University of Kentucky College of Agriculture, Food and Environment ID-241 Cooperative Extension Service After Your Ash Has Died Making an Informed Decision on what to Replant William M. Fountain, Horticulture, Ellen V. Crocker, Forestry, Lee Townsend, Entomology, Nicole Ward Gauthier, Plant Pathology s a homeowner, even under the opy cover in parts of the east. The death and red maple (Acer rubrum) became best of circumstances it can be of these trees had dire consequences popular as urban trees. challengingA to know how to provide the for the ecology of the woodlands and Green ash (Fraxinus pennsylva- best care for your trees. The arrival of wildlife that depended on their nuts nica), black ash (F. nigra), white ash (F. the emerald ash borer to Kentucky has for food. However, there was minimal americana), and a few other species presented extra challenges. As the em- impact to the urban landscape since of ash replaced the American elm. The erald ash borer rapidly kills young and this species was rarely found in cities. form of these trees was not as attractive old, healthy and struggling ash trees, A disease of elms was found in Eu- but they were easy to propagate and homeowners must decide whether rope and later brought to North Ameri- grew rapidly in nurseries. In addition to save their trees with insecticides. ca later in the same decade. Throughout they tolerated the harsh urban condi- Untreated trees will die and should be the 20th century, millions of urban elms tions that prevented many other tree removed if there is a chance that their would die across North America and species from growing in our industrial- failure could result in harm to people Europe. This disease became known as ized cities. or property. The question then arises: Dutch elm disease (DED), not because Mark Twain is often credited with What should be planted to replace it? it came from the Netherlands but for the statement, “History doesn’t repeat Unfortunately the emerald ash borer the person who discovered it. It was itself but it often rhymes.” The emerald is only the latest in a series of invasive actually three different but closely re- ash borer (Agrilus planipennis) was pests that have recently decimated our lated diseases (Ophiostoma ulmi, O. unintentionally introduced into greater trees. Here, we provide basic informa- himalulmi, and O. novoulmi). Like Detroit in the late 1990s. By 2016, hun- tion on the death of our ash trees and chestnut blight, DED was also intro- dreds of millions of ash trees native to what types of species are less likely to duced from Asia. This time the affected North America and Europe were dead be impacted by invasive insects and trees were the dominant species in the or dying and many millions more were diseases in the future. urban landscape. threatened. Like chestnut blight and The American elm Ulmus( ameri- Dutch elm disease, this insect borer The Loss of Two Giants cana) and related elms were planted was an unintentional introduction The year was about 1904. A fungus extensively in cities during and after the from Asia. Three different problems that became known as chestnut blight Industrial Revolution. Elms were easy were attacking three unrelated groups (Cryphonectria parasitica) was unin- to grow and had high branching that of plants. Our ability to see the connec- tentionally introduced into the US on allowed traffic to pass below a canopy tions and understand the problems can nursery stock imported from Japan. that spread, providing cooling shade in allow us to avoid making the same mis- Within the coming decades it caused the days before air conditioning. These take as we replace our dying ash trees. the death of 4 billion American chest- trees were tolerant of urban pollution nuts (Castanea dentata) growing and compacted soils. They could live Continental Drift and throughout the Appalachian wood- in wet or dry sites and in soils that Species Diversification lands and eastern North American for- were acid or alkaline. In effect, they We need to look back about 200 mil- ests. American chestnuts were massive; were the perfect tree. This majestic lion years ago to the breakup of large clearly the largest species in eastern tree lined entire streets, exceeding 50 super continents. Laurasia formed North America. They routinely reach- percent of the total urban tree canopy North America, Europe, and Asia; ing heights of 100 feet with trunks 14 in some cities. With the demise of the Gondwana separated into India, Africa, feet in diameter. Their branches made elms, people began looking for the South America, Australia, and Ant- up an estimated 25 percent of the can- best replacement. This is when the ash arctica. Plant and animal life on both Cooperative Extension Service | Agriculture and Natural Resources | Family and Consumer Sciences | 