Before even looking at applications to treat plant diseases, try and prevent initial infection by being wise about proper garden hygiene. Constant and complete soil cultivation will vastly reduce the risks of disease occurrence. Choosing vigorous and hardy plants will also safeguard against a range of diseases. Many plant producers subscribe to virus-free certification schemes. So, when you come across them, it is a great idea to use only certified specimens.
The majority of chemicals that deal with disease are preventative only. So when disease is identified make absolutely sure that you cut out affected areas of the plant or grub out entire plants if the disease has got out of control. Never fail to thoroughly burn all resultant debris.
Keep plants in the prime of health and nourish them well. This will make them more resistant to disease attacks. But, if disease does occur, here is an alphabetical guide to identifying diseases and the correct methods of treatment.