While not a pest per se, chlorosis is a troublesome garden condition in which leaves produce insufficient chlorophyll. As chlorophyll is responsible for the green colour of leaves, chlorotic leaves are pale, yellow, or yellow-white. The affected plant has little or no ability to manufacture carbohydrates through photosynthesis and may die unless the cause of its chlorophyll insufficiency is treated

To combat it, when leaves turn pale yellow, bring out 1 tablespoon of epsom salts and two pints of water and spray the foliage. Spreading a couple of handfuls around the base also will do the trick. But the wiki entry also states that it may be corrected by supplemental feedings of iron, magnesium or nitrogen compounds in various combinations.