Why Are There White Spots on Tomato Plants?

As a backyard vegetable garden staple, a tomato plant (Lycopersicon esculentum) naturally has green leaves and stems. So if you notice white spots on your plant's leaves, you'll know something is amiss. Look closely to determine if the spots are a symptom of disease or if they may be telltale signs of insects -- their skins or excrement.

Although it's a tender perennial in U.S. Department of Agriculture plant hardiness zones 10 through 11, the tomato is grown as an annual in other areas.

Several fungal diseases may cause tomato leaves to have white spots. These spots are fungal spores, one stage of the disease.

Typically common when the humidity is high, powdery mildew initially causes white spots on tomato leaves that typically spread across a leaf's surface. Most fungicides are applied only as protective measures -- before the disease takes hold -- or they're generally ineffective in controlling the disease. But one biological control may be successful in controlling powdery mildew after it appears. Look for the active ingredient Bacillus subtilis, a bacterium approved for organic gardening. Typically, you'll mix 2 to 4 ounces in 1 gallon of water and spray all surfaces of a tomato plant thoroughly until the products runs off. You may need to repeat this application every seven days to keep the disease under control.

Applying this bacterium as a fungicide works best when you begin treating plants at the first signs of powdery mildew.

Even though Bacillus subtilis is approved for organic gardening, exercise caution when using it. Wear a long-sleeved shirt, long pants, socks, shoes and waterproof gloves. Don't apply the product in windy weather because it's harmful if inhaled.

Septoria leaf spot is another fungal disease that causes white spots on tomato leaves. You can tell the difference between this disease and powdery mildew because septoria leaf spots typically have brown borders and powdery mildew spots are solid white. Apply a product with chlorothalonil as the primary ingredient, typically at the rate of 2 teaspoons per 1 gallon of water, and coat all plant surfaces thoroughly. Reapply every seven to 10 days. Wash your hands after using and keep the product out of reach of children or pets.

Don't plant tomatoes in the same spot where last year's plants were infected with this disease. The fungus can overwinter in the ground and infect the following year's plants.

Aphids are insects that feed on the cell sap in tomato leaves. Although the insects are not white, they shed their skins as they grow. These skin castings are white and are typically left on leaf surfaces, like a sprinkling of white spots. Control aphids without using chemicals by spraying them with a jet of water from a garden hose, which knocks them off plants.

Psyllids are insects that also feed on tomato leaf cell sap. They excrete white waste, which looks like grains of sugar on the leaves. Dust leaves with sulfur, particularly the undersides of leaves, to control psyllids.

Using a horticultural oil after applying sulfur to tomatoes can harm the plants.

When applying sulfur, do not breathe the product or apply on a windy day and wash your hands after using. Store out of the reach of children or pets.