If houseplants, trees, plants, or shrubs are repeatedly overwatered, their leaves can start to get dull, turn yellow, and the plants can look distressed. Typically when this happens to any plant, it is suffering from root rot.
Root rot can be caused two ways:
1) is prolonged exposure to overwatered conditions that can cause some of the roots to die back due to a lack of oxygen. As they die, they decay and rot away. The rot can spread to healthier roots killing them too, even if the soil conditions are corrected.
2) Root rot can be from a fungus in the soil. The fungus may lie dormant in the soil for years and then may suddenly come to life when a plant is overwatered once or twice. The root rot fungus attacks the roots and causes them to die and rot away.
Root rot symptoms are most obvious on the above-ground plant parts. Look for wilted leaves and branch dieback.
Symptoms of root rot include leaves that are smaller than usual that may wilt and turn yellow or brown. The plant or tree begins to decline gradually or quickly for no obvious reason. Brown or blackened and damaged areas are visible on crowns and roots. Armillaria root rot on trees is indicated by white mats of fungi and dark brown fungal strands found on the roots or growing between the bark and wood.
If the plant is slowly wilting and the leaves are turning yellow for seemingly unknown reasons, check the roots. Remove the plant from the soil and feel the roots. The roots affected by root rot will look black and mushy. Affected roots can actually fall off the plant when touched. Healthy roots may be black or pale, but will feel firm.
ROOT ROT PREVENTION & CONTROL
1.Plant resistant cultivars when available.
2. Set out transplants when soil has warmed up enough for good growth.
3. Enrich soil with compost.
4. Try not to cultivate close to roots which can cause injury and allow disease to enter.
5. Plant in well-drained soil; avoid overwatering.
6. Make sure water doesn’t pool around the base of trees by forming irrigation moats to hold water at least 12 inches (30 cm) away from the stems or trunks.
1. Remove and destroy badly infected plants.
2. Moderately affected plants may be saved by pruning away diseased roots and replanting in well-drained soil. Using clean shears, make sure to also prune back a 1/3 to 1/2 of the top growth on the plant. This will give the plant a better chance to regrow the roots since it will not need to support as much top growth.
3. Disinfect all pots or tools used on or around infected plants using a 1 part bleach to 9 parts water solution.
4. Use soil solarization before planting as it has been shown to give good results in controlling root rot.
For more about Soil Solariztion read:
Soil Solarization Will Kill Fungus
Soil Solarization – Use Clear or Black Plastic