Root rot
Roots decay in waterlogged soil, so the plant wilts and yellows even though the soil is wet.
Also known as: Pythium root rot, Phytophthora root rot
Symptoms and identification
Leaves yellow, wilt or drop while the soil is still moist. Growth stalls, and the base of the stem may turn dark and soft. Roots are brown or black, mushy and often smell sour instead of being firm and pale.
Causes
Overwatering, pots without drainage holes, saucers left full of water and heavy, compacted soil keep the roots short of air. Soil-borne water moulds and fungi such as Pythium, Phytophthora and Fusarium then attack the weakened roots. Cool or cloudy weather, when plants use less water, raises the risk.
Organic treatment
Remove the plant from its pot and wash away the old soil. Cut off every soft, dark root with clean scissors, then repot in fresh, barely moist mix in a clean pot. Mixing in compost enriched with Trichoderma helps protect the new roots. Water sparingly until new growth appears. If most roots are gone, take healthy stem cuttings and start again.
Prevention
Use pots with drainage holes and an airy potting mix. Water only when the top few centimetres of soil are dry, empty saucers after watering, and water less during the monsoon and cooler months. Choose a pot only slightly larger than the root ball.
Chemical treatment (use with care)
After trimming the roots, a soil drench of a fungicide labelled for root rot (for example, one based on copper oxychloride) can be applied at the label dose.