I received this inquiry regarding a very common and avoidable problem, so common yet so apparently unknown I feel it bears discussion here.

Hi Bill,

They told me at the garden center that our Japanese maple may have heart rot and suggested I contact you. The soil is very moist. Attached is a picture. Do you think they are right, and, if so, is there anything we can do to save the tree?

KC, Napa

The photograph showed a young coral bark (Sango Kaku) Japanese maple with a straight single trunk, perhaps 4 feet tall and branching out into the very upright limb structure typical of this variety.

One side of the trunk, for much of its length, was encrusted with grayish-white, fuzzy brackets — stemless mushroom-like structures, technically known as basidiocarps — growing out of the bark.

I immediately recognized it as the common sapwood rot — not heart rot — fungus Schizophyllum commune. It is also known as split gill fungus, which refers to the finely divided gills on the lower surface of the basidiocarps, the fruiting bodies of the fungus.

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.

It is critical to note the fungus typically grows on one side of the trunk. In my experience it is always the sunny south to west side where tender bark is exposed to direct sunlight in the afternoon (here in the Northern Hemisphere).

Susceptibility of the southwest facing stem surfaces is probably related to reduced water status as the day wears on. And it is most common where tender bark trees like Japanese maple and apple have no low branches or foliage shading the bark.

Schizophyllum is a “saprophyte and opportunistic pathogen” as described in Diseases of Trees and Shrubs, (Sinclair, Lyon, Johnson). In other words, it grows on dead and weak or injured tissue. The reference lists seventy-three tree and plant species susceptible to infection.

Drought stress and fire wounds are major factors predisposing trees to infection. In the instance of this Sango Kaku maple in the very moist, green lawn, susceptibility resulted probably not from drought stress, but from sun injury where the lack of shade from low branches and foliage allowed the living tissue under the bark to heat up and bake to death.

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

Here are a few tips to help prevent this problem:

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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. 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. Employ 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 well established and the lower trunk is well shaded by the canopy.

3. Apply white water-base 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 wrapping that can provide a hiding and feeding space for voles, which may feed on the bark.

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.

Bill Pramuk is a registered consulting arborist. Visit his website, BillPramuk.com, email questions to info@billpramuk.com, or call him at 707-226-2884.

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