Invasive fungi and insects have decimated ash trees in North America and Europe over the past few decades. Researchers in both continents have been feverishly working on mapping these trees’ genomes and disease pathways to breed trees resistant to these pests, states Dennis Sons of Garden Delights Nursery in Tennessee.
Ash trees are among the most dominant trees across the U.S. and account for up to 25 percent of the trees in big cities. While there are an estimated 7 billion ash trees in the U.S., experts believe that many will be killed off in the next few decades because of a non-native beetle called the emerald ash borer.
First appearing in the U.S. in 2002, the emerald ash borer is a slender beetle native of East Asia. It is believed to have first arrived in the U.S. in packing material and has devastated entire ash tree populations in Detroit. This pest has spread to 22 states, including New York, Missouri, Tennessee, and numerous others.
Europe has also been battling its ash tree disease called ash dieback. An infestation of the fungus Chalara causes this disease. Researchers from 34 different countries focused on the fungal disease in a four-year effort to determine how to combat it. The fungus originated in East Asia and spread to Eastern Europe in the early 1990s. Since that time, it has spread across Europe. By 2012, as much as 90 percent of the ash tree population of Denmark had succumbed to ash dieback.
Researchers mapped the genome of the Chalara fungus and then studied the ash trees of Europe. They found that some ash trees in the United Kingdom were more resistant to Chalara than in other parts of Europe. One team of researchers from Denmark, the U.K., and Ireland, began sequencing the ash tree genome in 2013. Their work in mapping ash tree variations across Europe and determining that some U.K. trees were more resistant was considered an essential step toward breeding trees resistant to Chalara.
The European researchers identified that ash trees that are more resistant to Chalara are less resistant to infestations of the emerald ash borer. You can find trees in Denmark resistant to Chalara to produce chemicals that made them less resistant to the emerald ash borer. Researchers are now working to find ways to breed ash trees that are resistant to both pests.
The devastation of ash trees in the U.S. by the emerald ash borer has left cities struggling with multiple problems. When the emerald ash borer is first introduced to a new area, half of all of the ash trees will die in about eight years. The trees that remain will succumb within three more years, leaving cities to struggle with increased stormwater runoff, disposing of millions of dead trees, and liability issues.
Some cities spray the ash trees with insecticides, which are costly but effective for about three years. With strategic applications, the cities can give themselves more time to address the problem by planting programs and establishing partnerships with companies to process the lumber from diseased trees. In New York, for instance, companies that specialize in distressed tree milling have gained considerable traction in the market.
U.S. researchers are working to develop hybrid ash trees that are resistant to the emerald ash borer. They have found that few ash trees in Asia are susceptible to the beetle because they co-evolved. For example, Manchurian ash is much more resistant to the emerald ash borer than white ash or green ash. Researchers have also found a small subset of ash trees that remain alive in areas in which more than 98 percent of the ash tree population has died. They are reviewing the phenotypes of these trees to identify those that are resistant and those that are susceptible.
The U.S. Department of Agriculture has been conducting cross-pollination studies of ash trees since 2005 to develop hybrid trees that are resistant to the emerald ash borer. The government is aided in its efforts by researching seedlings in its nursery laboratories. The USDA hopes to introduce hybrids that contain the Asian trees’ resistant genes so that ash trees may again flourish across the U.S. and in urban landscapes.
Trees that are resistant to diseases, infestations, and weather are essential for maintaining ecological systems. Some nurseries offer various resistant trees that can withstand drought, deer, and other difficulties. Nurseries also support governmental and university researchers in their efforts to develop hybrid species by donating plants. For example, one nursery in Tennessee called the TN Nursery donates seedlings and plants to nonprofits, governmental organizations, and university researchers.
While there have been promising developments in research in both the U.S. and Europe for developing resistant ash trees, it may take years before the populations’ rebound. Communities across the nation may want to focus on replanting programs to introduce greater diversity in their extant tree populations. Mapping the disease pathways and the tree genomes is only the beginning.
Author Tammy Sons – A Horticulture Major, Gardener, Dairy Farmer and Nursery Owner Near Nashville Tn – https://www.onlineplantnursery.com