Answer from Pat:
I need more information before i can answer this question intelligently: Where are the concentric rings on your plants. On the leaves? Under leaves or on top of leaves? on the stems? or on the fruit? What color are the concentric rings? white, black, yellow, or brown? Have you looked under magnifier? Are they eggs? About how large are the concentric rings? If on fruit are they located on stem end or on the blossom end? Where are your tomatoes growing? (Full sun? What climate zone or location?)
If you send back this information, I can answer the question with a chance of being correct, but in the meanwhile, here are some ideas. You may find the answer to your problem here and won’t have to write back with exact description of problem, but next time please give more information:
If the concentric rings are on fruit and and are large, firm, smooth, brown spots with concentric zones, then your tomatoes have Buckey rot. (Concentric rings from Buckeye rot can occur on stem end or on blossom end of fruit.) Mulching the soil can help. Sterilize soil by soil solarization (Pages 295-296 in my book) to prevent reoccurrence next year. It is possible that Serenade® might help you protect plants and other fruit from this fungal disease.
If the concentric rings are underneath the leaves and if they are white and beginning to give off white, dusty threads, they are eggs of the larvae of giant white fly and the best defense is to cover the soil under the plant with an inch or two of dry, bagged earthworm castings. This gets rid of giant white fly in about 2 weeks. (But problems with giant white fly are less this year which makes me think the Department of Agriculture campaign to introduce beneficials to kill them is working. Cool weather has also reduced their numbers at least along the coast.)
If there is dark brown to black color surrounding the stem end of the fruit and concentric rings at the stem end your fruit has Early Blight. Spray the plant and fruit with Serenade®, start earlier as a preventative next year. Bordeaux mixture also controls Early Blight. Keep garden clean. Use crop rotation. Use soil solarization if you can before planting tomatoes in that spot again.
All the above (except giant white fly) are diseases, but there is also a condition of “Concentric Rings” that is brown or black concentric curved cracks radiating outwards from the stem end. This comes from splitting of the epidermis of the fruit caused by swings of temperature, bad weather conditions, and or uneven watering. Fix the watering if you can by watering deeply once a week or less often instead of using drip. You can’t do anything about the weather other than making sure the fruit is well protected by the foliage. Pruning off too many leaves can leave fruit exposed to weather. Sometimes that causes cracking. Mulching soil can help maintain even moisture.
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