Mold on a Christmas Cactus

Botrytis Blight

• This fungal infection affects Christmas cactus grown in high temperatures and humidity. It appears on the plant’s delicate parts, such as flowers, buds and fresh leaves, although it can attack healthy stems. Symptoms begin with wet spots that grow into patches of soft, wilting tissue. These are followed by a layer of grayish, fungal threads.

Stem and Root Rot

• Christmas cactus stem rot is caused the fungus Fusarium oxysporum. It starts as a brown patch at or just above the soil line. It spread through the healthy stem tissue until it cuts the plant off from its roots. Christmas cacti roots are vulnerable to attack from oomycetes or water molds, such as Phytophthora parasitica and Pythiums. These destroy the roots and cause the plant to wilt and die.

Treatment

• Treat botrytis blight with a spray containing an antifungal agent such as iprodione. Root rots require treatment with a spray or soil drench containing antifungal chemicals such as metalaxyl or etridiazole.

Avoiding Mold

• Most cases of mold and fungal attack are due to excessive watering and high humidity. Keep your Christmas cactus in a well-ventilated spot, such as next to an open window. Water it only when the top inch of its soil has dried out and make sure that all containers have a large drainage hole. Never stand Christmas cactus in a dish of water and avoid misting the foliage on cool or damp days. Reduce watering during the colder months of the year. Pot your Christmas cactus in free-draining compost formulated for succulents or make up a mix using one part soil, two parts peat and one part sand or perlite.

Rescue

• Remove sections of healthy stem from infected cactus and plant them in sterile compost. Even single sections of stem will root if their lower ends are planted in the soil. Keep cuttings on a bright windowsill with the compost barely moist until new growth appears.