Pathogens
Several native and non-native species of Phytophthora cause Phytophthora bleeding canker in the northeast, such as P. cactorum, P. cambivora, P. gonapodyides, P. pini, and P. plurivora (Nelson et al. 2010). Phytophthora ramorum, a highly destructive species introduced from Asia, is also responsible for bleeding canker but is not established in New England. There are many other Phytophthora species that can cause bleeding canker diseases on trees and shrubs across the globe (Jung et al. 2024).
Hosts
The most common host of Phytophthora bleeding canker across managed landscapes in the northeast is European beech (Fagus sylvatica) (Jung et al. 2005; Nelson et al. 2010). Additional hosts in the region include maple (Acer), horse-chestnut (Aesculus), birch (Betula), dogwood (Cornus), walnut (Juglans), magnolia (Magnolia), apple (Malus), cherry (Prunus), and oak (Quercus) (Sinclair and Lyon 2005; Nelson and Hudler 2007).
Symptoms & Damage
Phytophthora bleeding canker kills the phloem, cambium, and outer sapwood on susceptible trees and shrubs (Sinclair and Lyon 2005). The cankers often develop on the lower trunk but can develop on large scaffold branches. The most prominent symptom of the disease is dark-colored sap oozing from bark cankers (Sinclair and Lyon 2005; Nelson and Hudler 2007). The fluid is typically reddish-brown, and it stains the surrounding bark as it flows downward. Infected bark is often water soaked and stained while the inner sapwood can exhibit a range of abnormal colors (brown, bluish-green, orange, and pink) depending on the host and particular species of Phytophthora present. However, bleeding cankers by themselves are not diagnostic of Phytophthora, as other fungal and bacterial cankering pathogens can produce similar symptoms (Nelson and Hudler 2007). Additionally, internal decay by wood-rotting fungi (e.g. Armillaria) can also sometimes produce sap to weep from the lower trunk.
Cankers caused by Phytophthora typically have a well-defined margin that is clearly associated with the bleeding of sap observed from the trunk or scaffold branch. Phytophthora species target sugar-rich tissues in the host plant and lack the ability to decay wood. However, the resulting death of the bark and outer sapwood can provide an infection site for wood-rotting fungi to invade later (Jung 2009). Research has shown that once Phytophthora invades the outer sapwood, the pathogen can be drawn upwards in the vascular stream where it can create cankers higher on the trunk or on main scaffold branches (Brown and Brasier 2007; Parke et al. 2007).
Phytophthora species that cause bleeding canker originate from the soil, where they are typically causing root disease as well (Jung et al. 2005; Jung 2009). These pathogens thrive in wet soils, which stimulates spore production and dispersal. Splashing rainwater can spread infective spores from the soil, facilitating establishment on the lower trunk (Ristaino et al. 2000; Davidson et al. 2005). European beeches across managed landscapes often have exposed and compacted soil around the base where water pools after heavy rains. In addition, these trees frequently have numerous bark wounds from vandalism (e.g. the carving of initials) or lawn equipment, which provides a variety of infection sites.
Once a tree is infected, Phytophthora can survive within infected tissues by producing resting spores (oospores or chlamydospores), depending on the species present (Brown and Brasier 2007). Trees may harbor numerous trunk cankers and still maintain a healthy canopy; in some cases, canopy symptoms may not develop until 50–75% of the lower trunk is cankered (Jung et al. 2005). Foliar symptoms caused by Phytophthora bleeding canker reflect the disruption of water transport within the infected tree. Canopy symptoms can include undersized foliage, early fall color and wilt (even when there is sufficient water in the soil), along with stunted shoot growth and progressive branch dieback.
Management
Phytophthora bleeding canker is a chronic disease that progresses very slowly within infected trees (Sinclair and Lyon 2005). Research has shown that overland spread of the pathogen is rare, meaning that an infected tree does not pose an immediate threat to nearby trees at a site (Kenaley et al. 2014). As stated previously, fungal and bacterial cankering pathogens, such as Nectria, Diplodia, and Pseudomonas, are also capable of producing bleeding canker symptoms. Therefore, accurate diagnosis is critical prior to management, as Phytophthora does not produce any visible signs in the field that can be used for identification.
Unlike many other chronic diseases of landscape trees, effective treatment for Phytophthora bleeding canker is possible. Specifically, phosphite applications have proven effective against Phytophthora (Wilkinson et al. 2001; Dalio et al. 2014; Kasuga et al. 2021). Phosphites (mono- and di-potassium salts of Phosphorous acid) should be applied as a lower trunk drench, thoroughly coating the entire bark surface but especially the symptomatic portions where bleeding cankers are visible. Application should take place when trees are flushing new growth (roughly early to mid-May in southern New England). A second application can take place in early autumn if necessary. A variety of commercial phosphite products (e.g. Reliant, Fungi-Phite, ArborFos, Phospho-Jet, Rampart T&O, etc.) are available to arborists and landscapers. Note that phosphites applied at the bark drench rate are phytotoxic to foliage and turfgrasses.
