I just stumbled upon this free and interactive tool for diagnosing the problems (pests and diseases) with your plants and finding the right treatment. This online tool provides both organic and chemical treatments as well as useful pieces of advice to help you prevent problems in the future.
The Pest and Disease Identifier asks you some basic questions about the affected part of the plant and then diagnoses the problems by asking detailed questions about the whole plant and its growing conditions.
In the end, it suggests possible organic and chemical treatments to combat insects or cure the disease.
Try Pest and Disease Identifier yourself.