Over Watering and Cold Water
Brown spots on the leaves of African Violets are caused by a number of factors. Over watering will result in a brown, mushy rot or necrosis of the leaves. Applying cold water directly to the leaves will cause brown spots to form when the plant is exposed to sunlight. Applying warm water directly to the roots of the plant is the best method of watering an African violet. Misting the leaves with warm water creates a high humidity environment around the plant to which the violet will respond favorably. The mist will not create huge water droplets that lead to brown spots on the leaves.
Chlorinated Water and Bright Light
Using overly chlorinated water on African violets causes leaf spotting to occur and can severely restrict the plant’s blooming ability. If using chlorinated water, remember that if you can smell the chemical then it is too strong. Let it set out overnight to allow the chlorine to diminish. Violets prefer indirect lighting and too much bright light will cause the leaves to turn brown, a condition similar to that of sunburn in people. When using grow lamps on African violets, it is best to mount them at least 18 to 20 inches from the top leaves of the plant for the larger varieties and 10 to 12 inches for the small varieties.
Temperature, Leaf Nematodes and Fertilizer
Exposing the plant to too much variation in temperature higher or lower than 70 degrees is toxic to the plant and can cause crown rot, turning the top leaves brown and mushy. Leaf Nematodes are microscopic worms that cause a fatal condition in the plant, early symptoms of which are shiny, brown spots between the veins on the undersides of the leaves. Using too much fertilizer all at once causes leaf rot which makes brown spots that spread and eventually turn the leaves to mush. Too much boron and nitrogen in the soil or too little calcium, molybdenum or potassium can cause leaf tip burn that turns the tips of the leaves dry, brown and crisp.