Phytophthora Bleeding Canker Disease
Disease Profile and Management
By Scott Sinner
ISA Board Certified Master Arborist WE-12480B
©Aspect Tree Care 2024
Early fall color in August 2024 is a symptom of tree stress.
©Aspect Tree Care 2024
Sap bleeding from cankers.
©Aspect Tree Care 2024
What is Phytophthora Bleeding Canker Disease?
It’s a fungal disease that was first discovered in Northern Nevada by local Certified Arborist Leslie Lyles, of Stewardship Horticultural in 1999. Nine silver maples were sampled between ‘99-2001 and the disease was confirmed by the state pathologist [1]. It went away for 10-12 years possibly due to weather-related reasons, and then reemerged. It’s caused by one or more species of the soil-borne fungus in the genus Phytophthora [1][2][3].
What tree species are susceptible?
It has been confirmed to affect silver maples [1]. Red maples and freeman maples appear to suffer from it as well. Other trees that are potentially susceptible include american beech, birch, dogwood, elm, magnolia, other maples, horsechestnut, linden, oak, sweet gum, walnut, and willow [2][3].
Predisposing factors
In 2008, Phytophthora Bleeding Canker Disease was reported to be very progressive and predisposition of trees by other factors wasn’t considered to be necessary for its infection [1]. However, based on the cultural controls that are recommended to prevent and slow its spread, as well as other research, it’s likely that trees that are not properly planted and maintained are more susceptible than trees that are managed properly.
Trees suffering aeration deficit
Tree roots require oxygen for growth and development [4][5]. Aeration deficit is a shortage of oxygen in the root zone that affects tree health and predisposes trees to root disease pathogens such as Phytophthora [4][5].
Poor Soil
Trees growing in clay soils, compacted urban soils, and soils low in organic matter are common in Reno and create the perfect environment for the disease. Phytophthora thrives in soil with poor drainage and low oxygen levels [6]. Lack of mulch around the base of a tree can promote fungal spore germination and dispersal [2].
Grass lawns may increase susceptibility
Trees growing in grass lawns may be more susceptible to Bleeding Canker Disease. Lawn maintenance equipment can cause mechanical injury to lower trunks, creating an entry for infection [1][3]. Lawn irrigation can create ideal soil conditions for phytophthora. Trees require a different watering schedule than grass lawns. When lawns are fertilized, trees may receive excess fertilizer, causing tree stress. High nitrogen rates may reduce the ability of a tree to manufacture defense compounds against insects and pathogens [7].
Covered root collars
The root collar is at the base of the tree where the trunk begins to flare out into roots, and is also known as the root flare or trunk flare [4]. The root collar should not be covered with soil, mulch, or ground-cover plants [4]. This can be caused by planting too deeply, grade changes, and improper landscaping practices. “Soil and mulch can retain high levels of moisture on the trunk that may soften the bark and allow easier entry of certain fungal pathogens” [4]. Several studies in the 80’s and 90’s suggested that trees planted too deeply (no visible root collar) led to phytophthora infection due to soil contact with trunk bark [8]. Trees with covered root collars usually suffer from aeration deficits as well [5].
Example of a root collar covered by gravel and landscape fabric. ©Aspect Tree Care 2024
Example of a visible root collar.
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Stem-girdling roots
Maples seem to be naturally prone to developing girdling roots [4]. Stem-girdling roots (SGRs) are likely to be primary stressors that prevent a tree from growing and functioning normally, and this reduced vitality makes trees more susceptible to pests and disease [9].In a five-year study concluded in 1999, more than 80% of declining sugar maples had SGRs, which were presumably associated with the decline in health or death of those trees (Johnson 1999) [9].
Girdling and circling roots on this maple tree likely suffering from Phytophthora Bleeding Canker Disease. ©Aspect Tree Care 2024
Consequences and Severity
In 2018, an extension article published by UMASS reported it to be a chronic disease that progresses very slowly within infected trees [2]. Trees with the disease should be considered a greater risk for tree failure that can potentially cause personal injury and property damage [4]. The bleeding, staining, branch dieback, and general decline affects the appearance of these trees as landscape specimens. Infected trees have been observed to die within a few years, but may be treated through a combination of management strategies.
Diagnostic Clues
Dark brown sap bleeds from cankers on the lower 8’ of trunks of infected trees [1]. This is commonly observed in the spring and fall. When not actively bleeding, there will be a permanent dark stain [1]. Early fall color, small leaves, and wilting are other common symptoms [2] Since phytophthora does not produce any visible signs in the field that can be used for identification, and other fungal and bacterial pathogens are capable of causing bleeding cankers that appear very similar, samples should be sent to a lab for proper diagnosis before management [2].
Closeup of a bleeding canker.
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Inconsistant fall color is a sign there may be something wrong with a tree part.
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Early fall color is a sign there may be something wrong with a tree.
©Aspect Tree Care 2024
Management
We recommend treating Bleeding Canker Disease with a variety of cultural and physical controls in combination with a systemic fungicide (chemical control) to improve tree health and prevent the spread of the disease within an individual tree. In trees where the disease has been detected, effective control may depend on how soon it is detected [1]. Trees with bleeding canker should have a Tree Risk Assessment performed if there are potential consequences of tree failure [4].
