Carefully consider the conditions in which the plant is growing. In addition, a plant growing in an environment that is not optimal may be more prone to injury from insects and disease. The first step in remedying the situation is determining the cause of the problem. After ruling out poor growing conditions, look for signs of insects or disease. Knowing the exact cause will help determine the treatment and whether a chemical is necessary at all.
Different pesticides control different problems.
Herbicides treat weed problems. Insecticides treat insect problems. Fungicides treat disease problems. A fungicide won't control an insect problem and an insecticide won't control blight on your tomatoes. If an insect is causing the problem, identification is of utmost importance.