DEPARTMENTS


Cross country briefs



SAD news for aspens

Researchers at last may have discovered the cause of sudden aspen decline, or SAD, which has been decimating stands of the iconic Western tree for years. Where fluttering green and waves of molten gold graced Rocky Mountain slopes, there now appear patches of gray, and forest scientists studying the phenomenon were stymied in their efforts to explain the die-off of massive stands. But a study published in the Proceedings of the National Academy of Sciences (www.pnas.org) reveals that drought is to blame.


The standing dead: Acres of aspen have fallen victim to drought-induced sudden aspen decline.
Courtesy of William M. Ciesla, Forest Health Management International; Bugwood.org


Quaking aspen
Courtesy of Dave Powell, USDA Forest Service; Bugwood.org
The scientific team, comprising researchers from Stanford University and the University of Utah, determined that the drought that plagued Colorado's mountains from 2000 to 2004 contributed to the failure of the trees' water transport systems, effectively killing the trees through dehydration. Further, the study found that in response to drought, the aspen were developing embolisms - similar to the kind of blockage found in human blood vessels - that blocked an average of 70 percent of the vascular system in trees suffering from SAD. Contrary to the "sudden" decline indicated by the term SAD, the process is a delayed reaction to drought resulting in a long, slow process of dehydration and death.

Citrus greening hits Texas

Call it Huanglongbing; call it yellow dragon disease; call it citrus greening. Whatever the moniker, the disease is among the most serious to hit citrus crops, and its presence was confirmed in Texas this January. Plant tissue samples taken from a symptomatic tree in a commercial citrus grove in Hidalgo County tested positive for the disease, which is caused by the bacterium Candidatus Liberibacter asiaticus, vectored by the Asian citrus psyllid. The USDA's Animal and Plant Health Inspection Service (APHIS) has regulated the state for the insect since 2009.


Mottled leaf shows characteristic signs of citrus greening.
Courtesy of J.M. Bové, INRA Centre de Recherches de Bordeaux; Bugwood.org

The most characteristic symptoms of citrus greening are a blotchy mottle and vein yellowing that appear on leaves, giving the branch an overall yellow appearance. As the bacteria move throughout the plant's system, the entire tree may appear yellow. What's tricky about diagnosing the disease is that leaf symptoms may mimic those of nutrition-related mottling. However, leaves subject to citrus greening display a mottled appearance that crosses leaf veins, whereas nutrition-related mottles normally are found between or along veins. The only sure-fire method of diagnosis remains DNA analysis.

At present, much of the Southeastern U.S. - from Texas to Florida - is under quarantine either for Asian citrus psyllid and citrus greening or for Asian citrus psyllid alone. On January 13, the Texas Department of Agriculture enacted a temporary, emergency quarantine in a 5-mile radius of the detection site; the agency planned to establish later a revised intrastate quarantine based upon updated information.


Left, entire shoots may appear yellow in response to infection by the bacterium Candidatus Liberibacter asiaticus.


Oregon gets a B+


Arundo donax
Courtesy of Jeffrey W. Lotz, Florida Department of Agriculture and Consumer Services; Bugwood.org
The Oregon Invasive Species Council gave the state an average B+ grade for its management of intruders in 2011. The grade is based on the evaluation of five areas of concern: reporting invasive species (A-); outreach and education (A); statewide action plan (A-); trust account (B); and success at preventing the establishment of invasive species in Oregon (B-). According to the agency's report, the lowest grade was levied because two plants and one disease on the 100 Worst List became established in the state in 2011. The list can be accessed at www.oregon.gov/OISC/most_dangerous.shtml.

On the bright side, the Council selected 10 accomplishments "to highlight the diversity and depth of activity in Oregon's invasive species arena in 2011." Among other items, these include Portland General Electric's planting of 85 acres of Arundo donax to test as an alternative fuel for a coal-fired power plant, and control programs for infestations of African rue, Patterson's curse, distaff thistle, giant hogweed and kudzu that showed continued success toward the goal of eventual eradication.