and running or splashing rain water.
Twig infections are known to occur on white, black, red, and scarlet oak and may occur on other species as well, especially if wet conditions persist into mid-summer. Twig dieback may lead to secondary colonization by wood-boring insects (e.g. Agrilus) and twig cankering fungi (e.g. Botryosphaeria, Diplodia, and Coryneum). Disease outbreaks usually subside by mid-summer when conditions become warmer and drier. In autumn, when cooler weather returns, there is often resurgence in disease development as the pathogen exploits senescing foliage. Resting structures then develop, which allows the pathogen to overwinter. In the spring, spores are produced from surviving propagules and the cycle repeats. When healthy trees are defoliated early in the season, most have the reserves to produce a second flush of foliage and suffer only minor growth losses. When oaks are weakened by other stresses, such as defoliation by spongy moth or root disease from Armillaria, the effects of oak anthracnose are amplified.
Management
In general, anthracnose fungi don't present a major threat to established, healthy trees. However, in conjunction with other stresses, anthracnose can play an important role in tree death. Prune and discard dead stems and branches and thoroughly remove all fallen leaves in autumn and spring as they harbor the fungus and allow inoculum to remain at the site. Anthracnose infections typically begin in the lower canopy and progress upwards. This is because shade and free moisture are more abundant in the lower canopy. For oaks that exhibit marcescence (incomplete leaf abscission after senescence), infected foliage can persist in the canopy over the winter. This can provide higher levels of primary inoculum for infection of newly developing foliage the following spring. Additionally, anthracnose fungi can live dormant within infected twigs until conditions become favorable for growth (mild and wet). As a result, management of anthracnose fungi can be very challenging, especially during years with persistent wet weather in spring and early summer.
Chemical control is not usually warranted, but foliar sprays can be effective when performed early in the season before bud break and on labeled intervals when immature foliage is most susceptible to infection. Fungicides labeled for use against anthracnose include: azoxystrobin, copper salts of fatty and rosin acids, copper hydroxide, copper sulfate, mancozeb, copper hydroxide + mancozeb, metconaozle and thiophanate-methyl + mancozeb. Some fungicides are labeled specifically for the red oak group. In southern New England, it's common for oaks suffering from anthracnose to also be diseased by the foliar pathogen Tubakia. Therefore, a combined management approach may be necessary. Maintaining tree vigor through adequate fertilization, supplemental watering (if possible), mulching to help moderate soil temperatures and pruning of dead branches will promote vigor. At the same time, it is important to note that spores can travel long distances and anthracnose fungi like Apiognomonia are abundant in both forest and landscape settings.
Citations
Boroń P, Grad B, Nawrot-Chorabik K, and Kowalski T. 2019. The genetic relationships within Apiognomonia errabunda and related species. Mycologia, 111(4): 541–550. https://doi.org/10.1080/00275514.2019.1631063
Sinclair WA and Lyon HH. 2005. Diseases of Trees and Shrubs, 2nd edn. Cornell University Press, Ithaca, NY.
Sogonov MV, Castlebury LA, Rossman AY, and White JF. 2007. The type species of Apiognomonia, A. veneta, with its Discula anamorph is distinct from A. errabunda. Mycological Research, 111(6) 693–709. https://doi.org/10.1016/j.mycres.2007.03.013