Casoron has very low toxicity. It is used primarily to eliminate grass and sprouting weed seeds in landscaped areas. Unless you were planning to plant your tomatos in an area casoron was applied to, I think you have little to worry about. It is not going to travel sideways several feet to get into your tomatos. Any tiny amount that might make its way into your mature tomatos would surely be so miniscule as not to be worth a second thought. This chemical is used massively by landscapers and home gardeners, and would simply not be permitted if it constituted a significant threat to anyone's health.