Cultural and Physical Control
Plant silver maples and other susceptible tree species in well-drained soil [1][2][3]. Don’t allow sprinkler irrigation to wet tree bark [1][3]. Avoid mechanical injuries and improper pruning [1][3]. Maintain mulch around the base of susceptible trees to prevent fungal spores from germinating and dispersing from the soil [2]. Avoid overwatering and underwatering [2]. According to Leslie Lyles, attempts to remove the canker disease through excising cankered trunk tissue and sanitizing with bleach did nothing to control the spread. Managing overall tree health is key.
Deep Watering
Overwatering and underwatering is often caused by frequent, shallow watering methods produced by irrigation systems designed for lawns and small shrubs. Trees require a different method called deep watering, where watering is done infrequently, and for a longer duration.
Root Management
Root Collar Excavation (RCX), and permanently exposing the root collar, should be recommended to remove mulch or soil covering the root collar so it is distinct and visible [4]. During RCX, stem-girdling roots, which reduce sap flow and can lead to tree decline, may be identified and removed by selective root pruning or progressive root pruning over a specified period of years [4]. From 2002-2004, root collar excavation to permanently expose root flares was studied on grapevines infected with Armillaria root disease, which is similar to Phytophthora. The conclusion was that it “appears to be a promising cultural approach for control of Armillaria root disease” [10].
Performing a Root Collar Excavation using an AirSpade.
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Soil Improvement
Soil quality directly influences tree health and longevity [11]. Arborjet, a leading manufacturer of Plant Health Care products, recommends amending the soil around the tree with compost, to improve drainage and soil aeration, as “Phytophthora thrives in poorly drained and anaerobic soils” [6].
Mulching
Applying 2-4” of wood chip mulch to a tree’s root zone is great for trees for dozens of reasons. In particular, it can reduce fungal spore germination and dispersal [2]. Replacing grass with mulch near the trunk can reduce the chance of mechanical injury. Replacing grass with mulch can prevent accidental overfertilizing of trees. Mulching can reduce soil compaction over a longer period of time.
Apply mulch to the root zone to improve tree health.
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Chemical Control
Although most chronic diseases in landscapes are untreatable, chemical control can be an effective treatment for Phytophthora bleeding canker [2]. Contact pesticides work by making direct contact with a pathogen and have not been proven to be effective in treating Bleeding Canker Disease.
Phospho-Jet is a registered systemic fungicide for trees and rates are based on tree size [3][6][12]. It’s best as a preventative control, but it is also effective on infected trees [3][12]. It does not kill the fungus, but can prevent it from spreading within the tree [3]. Arborjet, the manufacturer of Phospho-Jet, recommends applying it in spring and fall, along with cultural controls [6][12].
References
1) Wang S, Lyles L, Garneni S, Carlos WJ, McKie P. Bleeding Canker Disease of Silver Maple Trees: A Guideline for Nursery/Landscape Professionals and Homeowners. Nevada Department of Agriculture, Plant Pathology Dept; 2008. https://agri.nv.gov/uploadedFiles/agrinvgov/Content/Plant/Plant_Pathology/BleedingCankerDiseaseSilverMaple.pdf
2) Brazee N. Phytophthora Bleeding Canker. UMass Extension Landscape, Nursery and Urban Forestry Program; 2018. https://ag.umass.edu/landscape/fact-sheets/phytophthora-bleeding-canker
3) Maple (Acer spp.)-Bleeding Canker. Pacific Northwest Pest Management Handbooks. A Pacific Northwest Extension Publication (Oregon State University, Washington State University, University of Idaho); 2024
https://pnwhandbooks.org/plantdisease/host-disease/maple-acer-spp-bleeding-canker
4) Costello LR, Watson G, Smiley ET, Hauer R. Root Management. 2nd Ed. Best Management Practices. International Society of Arboriculture (ISA); 2023.
5) Costello LR, Perry EJ, Matheny NP, Henry MJ, Geisel PM. Abiotic Disorders of Landscape Plants: A Diagnostic Guide. University of CA; 2003.
6) Phytophthora. Arborjet. 2024 https://arborjet.com/problems_solutions/phytophthora/
7) Smiley ET, Werner L, Lilly SJ, Brantley B. Tree and Shrub Fertilization. 4th ed. Best Management Practices. International Society of Arboriculture (ISA); 2020.
8) Susan D. Day, J. Roger Harris. Growth, survival, and root system morphology of deeply planted Corylus colurna 7 years after transplanting and the effects of root collar excavation,
Urban Forestry & Urban Greening, Volume 7, Issue 2, 2008, Pages 119-128, ISSN 1618-8667.
https://doi.org/10.1016/j.ufug.2007.12.004
9) Johnson, Gary R.; Hauer, Richard J.. (2000). A Practitioner's Guide to Stem Girdling Roots of Trees. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/49810.
10) Baumgartner K. Root Collar Excavation for Postinfection Control of Armillaria Root Disease of Grapevine. Plant Disease 2004 88:11. https://doi.org/10.1094/PDIS.2004.88.11.1235
11) Scharenbroch BC, Smiley ET. Soil Management for Urban Trees. 2nd ed. Best Management Practices. International Society of Arboriculture (ISA); 2021.
12) PHOSPHO-jet Label. Systemic Fungicide and Plant Resistance Activator by Arborjet. https://arborjet.com/products/phospho-jet/