Raymond W. Schneider, Susan Useman, Irving Mendelssohn and Karen McKee
Louisiana’s coastal salt marshes, which are dominated by smooth cordgrass (Spartina alterniflora), suffered a catastrophic dieback in 2000. About 110,000 acres of the Barataria-Terrebonne estuarine salt marsh, which is positioned between the Mississippi and Atchafalaya rivers in south central Louisiana and covers approximately 390,000 acres, were severely damaged. Of those acres, at least 17,000, or about 4 percent, converted from dense vegetation to open mud flats with little or no vegetation. About 150,000 acres are considered moderately impacted because, even though green marsh can still be seen, the marsh has a significant degree of browning.
This dieback, or brown marsh syndrome as it is now known, is a concern because this highly productive ecosystem serves as habitat for many avian, invertebrate and fish species. These marshlands also serve to mitigate hurricane storm surges and to protect vital oil, natural gas and other industries along the Louisiana coast. Similar diebacks were recently documented in Georgia, Delaware, Connecticut, Massachusetts and other Atlantic seaboard states.
To determine the cause and extent of this dieback in Louisiana, a multidisciplinary, coordinated research effort was undertaken. The exact cause, however, has yet to be determined. Factors such as salinity, drought, high temperatures, rainfall, and soil chemical and physical properties were evaluated as possible causal agents. View conclusions from these studies.
Plant pathologists viewed this catastrophic event as possibly being caused in part by one or more plant pathogens with the expectation that there had been an interaction between unknown pathogens and stressful conditions, such as salinity and drought. Such conditions may have caused the plants to become susceptible to infection by weak pathogens that normally would not cause disease.
There are examples of similar diebacks in the plant pathology scientific literature. For example, stalk rot of corn, caused by several Fusarium species, can cause severe, rapid and widespread losses when the crop is predisposed to water stress early in the season. In this case the predisposing event may occur many weeks before first symptoms are apparent. Other examples include the jarrah forest decline in Australia and little leaf disease of pine in the southeastern United States. In all of these examples, a predisposing stressor is required for otherwise weak pathogens to cause severe symptoms that ultimately lead to plant death. Furthermore, these pathogens can be readily recovered from apparently healthy plants in which they may exist indefinitely as nonpathogenic parasites.
LSU AgCenter scientists isolated fungi from roots and lower stems of symptomatic and healthy smooth cordgrass and tested them in various ways for pathogenicity. In addition, the as yet unidentified fungus that causes a newly described black leaf spot on smooth cordgrass was included in these tests. Those isolates that caused vascular discoloration, stunting or leaf death were then identified and used in an experimental greenhouse system in which plants were subjected to stressors (drought and high salinity) to determine if predisposition accentuated disease severity. The results showed that these stressors alone did not cause symptoms or plant mortality, but in combination with certain strains of Fusarium sp., the entire brown marsh syndrome was reproduced.
At this point, it is not certain that pathogens are involved in every instance of smooth cordgrass dieback. It is clear, however, that these fungi are widespread, and in association with certain predisposing conditions, they can cause symptoms identical to those observed under brown marsh conditions.
The next step in the process of resolving the cause or causes of brown marsh is to conduct a wide-ranging survey to determine the geographic distribution of the suspected pathogenic fungi. Isolates would be recovered from apparently healthy and symptomatic plants, and these isolates would then be tested for pathogenicity under normal and stressful conditions. If the syndrome can be reproduced with a large number of isolates of the same fungus under the stressful conditions that existed in the marsh in 2000, but not under normal conditions, then we would conclude that pathogens played a role in the original catastrophic dieback.
Raymond W. Schneider, Professor, and Susan Useman, Research Associate, Department of Plant Pathology & Crop Physiology, LSU AgCenter, Baton Rouge, La.; Irving Mendelssohn, Department of Oceanography and Coastal Sciences, LSU, Baton Rouge, La.; Karen McKee, National Wetlands Research Center, U.S. Geological Survey, Lafayette, La.
(This article was published in the spring 2007 issue of Louisiana Agriculture.) |