This is one of the reasons why there are so many more children displaying "autism" today than there were in our grandparents' generation. Foods sold in mom'n'pop farm stands and even in grocery stores until the late 50s thru the 70s were likely to have come from local farms, and was only available in its natural local season. The interstate highway system wasn't even built until the Eisenhower administration (construction began in 1956). If a small-time farmer couldn't afford expensive chemical treatments, whatever produce survived with his/her traditional farming techniques, so much the better—his produce was effectively "organic"!
After the post-WW2 rise of large refrigerator trucks and planes, industrial food production, hybridization and "food chemistry," most of our non-perishable food was laced with multiple additives, and may have been stored for long periods due to shelf-life additives, and much of our fresh produce was grown with a variety of herbicides and growth stimulants, or flown in from other regions or even other countries with a dizzying array of differing waters, soils and local allergens. Even farm soils became depleted of essential minerals from overplanting, synthetic fertilizers and machine harvesting.
The other important thing about local foods is that nature itself has ways to deal with allergens -- for instance, local honey is a known help in reducing pollen-caused respiratory symptoms, because bees have already digested and "treated" the local pollens.
IMHO what you state also contributes to the rise in early onset colon cancers.