Plant diseases are caused by one of three organisms: bacteria, fungi or viruses. Although all groups may give trouble from time to time, bacterial and viral diseases are relatively rare. Diseases usually affect the growth or health of the plant (shown by distortion, discoloration or wilting), and may even ultimately lead to its death.
The key to disease prevention, like many other things, is good garden hygiene. Keeping the garden or greenhouse clear of decaying, diseased or dead material and disinfecting pots, trays, canes and other equipment at the end of each season can remove potential sources of infection. Choosing disease-resistant plant species will also help, as will keeping plants vigorous and healthy so that they are better equipped to fight off any problems.