Question
Dear Mr. Cawly,
I am doing a project in Science Class comparing plants grown in hydroponic solution to plants grown in soil. The plants in the hydroponic solution are getting discolored leaves- they aren't bright green like my teacher says they should be. He also says that the disease they have is called "yellow leaf", and he wants us to figure out the cause and what the cure is. Thank you very much for your help in advance.
Sincerely,
Jimmy

Answer
Hi Jimmy,

When plants don't have a good supply of nutrients (minerals), such as iron, magnesium, potassium, and others, the leaves can turn yellow.  This is called chlorosis.  This condition does not have to be disease-related; it can just be due to a shortage of an important mineral.

There are many diseases that can cause the same symptom (yellow leaves).  Any time a pathogen (disease-causing organism) causes the plant not to function properly, the effect is the same as a mineral deficiency.  There are very small worms called nematodes that can cause yellow leaves; there are many plant viruses that cause yellow leaves.

Your first step should be to determine what nutrients are provided in your hydroponic solution.  If there is a shortage of nutrients (nitrogen, phosphorous, potassium, iron, magnesium, and calcium.)  The most important nutrients (for maintaining good green leaf color) are nitrogen, iron, and magnesium.  When there are shortages of these nutrients, the yellow leaf symptoms appear.

If you find no deficiencies in those nutrients, then the plants may be diseased.  (Are the soil-grown plants in the same location as the hydroponic-grown plants?  If so, and there is no discoloration of the leaves, then you probably are not dealing with a plant disease.)

Please let me hear what you find out!