Diseases and physiological problems are it much more difficult to identify than pests, where you can usually see a definite culprit. Fungi and bacteria are microscopic organisms, which invade unnoticed; unless the symptoms of attack are seen and dealt with promptly, they can be fatal, not only to the initial host, but also to other nearby plants as the organism spreads.
A virus can be passed on in cuttings from one generation to the next, and while the resulting leaf markings tan be attractive (as in several varieties of camellia), they can sometimes cause serious distortion.
Physiological problems arise most often when the care instructions for a particular plant have not been followed closely enough. It is important to the health of the plant to try to give it the conditions it needs, regular food, sufficient water, and plenty of room for the roots to expand. If a problem is suspected, the plant in question should be isolated and treated as quickly as possible, before other plants can be affected.
Identification of the problem is not always straightforward because it is possible for different causes to produce the same symptoms on the plant. If there is no obvious pest, and the symptoms seem to suggest more than one possible cause, it may be necessary to try more than one course of treatment to help the plant recover.
Sometimes, the most efficient solution is to use a chemical control for the problem, and if this is the case, it is vitally important that the manufacturer’s instructions are followed exactly. Apply the chemical outdoors on a calm, windless day, keeping children and pets well out of the way and wearing protection if it is advised.