Answer: If it is the older leaves that are curling, there is not a concern. Tomatoes often do this as summer temperatures rise. Some varieties are more prone to this than others and high fertility levels increase the condition. It does not hurt the plants is any way.
If you are seeing a twisting of new leaves and growing shoots it may be a sign of virus infection or herbicide injury. Neither is treatable, and affected plants should be removed.
Viruses are brought in by insects (like mosquitos carry malaria). Herbicide injury can occur from drift from lawn applications, use of grass clippings or hay as a mulch from treated lawns or pastures, or even use of manure from cattle that grazed on treated pastures.
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