SWEET HOME, Ore. — Growing organic seeds for cabbage, squash and other crops in northwest Oregon gives Sarah Kleeger and Andrew Still plenty to worry about, from hungry insects to unexpected frosts.

A man-made concern is fast rising on their list: the encroachment of bio-engineered crops, which they say threatens the genetic purity essential to the organic and heirloom fruits and vegetables they produce in the Willamette Valley, the top U.S. seed-growing region.

“We want to be nice to our neighbors and work together, but we can’t take any chances,” said Still, 33, inspecting plants for damage after a frost on their farm near Sweet Home, about 80 miles south of Portland. Contamination from gene-altered plants could render worthless the seeds they grow for their catalog business, Adaptive Seeds.

Twenty years after the first commercial varieties of genetically modified foods went on sale, farmers have embraced biotech crops that now account for 90 percent of all U.S. cotton and corn and 93 percent of soybeans.

Paradoxically, this has boosted demand for organics by consumers seeking to avoid GMOs. Sales of such foods were $29 billion in 2012, up from $6.1 billion in 2000, according to the Organic Trade Association.