Hazards Associated with Pest Pathways and Economic Impacts-Seeds, Propagative Materials and Nursery Stock

Michael E. Ostry

Introduction
The arrival, establishment, and spread of exotic forest pests have had profound effects on forest ecosystems and continue to have significant economic effects long after their arrival. Nearly 400 exotic insect pests and diseases, many originating on planting stock, are established in North America. A review of several of the damaging pathogens that entered North America as hitchhikers on plant material reminds us of the serious consequences that accidental introductions of exotic organisms have on the environment. Although the most serious introductions occurred before the enactment of quarantines, the dramatic increase in world trade has made the likelihood of new invasions even greater.

Movement of infected plant material
In North America the majority of forest tree diseases that have had the largest impacts resulted from the accidental importation, propagation, shipment and planting of infected nursery stock. Pathogens, even well-known organisms, are often difficult to detect in nurseries. Diseased seedlings or other forms of plant materials are often asymptomatic when they are shipped and planted, resulting in rapid and widespread outbreaks of exotic organisms. In addition, hitchhiking organisms not damaging to the plant material that harbor them can be moved into new areas and spread to nearby susceptible host plants, resulting in new pest problems of unrelated plant species.

Figure 1. Many insect pests and pathogens are difficult to detect in nurseries resulting in the accidental shipment of damaging organisms.

Fortunately, the establishment and spread of damaging exotic organisms are probably rare events. We have no idea how many accidental introductions have occurred in the past and continue to occur but for one reason or another do not become pest problems. However, several past introductions have resulted in impacts of such magnitude they underscore the importance of minimizing the likelihood of future accidental arrivals. The following classical examples of the establishment of exotic forest tree pathogens illustrate the consequences of the accidental movement of pathogens on propagative materials.

Destruction of the American chestnut
No greater ecological impact in recent history has resulted from an introduced organism than occurred with the arrival of Cryphonectria parasitica, the pathogen causing chestnut blight. Since its discovery in New York in 1904, the disease has forever changed the eastern forests, rapidly destroying the ecologically and economically valuable American chestnut (Castanea dentata) resource.

Figure 2. Chestnut blight has resulted in the death of the once majestic chestnut, reducing it to multiple-stemmed stump sprouts.

Chestnuts and related species from many countries were being brought into the United States in the form of nuts, root cuttings and grafting wood during the late 1800s for orchard establishment and breeding work. After the identity of the fungus causing this disease was discovered, it was determined to be the same the fungus found in China and most likely was accidentally brought into North America on propagative material.

Introduction of white pine blister rust
White pine blister rust caused by Cronartium ribicola is considered to be the most damaging disease of eastern white pine (Pinus strobus), western white pine (P. monticola), and sugar pine (P. lambertiana). The fungus was introduced first to Europe from Asia and then from Europe to North America on infected plant material as the result of importation of pine seedlings to support early reforestation activities in Canada and the United States.

Figure 3. White pine blister rust kills trees of all ages and makes restoration of the species difficult in many parts of its original range.

C. ribicola produces five types of spores and requires two hosts, alternating between the pines and currants or gooseberries (Ribes spp.). First discovered during the early 1900s on Ribes in New York, attempts to control this disease were the most extensive in time, labor, and money in the history of forestry in the United States. The pathogen continues to spread in western North America, possibly from a shipment of infected seedlings from Europe to Vancouver, British Columbia.

Development of Beech Bark Disease
Beech bark disease is a damaging disease of American beech (Fagus grandifolia) caused by a scale insect and Nectria fungi. The beech scale insect (Cryptococcus fagisuga) was accidentally imported into Nova Scotia on ornamental beech from Europe sometime during the late 1800s. The insect is wingless but is easily disseminated by wind, birds, and other animals.

Figure 4. American beech affected by beech bark disease illustrates the white waxy material secreted by the scale insect and the roughened bark of trees severely affected by this insect-fungus complex.

The feeding wounds made by the scale insect are entry courts for the native Nectria galligena resulting in large areas of killed bark tissue and eventually the death of severely diseased trees. A second species, N. coccinea var. faginata, also native to Europe and perhaps a third species, N. ochroleuca, are also part of this disease complex. This illustrates how the accidental introduction of an insect that itself is not a serious pest can interact with a native fungus that previously was not damaging and result in a new disease with serious ecological and economic impacts. The disease has recently been found in Michigan near the northwestern limit of the natural range of beech.

There are numerous other examples of exotic organisms that have been moved on nursery stock and also of organisms that have moved to new locations within North America. The role of infected nursery stock in outbreaks of diseases of red pine (P. resinosa) and jack pine (P. banksiana) is well established. Diseases such as Scleroderris canker caused by Gremeniella abietina and shoot blights caused by Sirococcus conigenus and Sphaeropsis sapinea were directly linked to the movement of infected seedlings from nurseries. More recently, evidence strongly indicates that anthracnose of dogwood (Cornus florida and C. nuttallii) caused by Discula destructiva and canker of butternut (Juglans cinerea) caused by Sirococcus clavigignenti-juglandacearum are diseases that may also have been introduced into North America on nursery stock or seed.

These examples of inadvertent movement of damaging forest pests and their consequences are ample evidence for the necessity of increasing our efforts to minimize future problems not only from known pathogens and insect pests, but more importantly from unknown pests or new races of pathogens. The risk of the spread of exotic insect pests and pathogens throughout the world is high, especially in light of the increasing international movement of woody plant material for forestry and horticultural uses.

References

Campbell, F.T., and S.E. Schlarbaum. 1994. Fading Forests North American Trees and the Threat of Exotic Pests. Natural Resources Defense Council, New York, NY. 47p.

Gibbs, J.N., and D. Wainhouse. 1986. Spread of forest pests and pathogens in the Northern hemisphere. Forestry 59:141-153.

Liebhold, A.M., W.L. MacDonald, D. Bergdahl, and V.C. Mastro. 1995. Invasion by Exotic Forest Pests: A Threat to Forest Ecosystems. For. Sci. M.onogr. 30 Vol. 41, 49p.