Part II: Tree and ornamental pathogens
Sudden oak death
Researchers believe that the primary pathogen of this new disease is a new species of Phytophthora. First discovered in California in 1995, this disease affects tanoak (Lithocarpus densiflorus), coast live oak (Quercus agrifolia) and black oak (Q. kelloggii).
Until recently, the disease was positively identified only in these trees. However, in January 2001, researchers confirmed that the new Phytophthora has been found on rhododendrons in California. They also confirmed that the same fungus is infecting rhododendrons in Germany and the Netherlands. Researchers don't know where the pathogen found in the rhododendrons originated. However, it is similar to a Phytophthora species that infected rhododendrons in Germany and the Netherlands in the 1990s.
Other pests, such as the fungus Hypoxylon thouarsianum and Ambrosia beetles then attack the weakened trees. Also, another fungus species, Diplodia, may contribute to the symptomatology.
Plum pox potyvirus
Plum pox was discovered for the first time in the United States in the fall of 1999 in an orchard in Pennsylvania. The virus causes stone-fruit tree species to produce worthless, blotched or misshapen fruit. It is spread through seed and via insect vectors, but it does not kill trees. Infected plants may not show symptoms for several years, so determining when infections occur is nearly impossible. However, researchers believe that the virus may have existed in Canada before it was discovered in the United States.
“Fortunately, we have the D-strain which is less virulent than some of the strains seen in Europe and Egypt,” says Gary Clement with the USDA, Animal and Plant Health Inspection Service (APHIS) in Carlisle, Pennsylvania. Further, Clement states that although this virus has infected ornamental plant species such as purple-leaf plum in Europe, researchers have not discovered the virus in wild or residential settings or in ornamental nurseries.
This disease was first identified in the United States in August 2000 in Georgia. At this time, the exact identity is unknown, but samples indicate that it is a Puccinia species and perhaps Puccinia hemerocallids. It has also been positively identified in three other states: Florida, Alabama and South Carolina. However, a survey is being conducted this spring and summer by state Department of Agriculture personnel to determine the presence of the disease elsewhere. It could be more widespread than currently known.
For more information and to keep current with this disease, go to www.ces.uga.edu/agriculture/plantpath/daylilyrust.html. This website is maintained by Dr. Jean Williams-Woodward, a University of Georgia extension plant pathologist.
An outbreak of elm yellows has occurred in the Chicago area, according to Karel Jacobs, a research pathologist with the Morton Arboretum (Lisle, Ill.). The disease, usually not found as far north as Chicago, is caused by a bacteria-like organism called a phytoplasma. A phytoplasma is a plant-infecting, wall-less cluster of organelles that behaves like an organism. It is more closely related to bacteria than fungi and needs a living host to survive.
Elm yellows, also called elm phloem necrosis, differs from Dutch elm disease (DED). Once infected, the leaves turn yellow and may eventually fall off. Unlike DED, this disease infects the entire crown and does not discolor the sapwood. As the alternate name suggests, this pathogen infects phloem tissue and can be spread by phloem-feeding insects.
Other diseases reported
Researchers indicate that some well-known diseases are re-emerging in locations where they are not typically seen or have not been a problem. Mary Small from the Colorado State University cooperative extension has seen higher incidence of apple scab, Venturia inaequalis, the past few years. Although common in humid climates east of the Rocky Mountains, this disease usually is not seen in the drier elevations of Colorado. Small sees the disease most often in mature, overcrowded landscapes that need pruning. Small attributes the increased incidence of apple scab to moist weather patterns and says that the disease was a particular problem 2 years ago during an excessively moist period.
Melodie Putnam with the Oregon State University plant clinic has recently seen a strain of Phytophthora syringae emerge that causes stem cankers on some species of Malus. Researchers originally saw the stem canker symptoms during 1997 when the region experienced flooding and left many trees in standing water.
Putnam, like Small, sees weather being the major influence in the incidence of these diseases. “We had some wet weather in 1997 and 1998. However, last year was relatively dry, and we experienced no serious disease problems,” states Putnam.
Another phytoplasma, lethal yellowing (LY) of palm trees, continues to threaten non-resistant palm populations in Florida. The disease was first noticed in the Florida Keys in the 1950s and soon decimated palm populations in Key West. By the early 1970s, the disease was seriously affecting palms in the Miami area, and in 1979, it was found in Texas. The disease is slowed by chemical treatments, but eradication has not been possible. Most agencies stress the planting of resistant palm species as the most important control strategy.
The factors affecting the emergence and re-emergence of diseases are complex and not fully understood. For instance, weather conditions greatly affect disease development, but quantifying temperature and moisture in relation to disease cycles is at best difficult. Besides, weather is uncontrollable and can only be used abstractly to predict pathogen development.
Most of the researchers I spoke with, feel that you should be aware of cultural practices that promote healthy, vigorous, but not luxuriant, plant growth. These practices can provide a front-line defense against the unseen presence of pathogens.
I James Houx is the technical editor for Grounds Maintenance.
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