Diseases  Diseases are plant disorders caused by an infectious pathogen or agent.  Three conditions necessary for diseases in plants.  Host plant is the plant the disease attacks.  Disease causing organism or pathogen must be present.  A favorable environment for disease organism to develop (moist and warm).

Bacteria  Bacteria are single celled microorganisms.  Examples of common bacteria diseases:  Bacterial leaf spot are rings of different shades of brown, green or yellow spots on leaves.  Bacterial blight cause plant to quickly turn brown or black as if they had been burned.

Fungi  Fungi cannot make their own food, they develop hyphae, structures that grow and absorb nutrients from the host plant. Many fungi are spread by spores.  Examples of common fungi diseases:  Damping off causes young plants and seedling to rot off at the soil level.  Rust cause small spots on the leaves that resemble yellow, orange, brown or red rust mainly on the underside of leaves.  Powdery mildew grows on the upper and lower leaf surface as white or gray powdery substance. It is a common disease of houseplants.  Galls are round swellings or growths usually on tree branches or leaves.  Examples caused by bacteria or fungi are powdery mildew, blights, canker, rots, wilts and smut, canker, galls, leaf spot, mildew, rots and smut.

Viruses  Viruses are pathogens with an extremely narrow host range and are usually a threat to crops.  Examples of common viruses:  Tobacco mosaic virus which attacks tomatoes, peppers, poinsettias and tobacco.  Tobacco mosaic virus can be transfer from human hands of a smoker. Be sure to wash your hands before working with plants to control the spread of this virus.  Mosaic virus cause leave to have irregular mottled areas with patterns ranging from dark to light green and yellow to white.  Examples caused by viruses, bacteria or fungi are wilts and blights.