As with insect and animal pests, homeowners typically wait until their first encounter with a plant disease problem on their property and then seek out control measures. Once the crisis has passed, they should then think in terms of prevention.
You have fewer tools among the plants in your landscape for fighting disease than you have available for use against insect and animal pests. Often, by the time a disease makes its presence known, effective or practical control steps for individual plants do not exist. There is little choice but to consign each particular sick plant to the trash. However, the removal of infected plants does serve as an important control of disease in your landscape as a whole.
Remember, a number of fungal diseases can occur that cause spots or some yellowing but are not lethal to the plant or seriously detrimental to the performance of that plant in the landscape. So do not panic at first sight of a single yellow leaf on your azalea bush. Look at the general vigor of the plant. If it looks otherwise healthy, wait for a few days or even weeks to see if in fact you have a serious problem. Cultural or environmental disease and fungal diseases occur slowly. If your plants are deteriorating very rapidly (within a few days), it is more likely a sign of a viral or bacterial disease, and in that case the infected plants should be immediately removed and destroyed.
The first step in controlling diseases in the landscape is to spot and identify them. Look for symptoms. Examine plants for any changes in their general appearance. Many, if not most, of the problem symptoms you will find on your plants are caused by insects rather than disease. By reading the descriptions given for diseases and insect pest problems on this web site, and with some experience, you will learn to distinguish insect damage from disease damage.
Once you've decided that the problem is caused by a disease rather than an insect pest, you will need to determine the exact disease that has infected your plants. Unfortunately, not every disease exhibits its own specific pattern of symptoms. Many symptoms can be caused by more than one disease.
Plant diseases in the landscape can be divided into five broad categories:
Unfortunately, it is not always easy to distinguish symptoms among the categories, since many of the symptoms can be a result of two or three different types of disease. Nevertheless, some problems are more common than others, and homeowners learn to diagnose disease problems from their experience of what has happened in their yard in previous years. Although there is some overlap, each type of disease, environmental, fungal, bacterial, and so forth, can be recognized by a general set of symptoms.
If you have just begun to work with a yard and garden, you will find that most of your disease problems will be environmental (watering, feeding, location, etc.). The next group most likely to attack your landscape will be the fungal diseases, with the bacterial, viral, and nematode diseases occurring least often on the average.
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