Overwatering

• Overwatering increases stress on the plant and causes the foliage to turn yellow. Allowing the sensitive roots of the lily plant to sit in waterlogged or poorly draining soils prevents the roots from absorbing oxygen, thus causing the plants to turn yellow. Root rot causes the roots to appear black and slimy, unlike the creamy or white roots of healthy plants, and causes leaves to turn yellow and droop. The tropical plants thrive in slightly dry soils, so irrigate the garden or potted lily when the surface of soil feels dry to the touch. Avoid irrigating the plant with unfiltered tap water or water that contains high sodium or chlorine content. Soils with a thin layer of white crust indicate excessive salt buildup. Flush potted lilies with water to leach accumulated salts from the soil and use distilled or filtered water whenever possible.
  
  

Overfertilizing

• Most lilies generally thrive with two applications of an all-purpose fertilizer every year -- once during spring and then again in fall. Overfertilizing exposes the plant to excessive salts and nitrogen that it cannot absorb, resulting in yellowing foliage and burned roots.
  
  

Lighting

• Leaves of lilies exposed to direct sunlight for most of the day turn yellow and become sunscalded. Exposure to full sunlight causes the stems and leaves of the lily plant to pale in color, become yellow and wilt. Ideally, grow your lily plant in a south- or north-facing spot exposed to filtered sunlight. Place your potted lily a few feet behind if positioned near a window, or hang a light curtain to provide it filtered light.
  
  

Other Factors

• Lilies also turn yellow when they become dormant, the natural process of ensuring the plant rests every year. Yellowing foliage bounces back to life and regains natural color in spring when the plant turns green again. Lilies turn yellow when exposed to a sudden wind or freezing temperatures. Your lily may also turn yellow as a result of transplant shock when you move it from one place to another. Sometimes older foliage on the lily turns yellow and falls off to make way for new leaves.