Plant Diseases - Recongition
A clear understanding of the appearance of a healthy, "normal" plant gives you the basis for describing symptoms caused by a plant pathogen. Symptoms caused by plant pathogens include leaf spots, galls (overgrowths), mildews, rusts, rots, chlorosis (yellowing) and wilting. Wilting is a general symptom that should spur a number of questions. Wilting can be caused by environmental conditions (too much or too little water), mechanical injury to the roots or stem (trunk), insect damage and plant pathogens (including fungi, bacteria, and nematodes).
Like insects, most plant pathogens can infect only one or a few related kinds of plants. So if unrelated plants, for example turf, woody ornamentals, and bedding plants, are all affected, a plant pathogen is probably not the cause. There are a few exceptions such as cotton root rot, a fungal disease, which attacks a large number of plants (but not grasses).
It is important to check all parts of the plant, including the roots. Infections by plant pathogens can result in symptoms similar to those from other causes. Root damage caused by fungi, bacteria or nematodes may result in symptoms similar to drought or nutritional deficiencies. A spotted or yellowed rose leaf does not necessarily mean rose blackspot. It may be the result of mite injury or spray injury or a reaction to the weather. Careful observation of the entire plant and planting may help eliminate some of the possibilities.
The pattern of damage or affected tissue may provide a clue as to the cause. Most plant pathogens will attack either older leaves or younger leaves but not both at the same time. Do newly emerged leaves also show symptoms? Spray injury, a one-time occurrence, will not continue to spread to new leaves, while, plant pathogens will.
Diseases caused by plant pathogens take time to develop. Seldom do plants die overnight from a microorganisms. For example, leaf spots of different ages are usually obvious if caused by fungi and bacteria. Spots that occur overnight may be the result of spray damage or pollution injury. In addition, diseases caused by plant pathogens continue to develop and spread (on the same plant or between plants) over time if environmental conditions remain favorable.
Recognizing Cultural and Environmental Problems
On average, 65 - 75% of all the plant problems you see will be caused by cultural/environmental factors. Once you've eliminated insect, mite, and plant disease causing pathogens from consideration, chances are some environmental factor is the source of your problem. At this point you must begin asking lots of questions. Categories of cultural and environmental problems include problems with soil, problems with moisture, misapplication of chemicals, physical damage to the plant, weather conditions and a variety of other potential problems.
One good clue to an environmental problem is to observe whether several plant types in the landscape are exhibiting similar symptoms. If this is true, suspect either: (1) environmental stresses (e.g. drought), (2) chemical injury, or (3) if occurring on dicotyledonous plants (as opposed to grasses) in alkaline clay soils from June through September, cotton root rot. Other than cotton root rot, most diseases will attack only a narrow range of plant species. Insects, likewise, usually prefer one or a few related plants as food.