What Is the White Powdery Stuff on My Tomato Plant Leaves?
- According to The World Vegetable Center, there are two forms of powdery mildew, Oidiopsis taurica, which affects the bottoms of the leaves, and Oidium neolycopersici which affects the tops. If there are yellow spots on the upper surface of the leaf and a white powdery substance on the bottoms of the leaves, it is likely Oidiopsis taurica.
- Yellow spots and a powdery substance on either the tops or bottoms of the leaves are symptoms of powdery mildew. These affected leaves will turn yellow, then brown and eventually dry up if the plant is not treated. If the disease is caught early enough, only the lower leaves will be affected as the disease travels upwards from the bottom of the plant. These symptoms can appear at any stage in the plant's growth so frequent inspection is very important.
- This fungus's spores are spread by the wind, rain and insects as reported by The World Vegetable Center. Like many fungi, powdery mildew thrives in a warm, moist environment. Fluctuations in temperature and humidity cause the spores to germinate and the fungus to take hold. Plants growing in soil that is too high in nitrogen are also prime candidates for powdery mildew.
- Choose fertilizer that is not high in nitrogen. The lush growth that excess nitrogen promotes is particularly vulnerable to infestations of powdery mildew.
Do not overcrowd the plants; ensure there is enough space and air flow for the lower leaves to dry properly. Avoid watering from the top as this added dampness will promote infection of the disease. Also, try to water in the morning so that if the leaves do get wet, they'll have plenty of time to dry.
Plant tomato varieties that are resistant to disease. They are becoming easier to find and they may save you a lot of trouble. - Sulphur dust or sprays as well as other fungicides can be used to control powdery mildew. Ensure the affected sides of the leaves are well covered in the fungicide, but be careful to not spray so heavily that the fungicide drips excessively on to the soil, where beneficial fungi live. Avoid applying the sulphur during periods of extreme heat so you don't burn the leaves. Follow the instructions on the product you buy to ensure best treatment. Although affected leaves will not resume their previous form, no new leaves will be affected.