Powdery mildew presents a common tomato plant ailment that strikes the stems and foliage. Affected leaves turn yellow and become distorted. Severe cases can defoliate the plant, and cause premature fruit drop and even kill seedlings. Warm days and cool, damp nights enable the fungal infection to flourish. Overcrowded plants that hinder adequate air circulation further encourage its growth. While there's no cure for this disease, there are steps that you can take to treat powdery mildew on tomato plants and minimize the devastation.
Inspect the leaves and stems of your tomato plants daily. Look for a white or gray powdery-looking substance. If caught early enough, small patches can be briskly rubbed off of the leaves with your fingers.
Spray infected tomato plants with fungicide approved for treating powdery mildew. Commercial products that contain triforine, chlorothalonil or triademefon help fight infestations. Follow the packaging instructions carefully.
Add 1 tbsp. baking soda and 3 tbsp. dormant horticultural oil to a gallon of water. Mix well to dissolve the baking soda. Stir in ½ tbsp. hand dish-washing soap. Spray on affected tomato plants once a week throughout the growing season.
Prune excess foliage from your tomato plants to increase air circulation. Allow plenty of open space between each plant. Prune limbs from larger plants or trees that obstruct sunlight.
Reduce watering as much as possible without harming your tomato plants. Excess water increases humidity and encourages powdery mildew growth.
Fertilize your tomato plants correctly--do not overfeed them. Too much fertilizer promotes the growth of excessive foliage and further encourages fungal infection.
Pick up and dispose of infected vegetative material. Don't toss it onto the compost pile, creating a breeding ground and over-winter nursery for powdery mildew.