Root & crown rot

Pythium and Phytophthora spp., with Rhizoctonia solani

also called Wet feet, Collar rot

Roots or crown turn brown and mushy in waterlogged soil or compost; the plant wilts even when wet. Improve drainage, water less, and pot up only into free-draining mixes.

Water mold No cure Soil-borneSpread in waterSpread by contact Spreads to neighbors

What you'll see

The plant wilts in the middle of the day and does not recover overnight, although the compost is wet.
Knocked out of the pot, the roots are brown, soft and sparse, and the outer sheath slides off leaving a bare thread.
The stem base or crown is dark, sunken and squashy, often with a sour smell.
Foliage yellows from the bottom, growth stops, and the whole plant becomes easy to lift from the ground.
On woody plants a slice under the bark at the collar shows brown-stained wood instead of clean green.

Why it happens

Water molds that live in soil, compost and standing water, producing swimming spores that seek out roots through a film of water. They are almost always present; the trouble starts when the soil stays saturated and the roots, short of oxygen, can no longer defend themselves. Rhizoctonia and honest bacterial rots often join in once the tissue has gone.

Cold, wet ground, heavy clay, compacted borders, pots standing in saucers of water, and blocked drainage after a wet winter. Overpotting into a large volume of compost the plant cannot dry out is a common trigger indoors.

Treating it

Stop watering at once and let the compost dry; move pots off saucers and into shelter.
Lift the plant, wash off the soil, cut away every brown root and stem base back to firm white tissue, and repot into fresh gritty compost in a smaller pot.
Take cuttings from any clean top growth as insurance while the plant still has some.
Improve the drainage of the bed — a raised bed, added grit or a French drain — before replanting anything.
If the crown itself has gone, dig the plant out; there is no cure once the collar is rotted through.

Keeping it away

Plant into free-draining soil at the depth the plant grew before, never deeper.
Use fresh compost and clean pots, and choose pots matched to the root ball.
Water when the surface has dried rather than to a schedule, and less in winter.
Break up compacted ground and add coarse organic matter before planting.
Avoid replanting the same susceptible group into ground that has already lost a plant to rot.

Who gets it

Almost anything can be taken, but lavender, rosemary, sage, box, heuchera, container shrubs and seedlings are the usual casualties.

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