Strides in breeding have made tomato plants resistant to some debilitating diseases, thus increasing gardeners' success with them. But they are still prone to problems from pests, large and small. It may be hard to spot fat tomato horn worms but it's easy to defeat them with products containing Bt (Bacillus thuringiensis). Aphids and whiteflies can be blasted off vines with water or insecticidal soap spray. Russet mites, though, are too small to see without a magnifying glass, but they frequently are the cause of leaf death and drop on what seemingly are healthy plants. Sulfur dust or wettable powder is the best defense and treatment. Follow package directions to apply.