“Exclusively” is too strong. The author never states North American farmers only use commercial varieties.

The author never states that disease and insects of killed off some heirloom varieties.

The author never states that researchers have anything to do with causing the shrinking stock of heirloom crops. Although the author does suggest that heirloom crops produce lower yields, the author never connects researchers’ consideration of that lower yield to causing fewer farmers to plant heirloom crops.

The author never states that difficulty of adaptation to large-scale production has caused shrinking stock of heirloom crops.

Stated in the last paragraph. There are fewer small farmers, and many of those farmers plant non-heirloom crops. In addition, there’s a breakdown in the community and regional exchange networks that transfer information about heirloom crops.