Some of the leaves on my plants are turning yellow. What is causing this?

Yellowing leaves can be caused by several situations. The following are the most common:

1) Early Blight (fungus) - If this is the cause, you should see small circular brown lesions on the leaves as well. The spots become larger. Eventually, bottom leaves fall off.

2) Shading - As the plants mature, the bottom leaves get heavily shaded and naturally turn yellow as they senesce (age). This is not a problem. Any yellowing leaves are no longer productive and should be removed to improve air circulation.

3) Low nitrogen or potassium - If the nitrogen or potassium level is too low, yellowing will occur. Nitrogen deficiency shows up as a general yellowing of the entire plant. Potassium deficiency will appear as bright yellow leaf margins (edges) on otherwise green leaves.

4) Low Magnesium - Interveinal yellowing on older leaves is from low magnesium. If this is suspected, spray 2 TBL per gallon of magnesium sulfate (epsom salt) on the plants and see if they "green up" in a few days. For all suspected nutrient problems, a tissue analysis is recommended.