Recently I was reading a thread in a forum I frequent, the discussion was on identifying brown scale; One of the entries provided a reference to the following article on:
Physiological disorders of tropical foliage plants 
This month we are taking a look at two physiological disorders commonly seen on foliage plants here in south Florida during the colder months. Physiological disorders are not caused by pathogens, but by environmental factors instead. Because these disorders are not caused by biotic agents, they are not infectious and will not spread.
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Be sure to read the related articles following the main article.