Pythium root rot
A water mould that attacks fine roots in wet, poorly drained or warm stagnant conditions, causing wilting and stunted plants.
Also known as: Pythium, Water mould root rot
Symptoms and identification
Roots turn soft, brown or black and slimy, and the outer layer slips off the white core like a sleeve. Above ground, plants wilt, stop growing, yellow and may die, even when the soil is wet. Seedlings fall over at the base.
Causes
Pythium lives in soil and water, and it spreads quickly in waterlogged or overwatered mixes, in pots standing in water, and in warm hydroponic solutions. Dirty pots and tools carry it.
Organic treatment
Repot affected plants in fresh mix after trimming off rotted roots, and water less often. Biological products containing Trichoderma or Bacillus species may help protect new roots.
Prevention
Use pots with drainage holes and a free-draining mix, never leave pots standing in water, and clean pots and tools between uses. Keep hydroponic water cool and clean.
Chemical treatment (use with care)
Drenches with a fungicide labelled for Pythium can protect healthy plants, but they do not repair rotted roots. Follow the label.