8 Coral Spot Cankers Cut Out of an Acer campestre Hedge with a Silky Gomtaro Saw

Coral spot shows itself in tiny raised cushions, salmon to brick-red, clustered on dead or dying wood. On the Acer campestre in question they had colonised eight separate points along a 14-metre run, most of them on stubs left from a hurried trim two seasons earlier. Nectria cinnabarina is a weak parasite. It rarely forces its way into healthy tissue, but a torn twig, a frost-split branch, or a badly angled cut gives it the wound it needs, and from there it works back into living wood.

The Silky Gomtaro 240 with its 8-tooth-per-30mm blade cuts on the pull stroke, which keeps the kerf clean and avoids the crushing that a blunt bypass lopper leaves behind. A crushed cut is a slow-healing cut, and a slow-healing cut is an open invitation. That relationship, between wound quality and infection, sits underneath everything that follows.

Reading the Lesions Before You Cut

Before any blade touched the maple, each of the eight sites was mapped. Coral spot fruiting bodies appear on wood that is already dead, so the visible pustules are lagging indicators. The fungus has usually run some distance further into the branch than the coloured cushions suggest. Cutting flush to the last visible pustule leaves infected xylem in place, and the canker simply reappears the following autumn.

The working rule on this hedge was to cut back to a point 15 to 20cm below the lowest visible fruiting body, then check the cut face. Healthy field maple wood is pale cream, evenly coloured across the cross-section. Infected wood carries a brown or greyish stain, sometimes as a wedge, sometimes as a full ring. Three of the eight cuts had to be extended a further 10cm because the first cut face still showed staining. Two of them tracked back into a main stem and forced a decision about whether the stem could be kept at all.

The timing was deliberate. Nectria sporulates most heavily in damp autumn conditions, and cutting during a dry spell in late winter meant far fewer spores in the air to reinfect the fresh wounds. Field maple is fully dormant by then, so the sap loss that plagues Acer pruning in spring was not a factor.

Tool Hygiene Between Cuts

Each cut face was a potential transfer point. The Gomtaro blade was wiped and sprayed with 70 percent isopropyl alcohol between every one of the eight lesions, and the folded prunings went straight into a rubble sack rather than onto the border where the fruiting bodies could keep releasing spores. Burning the infected material is the cleanest disposal. Council green-waste collection will compost it, but a cool home heap will not reliably kill Nectria.

The Two Stems That Nearly Came Out

The decision that took longest was not about the small lesions at all. It was about a pair of structural stems near the middle of the run where the brown stain, on the first exploratory cut, extended into wood thicker than a thumb. Removing both outright would have punched a visible gap into the hedge face that Acer campestre, being only moderately vigorous, would take two or three growing seasons to fill.

The first stem was traced upward with successive cuts. At roughly 40cm above the lowest pustule the cut face finally ran clean, pale cream edge to edge, and the stem was kept from that point. The regrowth would come from dormant buds below, and field maple breaks readily from old wood, which is why it tolerates hard renovation better than beech does.

The second stem was worse. The stain followed the grain down past the point where the stem joined a larger limb, and there was no clean cut face to be found above the union. Keeping it meant leaving a reservoir of infected wood inside a load-bearing part of the framework. That stem came out entirely, cut back to the collar of the parent limb so the wound would occlude naturally. The collar cut matters: a flush cut into the parent removes the branch collar tissue that drives callus formation, and on a species prone to coral spot that is precisely the tissue you want intact and working.

With the stem gone, the gap was real but not fatal. The neighbouring growth was left unclipped on that face for the season so it could lean into the space, and the plan was to bring it back to line only once the void had partially closed. This is the slow part of hedge disease work. The cutting takes an afternoon; the recovery of the face takes two summers of restraint.

One further check went into both stems: the surrounding bark was examined for the sunken, cracked margins that mark an established perennial canker as opposed to a fresh colonisation. Neither showed the concentric ridging of a canker that had been overwintering for several years, which meant the infection was recent and the odds of a clean recovery were good.

Why the Hedge Was Vulnerable in the First Place

Nectria cinnabarina does not attack thriving plants at random. The eight infection points clustered on the shaded, congested lower half of the hedge, where air movement was poor and dead twiggy material had accumulated inside the canopy. That interior deadwood is the fungus’s reservoir. It colonises the dead stubs first, fruits there, and then reaches into adjacent living tissue through any wound within spore range.

Two cultural failures had set it up. The trim two seasons before had been done with a hedge trimmer run too fast through wood that was too thick, leaving ragged stubs instead of clean cuts. And the base of the hedge had never been thinned, so light and air never reached the interior. Coral spot on ornamental Acers is documented across RHS advisory material as an opportunist of stressed, wounded, or neglected plants, and this run ticked every box.

The corrective work went beyond removing the lesions. The interior deadwood was cleared out by hand, opening the canopy so the lower stems dried faster after rain. A hedge that dries within an hour of a shower gives Nectria far less opportunity to sporulate and germinate than one that stays damp all morning.

After the Cuts: Feeding and the First Season

The instinct after heavy pruning is to feed hard and push growth. On a maple recovering from canker that instinct works against you. A nitrogen surge produces soft, sappy wood that Nectria colonises more readily than firm, ripened growth. The feeding here was deliberately restrained: a balanced slow-release fertiliser at the manufacturer’s lowest rate in spring, no high-nitrogen top-up, and a mulch to hold soil moisture steady so the roots were not stressed by drought on top of the pruning shock.

A Felco No 2 handled the lighter follow-up cuts through the season, the small stuff too thin to warrant the Gomtaro. Keeping the secateurs sharp and clean mattered as much on those minor cuts as the saw hygiene had on the main ones, because a ragged secateur cut on soft summer growth is exactly the entry point coral spot exploits next.

By the end of the first summer, seven of the eight sites had begun to callus over cleanly with no return of the salmon pustules. The eighth, on the stem that had been cut back 40cm, showed a faint reappearance of fruiting bodies at the cut margin, which meant the infection had run further than the clean-looking cut face suggested. That one will be cut again, lower, in the coming dormant season.

What the Recut Will Have to Answer

That single reappearing lesion is the open question the whole job now turns on. If the stain has tracked into the main framework the way the second stem’s did, the maple loses a second structural limb and the gap in the face doubles. Whether Acer campestre can carry two adjacent voids and still read as a solid hedge from three metres away is not something the first season’s work settled, and it is the thing the next dormant-season cut will decide.