The organisms which cause plant disease can destroy crops from the time the seed is put into the ground until the crop is harvested and in storage. Some diseases are capable of totally destroying a crop, while others may cause only cosmetic damage. However, cosmetic damage may be equivalent to total destruction in the case of ornamental plants.

A basic understanding of diseases and how they develop will give an appreciation for the complexity of the problem and demonstrate the importance of cultural practices in control of diseases.