It sounds like you could have either one of two problems, curly top virus or early blight. Curly top virus is transmitted by the beet leafhopper. Infected plants turn yellow and stop growing. Upper leaflets roll and develop a purplish color, especially along the veins. Leaves and stems become stiff; fruit ripens prematurely. Early blight produces brown to black, target-like spots on older leaves. If severe, the fungus also attacks stems and fruit. Affected leaves may turn yellow, then drop, leaving the fruit exposed. Visit The University of California’s Agriculture and Natural Resources page to learn about treatment of these problems and make sure to sign up for our Gardening How-To Guide for more answer to common gardening issues, monthly updates, and more!