Why do my ripe tomatoes have yellow color?

by Tambra Baron
(Seguin, TX)

Q. All my tomato plants are full of green tomatoes, and the bushes look OK. The fruit doesn't seem to be ripening all the way to red. Instead, the fruit is turning red with yellow spots and streaks. The weather here is cool in the mornings and evening and extremely hot in the middle of the day. I'm watering every other day.

A. Carotene, a pigment in tomatoes, produces yellow and orange. Lycopene, on the other hand, is the pigment that produces a red color in your tomatoes. Unfortunately, lycopene is inhibited by excessive direct sun.

When higher temps and hot sun strike tomato tops, carotene (yellow) shines through while lycopene (red) is squelched. The lower part of the tomato is often protected from direct exposure by the top of the fruit. Thus the sun inhibits lycopene (red) in the parts of your tomatoes that receive the most direct sun. Carotene will dominate in those areas.

What you can do

• Maintain good leaf cover if possible. Plant more densely and prune less.
  
• Shade plants in the afternoons when temperatures are especially hot and fruit is ripening.
  
• Some tomato varieties have inherited more of a tendency to turn yellow in the direct sun. Choose varieties which are more heat-resistant.