4-H Youth Development | Community and Economic Development was already very advanced. However, Table 1. Common Kentucky landscape plants • A disease or insect pest is more likely as the new continents drifted apart, native to North America, Europe, and Asia with to attack a plant species in the same life-threatening disease/insect problems. different selection pressures sent evolu- Number of species genus than one belonging to a differ- tion along very different survival paths. in genus (Genera in ent genus. Likewise, a plant from a Consequently the chestnuts of North family/ total different genus in the same family is America and Europe were not exposed Genus (common name) species in family)1 more likely to be attacked than a plant to the same diseases and plant feeders Abies (fir) 48 (11/255) from a different family. Plants with as the Asian species. The same is true Acer (maple) 154 (137/1,751) close relatives on different continents for elms, ash and many other species. Amelanchier (serviceberry) 28 (104/4,828) are called disjunct species. Accidental or intentional transcon- Betula (birch) 119 (6/234) Carpinus (hornbeam) 42 (6/234) • The larger the number of species in tinental movement of organisms can Carya (pecan & hickory)a 27 (12/89) a genus, the larger the number of have devastating effects on native spe- Castanea (chestnut) 9 (9/1,101) diseases and insects that are likely to cies that lack effective survival traits. Cornus (dogwood) 45 (12/124) attack these plants. (See Table 1) We generally remain unaware of this Crataegus (hawthorn) 380 (104/4,828) • Genera with few or no relatives that worldwide problem unless we travel Fagus (beech) 11 (9/1,101) are native to other continents are to these other areas and study their Fraxinus (ash) 63 (25/688) less likely to have life-threatening environmental challenges. Juglans (walnut) 21 (12/89) problems. (See Table 2.) Juniperus (juniper) 75(32/166) Plums and cherries (Prunus), oaks Offsetting the Negative Larix (larch) 14 (11/255) (Quercus), and maples (Acer) are Impact of World Trade Liriodendron (tulip poplar)a 2 (6/250) Magnolia (magnolia)a 242 (6/250) examples of genera with hundreds One strategy for preventing the next Malus (apple/crabapple) 62 (104/4,828) of species across all three Laurasian big insect or disease catastrophe is to Picea (spruce) 40 (11/255) continents. These and similar groups avoid excess use of a species. Ideally, any Pinus (pine) 130 (11/255) of plants are widely used in landscapes one genus of plants should not exceed 5 Platanus (sycamore) 9 (1/9) in spite of having serious disease and percent or 10 percent of our landscape. Populus (poplar) 98 (54/1,269) insect problems. If, or more likely when, a new pest is Prunus (plum, cherry) 254 (104/4,828) Not all of these are large trees or aes- introduced into North America we do Pyrus (pear)b 69 (104/4,828) thetic replacements for American elm not want to see a rhyming of history as Quercus (oak) 597 (9/1,101) and ash. Some have traits, including the occurred with the chestnut, elm and Salix (willow) 552 (54/1,269) potential to be invasive, that may make Tilia (linden) 46 (245/4,465) ash. Developing a diverse landscape Tsuga (hemlock)a 10 (11/255) them unsuitable for use in specific land- can prevent the staggering losses that Ulmus (elm) 40 (8/64) scapes. Evaluate and match the cultural occurred with the American chestnut, 1 Nomenclature from: The Plant List: A Working List conditions of the site with the cultural elm, and now our ash trees and the of All Plant Species (http://www.theplantlist.org/) requirements of the plant. resulting loss in percent canopy cover. accessed 18 July 2016. There is no such thing as a perfect This practice also slows the spread of a Native to North America and Asia, but not Europe plant. Species listed in Table 3 are ac- problems that inevitably develop. It b Native to Europe and Asia, but not North America ceptable for use in urban landscapes gives green industry professionals time but should not exceed 5-10 percent of to develop pest management strategies. existing species in the region. Species in In addition it produces a more interest- Table 4 are already greatly over-planted ing landscape throughout the year. Basic Principles of Pathogenicity and should not be used except in ex- Second, we should avoid the overuse Some basic principals of pathology tenuating circumstances. of species that are found on all three and entomology strongly influence former Laurasian continents: North the potential for a disease or insect to The Value of Preservation America, Asia, and Europe. This in- attack a host plant. Insects such as the You may have mature trees in your cludes some of the most common trees Japanese beetle that feed on hundreds landscape that are on the “do-not-plant in our landscapes (Table 1).