This article by Darin Eastbourne of the University of Illinois appeared in The Great Lakes Vegetable Growers' News', March 1998. One of the most important aspects of any disease control program, whether it be for transplant production in the green house or crop production in the field, is the correct identification of the problem. There are some disease problems that growers can identify on their own, while others may require more expertise. In either case, there are things that the grower can do to narrow the possibilities and gather information that will make it easier to identify the problem, or at least give them a better idea of who they need to contact for help. Usually, a problem is first detected by the appearance of symptoms on the plants.
Symptoms can include spots on the leaves, stunting or wilting, root rots, and many other conditions.
It is important to make note not only of the symptoms themselves, but also where they occur on the plant, when they first appeared, and how quickly they develop. It is also important to look at the whole plant. Yellow and wilted leaves may be the result of a root rot or vascular wilt pathogen. It is important to observe the distribution of the problem in the planting. Likewise, if the problem appears all at once, and does not appear to be spreading, then a non-infectious problem might be a likely cause.