University of Manitoba - Faculty of Agricultural & Food Sciences - Rot (Crown, Stem and Root)
Rot (Crown, Stem and Root)

Problem type: Disease

Name of problem: Rot (crown, stem and root)

Plant name(s): All plants, especially African violet, begonia, cactus, dieffenbachia and Chinese evergreen; all container plants are extremely susceptible to rot

Symptoms / Characteristics:
Crown and stem tissue becomes dark, soft and spongy. Infected stems may eventually become girdled by dark lesions that form at the soil surface. Tissues above the infected area eventually collapse. Root rot is typically manifested through foliar symptoms. Leaves turn brown, wilt and fall from the plant. Symptoms begin in lower leaves and progress throughout the canopy. Plants appear wilted as though they are dehydrated but fail to recover after watering. Deterioration of the root system eventually causes the entire plant to collapse. Firm, white, healthy roots become mushy and dark. Decaying roots emit a rancid odor. Plants are poorly anchored and can be pulled easily from the soil. Infected plants exhibit an overall decrease in growth and vigour. Severe infections often result in plant death.

Crown, stem and root rot are caused by soil-inhabiting fungi. A complex of two or more fungal pathogens often causes root rot infections. Overwatering can be detrimental to a plant. Prolonged periods of high moisture not only facilitate the development of rot-causing fungi but also weaken root systems, rendering the plants extremely susceptible to attack. Waterlogged soil does not have enough oxygen to support the development of a strong, healthy root system and oxygen-deprived roots essentially suffocate. Stressed plants generally have weakened defense systems and are more susceptible to pest invasions.

Control / Preventions:
The best method of controlling rot is avoidance. Wash and sterilize pots prior to planting by soaking them in 10% bleach for a half hour. Clay pots are more porous and are preferred over plastic. If the pots do not have a drainage hole, line the bottom of the pot with a thick layer of gravel or stone. Use only sterilized potting soil that is light and well-drained.

Avoid overwatering, as this is the primary cause of rot infections. Allow the soil to dry slightly between waterings and water only when necessary. Remember that water requirements may vary according to plant species, soil type and light exposure.

Crown rot is extremely common in plants such as African violet that form a rosette near the soil surface. Never allow water to accumulate in the crown of a rosette-type plant as it leads to the rapid decay of sensitive crown tissue. African violet leaves and leaf stems (petioles) that come in direct contact with waterlogged soil are extremely susceptible to rot. Allow the top layer of the soil to dry out between waterings to prevent constant contact with wet soil. It is imperative that the violets are not planted too deep. Water plants at the soil surface instead of overhead and avoid wetting the leaves of African violet and other fuzzy-leaved plants.

Recovery may be possible if the infection is detected early. For crown and stem rot, remove and destroy infected tissues immediately and apply a recommended house plant fungicide in order to prevent further infection. Do not water wilted plants until is has been confirmed that dehydration is the cause. Watering infected plants will only fuel the rot-causing fungi and lead to further deterioration of plant tissue. Check the soil moisture and examine the roots if necessary. Look for mold growing on the surface of the soil. Although a separate organism, its growth is usually indicative of waterlogged conditions. If only slight root damage has occurred, remove the infested soil from around the roots and prune out infected areas. Repot into a smaller pot if a large amount of root area has been removed. Reduce watering, especially during the recovery phase. Unfortunately, severely infected plants must be discarded. If possible, take cuttings from healthy tissue and try to propagate new plants. Tools that have come in contact with infected plants must be thoroughly disinfected immediately after use. Never use infested tools to take cuttings from healthy tissue.