This is aggravated by the accumulation of fertilizer salts on the rim of the porous pot or on the soil surface. An aluminum foil collar on the rim of the pot and flushing the soil occasionally with heavy watering will prevent this problem. The unsightly yellow or white ring and line patterns that occasionally appear on the leaves are thought to be caused by a sudden chilling of the leaves by cold water or by the sun shining on wet leaves. This condition is known as ring spot or chlorosis, and can be avoided by carefully watering the plants from below with water slightly warmer than air temperature. African violets infested with root-knot nematodes are stunted and weakened.
Galls form on the roots, and the crown and leaves become thickened and distorted.
Blisterlike galls also develop on the leaves. Destroy all infested plants, sterilize their containers, and propagate new plants only from nematode-free plants. The leaves of plants grown under greenhouse conditions may become infected by the fungus Botrytis cinerea . Botrytis blight first appears as small water-soaked lesions on the underside of the leaf. These lesions enlarge until much of the leaf turns brown to black.