A bacterial inferno
In the mountains of Uzbekistan grows a wild pear that has specific genes. These trees, relatives of the common grocery store pear, possess natural defenses against one of the most destructive orchard diseases in the world: fire blight. Caused by the bacterium Erwinia amylovora, this disease can destroy an entire pear or apple orchard in a single growing season. The annual economic impact of fire blight in the United States alone is estimated to exceed $100 million.
Erwinia amylovora attacks with devastating speed. The bacteria enter through blossoms or small wounds on the tree. Once inside, they multiply rapidly in warm, humid conditions, moving through the tree's vascular system. This invasion causes blossoms and leaves to wilt and turn black, giving the tree a scorched appearance, which is how the disease gets its name. Infected branches develop a characteristic "shepherd's crook" bend at their tips. Pears are generally more susceptible than apples, and many popular commercial varieties have little to no natural resistance.
A genetic solution from Central Asia
Central Asia is a global center of genetic diversity for many fruit trees, including pears (Pyrus spp.). The wild pears in the Nurata mountain range, such as Pyrus korshinskyi, have co-evolved with local pathogens for millennia, resulting in a genetic traits for fighting off diseases. Scientists and plant breeders, including those from the U.S. Department of Agriculture's Agricultural Research Service (USDA-ARS), have turned to these wild relatives to find a solution to fire blight.
The process involves identifying the specific genes or quantitative trait loci (QTLs)—sections of DNA containing genes—that are responsible for fire blight resistance. Researchers cross the highly resistant wild Uzbek pears with susceptible but commercially desirable European pear varieties (Pyrus communis). The goal is to create new hybrid cultivars that produce delicious fruit while also carrying the wild genes for disease resistance. This work is part of the history of pear breeding, much of it focused on combating fire blight since the early 1900s. By using the ancient genetic diversity of Nurata's wild pears, this research may protect the future of pear cultivation worldwide.