Two Problems, One Set of Roots

Ask any British gardener what worries them most about mature trees and the answer is usually the same: something is going on underground, out of sight, and by the time you notice it above ground the tree is already in trouble. Honey fungus (Armillaria) and root rot, most often caused by Phytophthora species, are the two culprits behind most unexplained dieback in gardens and small woodlands.

They behave quite differently. Honey fungus is an active coloniser: it spreads through soil and along roots, sometimes travelling several metres through thick black strands called rhizomorphs, and it can kill a healthy tree outright. Root rot is more of a condition than an invader. It flourishes where soil stays wet and airless, so it tends to strike trees on heavy clay, in hollows, or wherever drainage has been compromised. In practice the two often overlap, and the remedies share a lot of common ground.

Reading the Symptoms

Both problems announce themselves above ground in similar ways: sparse, pale or unusually small leaves, dieback starting at the tips of branches and working inwards, heavy seed production as a stressed tree panics, and premature autumn colour. The differences show up when you look closely at the base of the trunk and the roots.

• Honey fungus: peel back the bark at the base of a dying tree and you may find a white, fan-shaped sheet of fungal tissue with a mushroomy smell. Look for black, bootlace-like strands in the soil and under bark, and for clusters of honey-coloured toadstools in autumn.
• Root rot: dig down and you will find roots that are dark, soft and brittle, breaking off in your hand, often with a sour or musty smell. On some species, such as apple and cherry, you may also see bleeding or dark, sunken cankers around the base.
• Buried root flares: if you cannot see where trunk meets roots, the tree is planted or mulched too deeply. That alone encourages both problems.

Sorting Out the Drainage

If root rot is your problem, drainage is where you win or lose. Before doing anything else, dig a test hole about 60cm deep and fill it with water. If it is still full the next morning, you have a drainage problem.

• Install a French drain — a perforated pipe in a gravel-filled trench, falling gently to a soakaway — along the uphill side of the affected area.
• Never plant into a hollow that collects water. If the only spot available is a sump, plant on a mound instead, raising the root ball 15–20cm above the surrounding soil.
• Improve structure with well-rotted organic matter, but do not dig heavy clay while it is wet; you will make a compacted pan that lasts for years.
• Keep mulch to 5–8cm and pull it back from the trunk. A mulch collar pressed against bark holds moisture exactly where you do not want it.
• Turn off automatic irrigation in heavy ground. More water is rarely the answer in a British clay garden.

Removing Infected Material Properly

With honey fungus, the fungus lives in the wood. Every stump and major root left in the ground is a larder that keeps it going. Grind out stumps where you can, and dig out the thickest roots within a couple of metres of the trunk. This is hard work but it is the single most effective step you can take.

• Do not chip infected wood and spread it as mulch. Burn what you can, or take it to a licensed waste site.
• Do not move soil, roots or chippings to other parts of the garden. Honey fungus travels happily on muddy boots and spades.
• Clean tools with methylated spirits or a garden disinfectant between jobs, and wash your boots before walking into clean ground.
• Where a diseased tree sits beside a healthy specimen, severing the connecting roots with a sharp spade in a narrow trench can buy a few years. It is a delaying tactic, not a cure.

Replanting: What to Grow and What to Avoid

Leave the spot empty for at least three to five years. Honey fungus can persist in buried wood for a decade or more, but the immediate risk in the soil around a removed tree falls sharply once the bulk of the root is gone. Sow grass or a green manure in the meantime and resist the temptation to fill the gap straight away.

When you do replant, choose species that are less susceptible. Yew, holly, box and hawthorn are among the most tolerant, and they will give you structure while the ground recovers. Avoid the worst offenders: lilac, willow, birch, privet, roses and most top fruit, including apple, plum and cherry.

If your site is wet as well as infected, the two lists pull in different directions — willow and alder cope with damp ground but willow is a honey fungus magnet. In that situation, plant yew or holly on a mound, improve the drainage, and give the ground time.

Woodland Planting and Long-Term Prevention

In a wood, honey fungus is part of the ecology rather than an enemy. It breaks down dead wood and recycles nutrients, and you will never eliminate it. What you can do is make your trees harder to kill.

• Plant in mixed groups rather than blocks of a single species, so disease cannot run through a plantation unchecked.
• Give trees space and thin overcrowded stands; vigorous trees resist attack far better than stressed ones.
• Protect trunks and roots from strimmer damage, mower scrapes and vehicle compaction. Wounds are open doors.
• Walk the wood in late summer and autumn, when toadstools and early dieback are easiest to spot, and act on small problems before they become large ones.

None of this is quick, and it rarely feels tidy. But drainage, hygiene and a sensible replanting plan will see most British gardens and small woods through, and the trees you put in next will have a far better start for it.