Phytophthora root rot

What you are looking at

A shrub or a young tree that looked well last month is wilting. The leaves are small, dull and pale, and they yellow and drop early. Branches die back, either from the top down or one at a time, and within a season or two the plant is dead.

The soil is damp. A wilting plant in damp ground looks like it needs water, so most people reach for the can, and watering it makes matters worse.

Look at the base of the stem. There is often dark staining on the bark at or just below ground level, sometimes with a gummy or wet ooze. Pare a sliver of bark away with a knife on the affected side and look at the tissue underneath. Healthy tissue is white or greenish and moist. Infected tissue is brown or reddish-brown, and the line between the two is sharp.

The other pattern to notice is where the dead plants are. They sit in the wettest part of the bed, along a low line, at the bottom of a slope, or beside a downpipe.

Not a fungus, which changes what works

Phytophthora is an oomycete, a water mould. It sits in a different kingdom from the true fungi and is more closely related to brown algae than to anything that produces a mushroom.

The practical consequence is that fungicides developed against true fungi disappoint against it, because the cell wall chemistry they target is not the chemistry it has. This is the same reason ordinary fungicides do so little against potato blight and downy mildew, which are oomycetes too.

The name comes from the Greek for plant destroyer, and Phytophthora cinnamomi is regarded as one of the most damaging plant pathogens in the world. It has over five thousand recorded hosts.

Water is how it moves

The spores swim. Zoospores have two flagella, they are released into films of water in the soil, and they travel towards root exudates until they reach a root tip and encyst on it.

Everything about the advice follows from that. Free water in the soil is the condition it requires, so ground that stays saturated after rain is where it does its damage and drainage is the whole of the prevention. Water moves it downhill, which is why plants die along a low line. It comes into a garden most often in the compost of a container, and it leaves a garden on boots, tools, tyres and barrowed soil.

Rainwater butts deserve a mention of their own. A butt collects runoff from anywhere in the garden that drains into it, spores included, and then distributes it to every pot you water. Where a plant matters, use the tap.

The organism survives in soil and in dead root fragments for years as thick walled resting spores, so replanting the same spot with the same genus repeats the outcome.

Telling it from honey fungus

Both kill a woody plant from the base and both are fatal, so the distinction gets made in the same five minutes.

Honey fungus leaves a sheet of white fungal growth between the bark and the wood that smells strongly of mushroom, and flat black rhizomorphs like bootlaces running through the soil and under the bark. Honey-coloured toadstools appear in tufts at the base in autumn.

Phytophthora leaves none of those. What it leaves is a sharp margin between brown dead tissue and white living tissue at the collar, and a root system stripped of its fine feeding roots.

The site tells you something too. Honey fungus spreads outward from a source along its rhizomorphs and works through a hedge or a border in sequence. Phytophthora picks out the wet ground, so the dead plants map the drainage rather than a line of advance.

What to do about it

Cut into the bark at the collar and look for the sharp margin before deciding anything.

Stop watering, and check the soil before you reach for a can.

Fix the drainage, raise the bed or plant on a mound before replacing anything.

Take the dead plant out with as much root as you can and bag it.

Replant with something that tolerates wet ground, and avoid the genus you lost.

Set new plants with the root flare at or above soil level and keep mulch off the stem.

Buy from a nursery that stands its pots on gravel rather than soil, and inspect the root ball.

Clean boots, spades and barrow wheels when moving between beds.

Water anything precious from the tap rather than the butt.

What it gets mistaken for

Honey fungus leaves white mushroom-smelling sheets under the bark and black bootlaces in the soil.

Waterlogging alone kills roots evenly by suffocation, with no sharp margin at the collar.

Overwatering a houseplant is the same word for a different problem, and there the fix is a change of habit.

Verticillium wilt stains a ring inside the wood higher up the stem and often takes one side.

Drought wilts a plant in dry soil and it recovers on watering.

Wind rock loosens the base and opens a socket in the soil while the bark stays sound.