Invasive Species of Concern in Maryland

Chestnut blight, Cryphonectria parasitica, is a very lethal organism. This parasitic fungus reached North America accidentally on chestnut trees shipped from Asia around the turn of the 20th century. It infects various oaks and some other hardwoods, but its principal victim is one of our region’s formerly dominant and most important hardwood species, American chestnut (Castanea dentata). Blight arrived in Maryland almost a century ago, within a decade of its first observance on trees growing in the New York Botanical Garden in 1904. Like many Eastern states in the US at that time, Maryland had an abundance of chestnut that dominated many of the forests and whose natural range included most of the state. There are signs of this story in the spring landscape, which is why MISC has chosen chestnut blight as the April Invader of the Month.

Blight kills chestnut trees by infiltrating a crevice or hole in the bark, growing strings of fungal tissue called hyphae throughout the vascular system of the plant and girdling the tree stem. The tree dies off above the infection site, called a canker, and may resprout below it. Because the fungus cannot grow well in the tree’s root system, and so sometimes does not completely kill a large tree, numerous small and medium-sized clumps of chestnut survive, with living stems growing from root collars of old, once large trees. Throughout the US, there are probably hundreds of millions of American chestnuts, but few reach a size and age when they can flower, and the blight continues to kill them back to the ground level in repeating cycles of disease. In Maryland forests, larger-sized, weathered American chestnut logs and stumps can still be found today. The great trees that once dominated the canopies and produced an abundance of edible nuts are absent, but the chestnuts continue to resprout from living roots and thereby survive in significant numbers in our forests.

Scientists believe that there were as many as 4 billion chestnuts growing in North America at the time of blight introduction. That would have been 25% of the trees. For an opportunistic pathogen it was a field of unexploited opportunity that spelled tragedy for American forests. The tree’s ecological and economic values were very great, so when the blight spread through the forests it was devastating to both wildlife and humans. Once a major source of food for animals and a frequently used building and furniture material for people, American chestnut has been replaced in the eastern forests by maple, oak and hickory.

Blight can be seen growing in the cambium of its hosts, just inside the outer bark layer, and is easily recognizable to an informed observer. Typically the infected bark will be discolored, usually rust-orange. Swelling and cracking of the bark are common, and numerous spore-producing bodies of the cankers are visible as 1mm sized rust-colored bumps. Many cankers can be found on the same tree, and it often takes years for the blight to girdle larger trees.

The blight is classified by scientists among the Ascomycota, a group of fungi that produce meiotic (sexual) spores called ascospores in a saclike structure called an ascus. Ascospores are dispersed by wind, often after rainstorms. Blight cankers also produce many asexual spores called conidia. Blight conidia are sticky and extruded on threadlike tendrils following rainy periods in spring, summer and fall. They attach to and are carried by animals such as insects and birds, sometimes for great distances. Because of these two kinds of spores, the blight spread quickly once it was introduced to North America, at an average distance of 60 miles a year. This means it dispersed over about 1/3 of the US in 50 years.

Blight is usually found wherever chestnuts and upland oaks grow, typically in acidic and well-drained soils. It often found in Maryland’s mountain forests, especially in locations where human land management practices such as frequent logging have given chestnuts an advantage over slower growing species and those which can’t resprout from their roots after cutting. Neither the fungus nor its hosts are likely to be found in wet sites like swamps and riparian habitats.

Given its prevalence in our forests, elimination of blight is unrealistic. Various topical and systemic treatments for individual cankers and trees are known to be effective in cases where they are accessible and resources available for their application. These include fungicides, mud packing, and the scientific use of certain strains of the blight infected with RNA virus that make the fungus hypovirulent (less virulent). Hypovirulence slows down the infection ability of the fungus and allows the tree to survive longer, and sometimes even to heal. It is a biological control method that some researchers feel holds promise as part of the solution to blight introduction.

Restoration of chestnut to Eastern forests is being pursued by the American Chestnut Foundation (ACF) utilizing backcross breeding. This involves breeding highly disease resistant Chinese and Japanese chestnuts with American chestnuts, increasing the percentage of American tree genes with each successive generation. ACF considers the production of these hybrids, which now are over 90% American, to be the best solution available to address the problem of blight.

There are ongoing efforts to better understand why some American chestnuts survive with disease for long periods of time, while their progeny don’t seem to inherit this relative longevity. It is also recognized that preserving pure American genotypes is important, and a number of public and private organizations as well as dedicated individuals are involved in this work. There are lots of ways to learn more about chestnut preservation and restoration and history, and research is ongoing on the great challenges of restoring these magnificent trees to Eastern forests in the face of a tenacious disease problem.

For more information about other Invasive Species of Concern, visit www.mdinvasivesp.org or call the Maryland Department of Agriculture at 410-841-5920.

For more information on the Internet:

Maryland Chapter, American Chestnut Foundation, www.mdtacf.org

American Chestnut Foundation, www.acf.org/