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On a theme of Trees and Fungi—my two most recent columns were on oak root fungus (harmful) and then “dead man’s foot” (beneficial)—I am continuing, based on this recent inquiry:

“Bill:

We have a red-bark Japanese maple that was injured in the winds at the time of the fires. It lost a branch. My husband treated the break with pruning material. It has been staked and supported and seems to be doing well. This winter we noticed these growths on the base of the tree and wondered if you know what it might be.”

She attached photos of the young tree showing white growths on the sunny side of the lower trunk.

I immediately recognized it as split gill fungus, a common saprot, technically: Schizophyllum commune. The common name refers to the finely divided gills on the lower surface of the basidiocarps, the fungal fruiting bodies.

I have seen this on trees innumerable times over the years, yet it does not seem to be well known to most gardeners and nursery people in general.

In my experience, it sometimes appears on fire-damaged stems but most often it is on the sunny south to west side where tender bark is exposed to direct sunlight in the afternoon. Because of the sunlight on the trunk in the photo, and the orientation in relation to our recognizable Mt. George in the background, I could tell the fungal infection was on the south and west sides.

Susceptibility of the southwest-facing stem surfaces is partially related to reduced water status in the tree. And it is most common where tender-bark trees like Japanese maple have no low branches or foliage shading the bark.

In this instance, the extreme wind that fanned the firestorm tore off a weakly attached limb (a common defect in the ‘Sango Kaku’ coral bark Japanese maples). Loss of the limb on the south to west side exposed previously shaded bark to direct midday and afternoon sunlight.

Split gill is a saprophyte and opportunistic pathogen. It grows on dead and weak or injured tissue. Numerous tree and plant species are susceptible.

Drought stress and fire wounds are major factors predisposing trees to infection. In the instance of this ‘Sango Kaku’ maple, sun injury resulted where sudden exposure to direct sunlight killed the shade-adapted tissue.

I see this constantly on fruit trees and shade trees, and quite often on mature trees that have been pruned excessively.

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Here are a few tips to help prevent this problem:

1. Prevent drought stress. Monitor root zone moisture, especially for young transplants and trees of water-needy species like maple, ash, and linden, using a soil probe or moisture meter. Irrigate before trees become stressed and too dry. Irrigate a generously wide area around the tree and verify soaking to an appropriate depth, at least the depth of the root ball of recently transplanted trees. Use mulch to help cool the soil and slow evaporation.

2. Retain temporary low branches and plenty of foliage to shade the bark on young transplanted trees. Low temporaries may be pruned to shorten them, but do not remove them until the tree is established and the lower trunk is well shaded by the canopy.

3. Apply white water-based paint to the trunks of susceptible trees including most fruit trees, especially apple, pear, and all stone fruits.

4. For susceptible flowering trees and shade trees, consider erecting temporary structures, such as bamboo screening or shade cloth attached to tree stakes to shade the bark of the trunk. Be wary of trunk shelters that can provide a hiding and feeding space for voles, which may feed on the bark. There is a white, breathable polypropylene tape product called Dewitt Tree Wrap that should be safe and effective.

5. Do not prune too heavily, especially where protected branch surfaces could be suddenly exposed to direct sunlight.

There is no direct treatment for the split gill fungus. Where the fruiting bodies are scraped off, they tend to reappear. Trees usually live on in spite of infection if they receive good cultural care, but it is best to prevent infection in the first place.

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Bill Pramuk is a registered consulting arborist. Visit his website, billpramuk.com. Email questions to info@billpramuk.com or call him at 707-363-0114.

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