Pathogen
Nectria canker is caused by the native fungal pathogen Nectria cinnabarina.
Hosts
Nectria cinnabarina attacks over 90 different genera of woody plants in landscape and forest settings (Sinclair and Lyon 2005). Common hosts in the managed landscape include: beech (Fagus), maple (Acer), elm (Ulmus) and honeylocust (Gledistia). Based on samples submitted to the UMass Plant Diagnostic Lab, Japanese maple (A. palmatum) are especially prone to the disease. Trees with thin bark that is easily wounded are most susceptible to attack.
Symptoms & Signs
Nectria canker (also known as coral-spot canker) is often readily identifiable due to the orange- to pink-colored pads of fungal tissue that erupt through the bark on infected stems and branches. The pads release large volumes of fungal spores that are splashed, washed and blown onto adjacent branches and other susceptible hosts nearby, allowing the pathogen to establish new disease centers. Typically, the pads strongly contrast with the bark of the tree and can be seen without the aid of a hand lens. Nectria primarily acts as an opportunistic colonizer of stressed or damaged trees and shrubs. Cankers often develop and expand when the host is dormant and branches may be girdled and fail to leaf out the following spring. Foliage on infected branches may be chlorotic, undersized or prematurely shed as a general canopy dieback develops. Branch lesions may appear sunken or water-soaked during the early stages of disease development. Over time, cankers with cracking, splitting and sloughing bark often develop. Wound wood may form on the margins of the canker depending on the vitality of the host. Like many cankering fungi, the pathogen kills the bark, cambium and outer sapwood as it obtains sugars from the host. The fungus does not actively decay wood, but the infection sites can, at times, be colonized by wood-rotting fungi.
Management
Pruning wounds are a common source of entry for the fungus, although many other types of wounding can facilitate infection. Once established, Nectria can be very difficult to eradicate. Pruning infected stems and branches, to physically remove the pathogen from the host, is critical for disease management. While this may create new wounds for establishment, reducing inoculum must be performed to control the disease. Nectria is often present in wood tissues distal to those with noticeable symptoms. Therefore, if possible, ensure 2–3′ of clean wood is present between the canker and the pruning site. For smaller trees and shrubs, prune as far from the symptomatic tissue as possible. Regularly sanitize pruning equipment when working with trees or shrubs known to be infected by Nectria. Once infected branches are removed, continue to scout the canopy in subsequent years to ensure the fungus was fully eradicated. When Nectria is present on a tree or shrub in the landscape, extreme care should be taken when pruning neighboring trees, to limit the potential spread of the pathogen. Any pruning to shape or form trees should be postponed until the disease can be controlled. Chemical management against cankering fungi is challenging, as fungicides may fail to contact the target pathogen within woody tissues. However, in certain cases a locally systemic fungicide, such as azoxystrobin, phosphites, pyraclostrobin or thiophanate-methyl, maybe applied in conjunction with pruning to protect high-value trees and shrubs. A lower bole or branch drench with phosphites may also be effective against Nectria, especially when infections are present on large scaffold branches or the main trunk, limiting the ability to prune these infected parts. Keep in mind that Nectria can survive as a saprophyte and may linger in the canopy if dead branches are not pruned.
Citation
Sinclair WA and Lyon HH. 2005. Diseases of Trees and Shrubs, 2nd edn. Cornell University Press, Ithaca, NY.