Avoid needless bark wounding from string trimmers, mowers and vandalism, especially close to the soil line where splashing spores are most likely to contact the bark. Maintaining a thick layer of mulch or wood chips around the base of susceptible trees can also help to prevent new infections by physically covering the pathogen in the soil. Avoid planting susceptible trees like beech in locations with persistently wet soils and poor drainage where Phytophthora thrives.
References
Brown AV and Brasier CM. 2007. Colonization of tree xylem by Phytophthora ramorum, P. kernoviae and other Phytophthora species. Plant Pathology 56(2): 227–241. https://doi.org/10.1111/j.1365-3059.2006.01511.x
Dalio RJ, Fleischmann F, Humez M, and Osswald W. 2014. Phosphite protects Fagus sylvatica seedlings towards Phytophthora plurivora via local toxicity, priming and facilitation of pathogen recognition. PLOS one 9(1): e87860. https://doi.org/10.1371/journal.pone.0087860
Davidson JM, Wickland AC, Patterson HA, Falk KR, and Rizzo DM. 2005. Transmission of Phytophthora ramorum in mixed-evergreen forest in California. Phytopathology 95(5): 587–596. https://doi.org/10.1094/PHYTO-95-0587
Kasuga T, Hayden KJ, Eyre CA, Croucher PJ, Schechter S, Wright JW, and Garbelotto M. 2021. Innate resistance and phosphite treatment affect both the pathogen’s and host’s transcriptomes in the tanoak-Phytophthora ramorum pathosystem. Journal of Fungi 7(3): 198. https://doi.org/10.3390/jof7030198
Jung T, Hudler GW, Jensen-Tracy SL, Griffiths HM, Fleischmann F, and Osswald W. 2005. Involvement of Phytophthora species in the decline of European beech in Europe and the USA. Mycologist 19(4): 159–166. https://doi.org/10.1017/S0269915X05004052
Jung T. 2009. Beech decline in Central Europe driven by the interaction between Phytophthora infections and climatic extremes. Forest pathology 39(2): 73–94. https://doi.org/10.1111/j.1439-0329.2008.00566.x
Jung T, Milenković I, Balci Y, Janoušek J, Kudláček T, Nagy ZÁ, Baharuddin B, Bakonyi J, Broders KD, Cacciola SO, Chang TT, et al. 2024. Worldwide forest surveys reveal forty-three new species in Phytophthora major Clade 2 with fundamental implications for the evolution and biogeography of the genus and global plant biosecurity. Studies in Mycology 107(1): 251–388. https://doi.org/10.3114/sim.2024.107.04
Kenaley SC, Rose C, Sullivan PJ, and Hudler GW. 2014. Bleeding canker of European beech in southeastern New York State: Phytophthora species, spatial analysis of disease, and periodic growth of affected trees. Journal of Environmental Horticulture 32(3): 113–125. https://doi.org/10.24266/0738-2898.32.3.113
Nelson AH and Hudler GW. 2007. A summary of North American hardwood tree diseases with bleeding canker symptoms. Arboriculture & Urban Forestry 33(2): 122–131. https://doi.org/10.48044/jauf.2007.013
Nelson AH, Weiland JE, and Hudler GW. 2010. Prevalence, distribution and identification of Phytophthora species from bleeding canker on European beech. Journal of Environmental Horticulture 28(3): 150–158. https://doi.org/10.24266/0738-2898-28.3.150
Parke JL, Oh E, Voelker S, Hansen EM, Buckles G, and Lachenbruch B. 2007. Phytophthora ramorum colonizes tanoak xylem and is associated with reduced stem water transport. Phytopathology 97(12): 1558–1567. https://doi.org/10.1094/PHYTO-97-12-1558
Ristaino JB and Gumpertz ML. 2000. New frontiers in the study of dispersal and spatial analysis of epidemics caused by species in the genus Phytophthora. Annual Review of Phytopathology 38: 541–576. https://doi.org/10.1146/annurev.phyto.38.1.541
Sinclair WA and Lyon HH. 2005. Diseases of Trees and Shrubs, 2nd edn. Cornell University Press, Ithaca, NY.
Wilkinson CJ, Holmes JM, Dell B, Tynan KM, McComb JA, Shearer BL, Colquhoun IJ, and Hardy GSJ. 2001. Effect of phosphite on in planta zoospore production of Phytophthora cinnamomi. Plant Pathology 50(5): 587–593. https://doi.org/10.1046/j.1365-3059.2001.00605.x