- Powdery mildew thrives in humid conditions. It appears on parts of the plant above the soil as a white, powdery substance. In its early stages powdery mildew doesn't typically cause harm to the plant. It can be controlled by moving infected plants to a dryer location with good ventilation and prune off infected parts. Spores are easily spread and do not need water to germinate so inspect adjacent plants as well.
- Micro-organisms that cause root, stem, or crown rot thrive in moist to wet soils. Symptoms begin as drooping leaves, and then the stem and root darken and turn mushy. Lower leaves may appear dark or soaked. If detected early, damage can be stopped and the plant can still thrive. Place infected plants in a dry, well-ventilated area and do not water until signs of infection have stopped appearing. Remove damaged parts and if necessary, re-root the top portion of the plant. Typically cacti and succulents are most vulnerable to stem and root rot.
- Various types of fungus and bacteria can enter new plants through the soil causing several wilt diseases. These organisms impede the plant's ability to soak up water and the leaves eventually wilt and turn brown. If the infection is severe enough, growth will stop and the plant will die. Healthy cuttings from an infected plant may be removed from the upper portion of the plant and rooted in new soil. Disinfect or discard the old pot.
- Botyritis blight, also called grey mold is typically caused by humid or crowded conditions or dead parts left on a plant. Symptoms may be different depending on the plant, but typically it botyritis blight appears as fuzzy grey mold-like growths. Removing old or infected leaves and flowers and moving the plant to a dry, well-ventilated area can stop the mold. Allow soil to dry somewhat between waterings as well. Fungicide treatment may be necessary in severe infections.
- Several types of fungi located in the soil may attack seedlings or cuttings resulting in rotting at the base of the plant. Damping off fungi can result in seedlings collapsing and dying, and in seed decay, where seeds fail to germinate. In proper conditions, damping off isn't likely. To prevent these fungi from infecting seedlings and cuttings use a sterile, porous soil, don't overcrowd seedlings and keep plants in a well-ventilated area.
- Rust diseases are caused by parasitic fungi and each type is usually specific to one or two species of plants. Rust infections begin as orange, yellow or white spots on the undersides of leaves. These spots eventually show on upper sides of leaves, stems, flowers and fruit and swelling and galls may appear. Rust weakens plants resulting in dropped leaves and plant death. Typically rust infections don't spread internally within the plant, but spores can be picked up and spread to other plants by the wind. They thrive in warm, wet conditions and new plant tissue is more vulnerable than old. Providing good air circulation and avoiding over-watering can prevent rust infections from developing.
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