Introduction

Invasive Species Defined

What's Being Done

 National Invasive Species Information Center

Submodule 6: Microbes and Fungi -- the Pathogens

Plant Pathogens

Viruses, bacteria, and fungi can all cause plant diseases. Invasive plant pathogens usually enter the U.S. in infected imported plants, but they can also enter in infected soil or on equipment.

There are several examples of plant pathogens changing the American landscape. Until the early 1900s, the American chestnut tree (Castanea dentate) was commonly found growing in forests from Maine to Georgia and from Michigan to Louisiana. These tall, stately trees reached over 100 feet with 3-4 foot diameter trunks. Their nuts were said to be some of the best tasting chestnuts available. In the early and mid-1900’s a non-native fungus, Cryphonectria parasitica, commonly known as Chestnut blight, decimated the chestnut tree population leading to one of the worst ecological disasters in U.S. history. This fungus enters the tree through cracks in the bark and eventually obstructs the tree’s food transport system, leading to death. It is commonly thought that the fungus entered the U.S. on chestnut trees imported from China and Japan; chestnut trees from this part of the world are quite resistant to the effects of blight. Once the native chestnut trees were gone, other native trees that once competed with them for resources and space were able to grow and flourish.

Another non-native fungus that invaded the U.S. in the 1900s causes Dutch Elm Disease (Ophiostoma (or Ceratocystis) ulmi). This fungus kills American elm trees (Ulmus americana) by causing them to produce substances that block their vascular systems, leading to death. The disease has spread to most of the contiguous 48 states and has changed the landscape of many American cities where the tree was commonly used for landscaping.

Currently, several plant pathogens are causing serious problems for wild and cultivated plants in the U.S. Some examples include:

• Phytophthora ramorum, the fungus responsible for causing Sudden Oak Death. Symptoms vary and diagnosis based solely on visual examination is nearly impossible. The fungus is reported to infect over 60 different plant species. First identified in the San Francisco area in 1995, it has since been spreading to other areas of California and into Oregon. It was first described in diseased rhododendrons in Europe in 1993. There is concern that the fungus will spread since several plants have been identified as susceptible to the fungus in other parts of the country.
• Plum Pox (Potyviruses: Potyviridae), a viral plant disease known throughout the world as sharka, causes the production of deformed fruit in trees of the genus Prunus; these include peaches, nectarines, plums, apricots, and almonds (cherries do not seem to be affected by most strains). Other types of plants are also known to be susceptible to the virus. Transmission occurs by aphids that transmit the virus between plants as they feed sucking the sap from trees, and by grafts taken from infected trees. It is not known how the virus was initially introduced into this country. Genetic engineers in the U.S. and France are currently working on ways to introduce resistance to this virus into plum trees.
• Ralstonia solanacearum is a bacterium that causes what is known as Southern wilt, bacterial wilt, and brown rot of potato. In 2003, some geraniums imported from Kenya were found to be infected with the bacteria and were subsequently destroyed. Similar incidents prompted the U.S. government to put careful controls on the importation of plants from countries known to harbor the bacterium. Infected plants, soil, human clothing, and equipment can carry the disease. These bacteria are widespread throughout most of the world.
• Soybean Rust can be caused by two fungi called Phakopsora pachyrhizi and Phakopsora meibomiae; P. pachyrhizi seems to be the more aggressive of these two species. Native to Asia, the fungi have been introduced to Africa, Australia, South America, Hawaii and along the Gulf Coast in the continental U.S. Fungal spores carried by wind currents over long distances may have led to introduction to the U.S. from Central America. Hosts of the fungi include soybeans (Glycine max) and other leguminous species.
• Karnal bunt, a disease of wheat, rye, durum what, and triticale, is caused by the fungus Tilletia indica (also known as Neovossia indica). In 1996, the Arizona Department of Agriculture detected this fungus in some imported durum wheat seed. Since then, governmental agencies including APHIS (Animal and Plant Health Inspection Service) have issued a quarantine for Arizona and surrounding areas. Fungus growing on one kernel of diseased grain can generate millions of spores, which is how the fungi reproduce and spread. These spores can be carried by wind, or on equipment and humans. The government is doing whatever it can to contain the spread of karnal bunt and to convince those who buy U.S. grain that the occurrence is isolated and grain is free disease-free. Interestingly, this fungus does little to damage crop production; the primary reason for alarm is international trading restrictions if the fungus is detected.
• Cronartium ribicola (White Pine Blister Rust) is a unique species of fungus that is heteroecious, meaning that to complete its life cycle, it requires two hosts. In this case, the hosts are American white pine (Pinus subgenus Strobus) and currant bushes (genus Ribes). This fungus, first introduced to North America in the early 1900s, is native to Europe and Asia where local species of white pine are more resistant. The U.S. Forest Service has undertaken a program to indentify native trees that are resistant to the fungi for use in a breeding program.

The best control of these and other plant pathogens involves preventing initial introductions and destroying infected plants. This requires scrupulous examination of all plants coming into the country to prevent introductions, and diligent monitoring of nurseries and orchards. Unfortunately, due to the sheer volume of plant materials entering the country, this is a daunting task considering limited resources and personnel.