Crown Rot (fungi – Pythium spp., Phytophthora spp., Fusarium spp.): Crown rot is probably the most serious disease of African violets and may cause loss of entire groups of plants. Older leaves droop and younger leaves showing stunting. Roots are killed rapidly and appear brown. Unless treatment is administered before massive root death, the plant will have to grow an entirely new root system before recovery occurs. Two alternatives are available for infected plants: One is to discard all affected and exposed plants and the other is to use fungicide drenches. Drenches should be administered when the first evidence of disease occurs. Caution should be used in getting the right dosage levels since excessive levels of some chemicals may damage plants. Preventive measures include using sterilized soil and avoiding plant introductions that may harbor crown rot organisms.
Botrytis Blight (fungus – Botrytis cinerea): Leaves, flowers and petioles develop small water-soaked spots that enlarge rapidly. A grayish fungal growth may be seen upon close examination of diseased tissue. The disease is more severe when atmospheric conditions are cool and damp, poor air circulation exists and when light intensity is low. All disease tissue should be removed as this serves as a source for new spores. Surface sterilize the area surrounding the plants with a household cleaner or bleach.
Powdery Mildew (fungus – Oidium spp.): A white powdery type substance may be observed on leaves, petioles, petals or flower stems. It is more easily seen on dark blossoms than white ones even though the white varieties may be slightly more susceptible. Spores of the fungus are air-borne from one plant to another. Control with fungicides is very effective. If the systemic fungicide is used, one may have to spray blossoms since the fungicide is not translocated to floral parts when the pot drench method is employed.
Petiole Rot (Physiogenic): An orange to brown, rust colored spot appears where the petiole touches the pot. The petiole and leaf may collapse. This damage results from salt accumulation on the rim of the pot and the soil surface. Use rain water or another salt-free source of water and avoid over fertilization. Construct a collar from aluminum foil to be fitted around the rim of the pot.
Ring Spot (Physiogenic): Light brown rings form on leaves with some running together to form irregularly shaped spots. This condition is caused by cold water coming in contact with the leaves. In some cases, damage may occur even if warm water is used. Such a possibility exists when a breeze blows across the wet area and produces an evaporative effect. Water should be kept off leaves.
Root Knot (nematode – Meloidogyne spp.): Galls form on roots of plants causing the root system to be inefficient in absorbing and translocating water and nutrients. This condition can be prevented by using nematode free propagating stock and a sterilized potting medium. The nematode may also be transmitted by petiole cuttings from infected plants. Do not use infected plants for propagation.
Crown Gall (bacterium – Agrobacterium tumefaciens): Fleshy galls form around the base of the plant with a profusion of leaves being produced at that point. Natural infection seems to be slight but infected tissue is perpetuated by some who sell this unusual looking plant under the name of “Witchcraft”. Infected plants should be discarded.
Viruses (virus – several): Infected plants have distorted leaves that are mottled in color. Infected plants are not killed but should be discarded because they may serve as a possible source of infection for other plants.