Answer: A number of possibilities come to mind. First, tomatoes are highly dependent on consistent soil moisture and don't like to dry out. Is the soil moist, but not wet? Determine if the plant is receiving sufficient water throughout its root system. You might also want to add several inches of organic mulch such as compost or dried leaves on top of the soil to maintain moisture and reduce temperatures. Second, tomatoes are susceptible to a variety of diseases.
Curly top virus is spread by leafhoppers and shows up as curling leaves. There is no cure for curly top once it strikes. However, if the plant survives to bear fruit, it may not be curly top. If the plants yellow and die, this is the likely cause. To avoid it on your other plants, cover them with floating row cover to prevent leafhoppers from feeding. Leafhoppers don't like shade, so they should go elsewhere to feed.
Tomatoes are also subject to fusarium or verticillum wilt, but the leaves would also be yellowing and browning. The best way to prevent these diseases is to plant resistant varieties, which will have the letters F or V after the variety name. It's always good to plant tomatoes in fresh soil, or rotate their location in the garden, to inhibit the spread of these diseases.
You might want to take a sample of the affected leaves to a local nursery or County Cooperative Extension office for diagnosis. I hope this info helps!
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