Coleus Diseases

Downy Mildew

• Downy mildew is more likely to infect coleus plants grown in a greenhouse, but it may also infect garden plants under the right conditions. The symptoms vary and may be as slight as small spots on the leaves or as severe as a complete loss of leaves. Remove and destroy infected parts of the plant as soon as you detect them. Wet leaves encourage the downy mildew, so water the soil rather than the plant and water early in the day so that any moisture on the plant has a chance to dry completely before nightfall.

Rots

• Microorganisms that live in the soil infect the roots of coleus plants and cause rot. Because the damage is underground, gardeners don't notice the problem until the plant fails because the roots have died or the problem spreads to the stems and foliage. There is no cure for rots. Removing the plant is the only option. Plant coleus in well-drained soil to help prevent rots from infecting the roots.

Gray Mold

• Like downy mildew, gray mold, also called botrytis blight, is primarily a problem for coleus grown in greenhouses. In the presence of high humidity and overcrowding, though, it may also infect garden plants. Look for brown, water soaked spots on the leaves. Remove and destroy infected parts of the plant and disinfect the tools you use to care for diseased plants with a 10 percent bleach solution. Thinning plants to allow good air circulation helps prevent gray mold.

Powdery Mildew

• Powdery mildew strikes shaded plants when days are warm and nights are cool. Infected coleus plants look as though they've been sprinkled with talcum powder and severely infected plants may have distorted leaves that drop from the plant. Avoid overcrowding to help prevent the disease. Most garden centers carry products labeled for control of powdery mildew, or you can make your own by mixing 3 tablespoons of horticultural oil and 1 1/2 teaspoons of baking soda in a gallon of water.