Plant defense mechanisms normally activate and “kick into gear” when attacked by disease or insects. One method is to wall off the pathogen by killing off surrounding cells, somewhat like a fire line around a forest fire, as is apparent with canker symptoms on citrus. This “halo effect” is commonly observed as the yellowing around a diseased area, such as with bacterial canker. But the plant responds further, by releasing various chemical compounds that alert the rest of the plant to begin producing other defensive compounds that increase plant resistance to infection or attack at other sites on the plant.
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