The Q&A Archives: Tomato leaves are yellowing

Question: Why are our tomato leaves yellowing? Mainly the lower older leaves--however some of the leaves in the middle of the plant are getting some pale yellow spots. We garden in a small solar heated greenhouse, the temperature stays between 68 and 85 degrees, and we water regularly.

Answer: It's natural for some of the older, lower leaves to yellow and die on tomato plants, but yellow spots on other leaves indicates a disease of some sort. Unfortunately there are several diseases that can attack tomato plants and cause yellow spots on the leaves. Anthracnose symptoms include yellowing of the older leaves plus small spots on other leaves that are dark in the center, surrounded by yellowing tissue. Early blight causes small, irregular spots on the leaves. The spots grow and you can see concentric rings in the spots. Tomato leaf mold, common in greenhouse environments, starts out as small white spots on leaves that eventually turn yellow. They're found on the upper sides of the leaves. Then patches on the lower surfaces become covered with a velvety, olive-brown fungus growth. This problem is associated with very high humidity and temperatures between 65 and 80 degrees F. To control, keep temperatures about 60 - 65 degrees and try to keep the humdity below 85%. If you can improve air circulation and keep infected plants from touching one another, you may be able to keep this fungus from spreading to your other tomato plants. Inspect your plants and try to figure out which problem might be causing the symptoms. Rogue out the diseased plants if necessary, to keep from infecting the other plants you're growing.

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