Answer: It is normal for older tomato leaves to curl upward in the heat. Some varieties are especially prone to this. High fertility seems to contribute to the condition.
If however the young leaves at the growing tips of shoots are curling, this is an indication of a problem. The culprit could be a virus infection or hormone type herbicide injury. Viruses usually are carried to plants by aphids and other insects in much the same fashion as mosquitoes would spread malaria. Pest control is therefore important for virus prevention.
Herbicide injury can occur from using a sprayer to spray tomato plants that had been used for herbicides in the past, by mulching with grass clippings from a treated lawn or hay from a treated pasture, by fertilizing with cow manure from an animal that had grazed on a treated pasture, or by drift of volatilized herbicide applied to a nearby area. There is no "cure" for virus or herbicide injury. In both cases plants should be pulled from the garden and discarded.
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