Hello Gardeners, I’m Amanda McNulty with Clemson Extension and Making It Grow. The first callery pears were brought to the United States by USDA scientists trying to find varieties resistant to fire blight – a native bacterium that was in the early 1900’s ruining 85% of the western states’ commercial pear crop. Tens of thousands of seeds collected in Asia were planted in Oregon and Washington and resistant individuals eventually became root stock for tasty pears varieties. In the 1950’s a tree with no thorns and sterile dry fruits collected from the Maryland seedlings was selected for a trial of hardy street trees. Surprise, surprise. These extremely adaptable, disease-resistant, and fast-growing trees were a hit and a new craze began. Other cultivars from those original trees were selected although still self-sterile, the trees could pollinate each other and create viable seeds. The resulting crosses, many armed with four-inch thorns, now are among our worst invasive species.