My tomato plant's leaves are yellow in the center

by Sher
(Apopka, FL, USA)

Q. My tomato plant is about 24" high and has approx 20 small (marble sized) fruit. The plant seems very sturdy and the leaves are very green. The center of the leaves are now turning light green and yellow. What can the problem be?

A. There are several reasons the centers of tomato leaves could be turning yellow. The first question to ask is whether or not you have sprayed nearby with a weed killer or herbicide? If so, the yellowing/white on the leaves could be due to drift from the glyphosate in products like RoundUp. Tomatoes are affected by very small quantities of application. Herbicides can take on a gaseous form after application in hot weather. Even if you spray from a distance, wind drift can carry the chemicals towards your tomato plants.

Other reasons your leaves could be affected:

• Over-watering. Maybe you haven't watered too much, but perhaps your soil mix (if plant is in a container) may be heavy. A looser mix with more peat, vermiculite, perlite, and coco coir will allow water to drain easily, avoiding drowning. If plants are in the garden, check to see if they have been planted in a low-lying area or if they have had too much rain.
  
• Pests. Aphids, spider mites and nematodes cause tomato leaves to turn yellow, then brown and die. Spray plants with
  insecticidal soap.
  
• Fusarium wilt or Septoria leaf spot. Both diseases present with gradual leaf yellowing. Destroy fusarium-infected plants. Treat Septoria-infected plants with fungicide.