Indescriminate spraying is never a recommended course of action. Know what is causing the damage first, then spray for that particular pest/fungus/disease. Or if you know for certain that a fungus/disease is prevalent in your area you can start a preventative spraying program.

In the case of your cucumbers, I'd bet that it's cucumber beetles transmitting bacterial wilt to your plants. Nothing you can do about the wilt once your plants have it but you can control the beetles. A search here will pull up a lot of info on them.

Rodney