Whether you are just starting to grow tomato plants or are a seasoned gardener, when the leaves of a healthy green plant turn color and drop off, you know something is wrong. Although the symptoms of many tomato plant leaf diseases sound quite similar, once you know what to look for, you will know what disease you are dealing with and how to treat it.
Early blight is caused by the fungus Alternaria solani and is one of the most common diseases that affects the tomato plant. It usually appears when there is constant moisture such as rain or fog and is first recognizable by the appearance of ¼- to ½- inch round brownish black to black spots that appear on the plant's lower leaves. When only a few spots are present, the leaves may turn yellow and begin to dry. As the disease progresses, however, the spots will join together and eventually the leaves drop off. Although removal of diseased leaves may slow early blight, to effectively control it will normally require a fungicide such as chlorothalonil. Apply the fungicide over a 7- to 10-day time span. Since the spores can remain in the ground for one or more years, cleanup and removal of affected plants after the growing season is essential. When possible, plant tomato plants for the following year in a different area to avoid the possibility of early blight returning.
Leaf spot is caused by the fungus Septoria lycopersici and is a common problem for tomato plants worldwide. Appearing at almost any stage of plant growth, leaf spot begins as 1/16- to 1/8-inch small circular spots that initially appear on the underside of the plant's more mature leaves. The spots begin with gray or tan centers and have darker margins. To control leaf spot, apply a fungicide for 7 to 10 days as soon as you notice the disease. Most fungicides registered for use on tomatoes will control leaf spot. At the end of the harvest season, clean the growing area and dispose of any debris remaining from the plants. For best results, do not use the area again for tomato plants for at least a year to ensure leaf spot does not return.
Powdery mildew is caused by the fungus Oidium neolycopersici and is usually seen on tomato plants that are grown in a greenhouse. Early signs of this disease are white areas on the upper surface part of the leaves that cause the leaves to drop as the affected areas turn to brown. Powdery mildew is primarily found in greenhouses that have poor circulation and plant spacing. To help prevent powdery mildew from continuing, properly space tomato plants and provide them with better air circulation. Apply a fungicide as soon as you notice the problem to help prevent it from spreading further. Fungicides containing tebuconazole and trifloxystrobin are appropriate for treating powdery mildew.