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. Insects are vulnerable at different stages in their life cycle. If you spray an adult insect that is only vulnerable in its larval stage, little or no control will be achieved. It is also important to know when the insects are causing the most damage.