Root rot: how to recognise and treat it

Disease

Symptoms

The plant wilts and does not recover when watered; leaves turn dull green, yellow, red or purplish, foliage thins and branches die back.
Fine roots have rotted away; larger roots are brown or black inside, softer than normal and break easily.
Dark decay at the stem base or crown, sometimes with gumming; in soft-stemmed plants the stem collapses and the whole plant dies.
Signs above ground appear late and look like waterlogging or drought — dig down and check the roots.

Destroy badly affected plants and replace the soil from their root run with fresh topsoil; replant with less susceptible plants.
If a tree is caught early, cut away the rotten bark, pull the soil back from the crown so it can dry out, and water only at the edge of the canopy.
For a pot plant, let the compost dry, then repot into fresh, well-drained potting mix in a pot with a drainage hole.

Most fungicides do not control it, and no chemical treatments are available to UK gardeners; in the US, fosetyl-Al or phosphorous-acid (phosphonate) products may be used on some ornamentals to help prevent infection, only together with better drainage and watering — follow the label.

Reference information, not a guarantee. Read and follow the label of any product: dose, safety and the days to wait before harvest.

Avoid long periods of saturated soil and standing water around stems; keep sprinklers and drippers off trunks.
Improve drainage; on wet ground grow vegetables in raised beds.
Do not plant deeper than the plant grew in the nursery, never bury the graft union, and buy healthy, certified stock.
Do not move soil from affected spots; after root rot in tomatoes, grow a non-susceptible crop such as sweet corn there for a year or two.

Figures checked against these sources.