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Monitoring Incidence of Fusiform Rust in the South and Change Over Time
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For any major disease, such as fusiform rust, it is vital that the distribution and incidence be estimated and change over time monitored. When dealing with an 11-State area and millions of acres of susceptible hosts, the task becomes exceedingly difficult. Distribution and incidence of rust as reported here is particularly useful because it is based on a relatively stable and recurring data set that will allow periodic southwide updates of rust status. As management strategies to reduce rust are more frequently applied, and as genetically resistant planting stock becomes more widely available, their effects on the status of fusiform rust can be monitored over time. In this analysis, slight regional trends toward a higher proportion of slash pine acreage with >10-percent rust and a lower proportion of loblolly pine acreage with >10-percent rust were detected. For individual States, the detected changes may be viewed as important by forest managers.

For instance, in Louisiana, the number of acres of natural slash pine decreased from 133,000 in 1984 to 107,000 in 1991 (a 20-percent reduction), but the proportion of acres with >10-percent rust infection increased 21 percent (table 3). Or, in Mississippi, the number of acres of planted loblolly pine increased from 1.1 million in 1987 to 2.5 million in 1994 (a 125-percent increase), while the proportion of acres with >10-percent rust infection decreased 14 percent. Unfortunately, the real reasons for changes in the number of acres of trees with rust infections cannot be discerned from survey data. Planting of resistant host species or genotypes, variations in local and regional weather, and the great variety of management activities imposed on surveyed stands can all affect infection levels as can the effect of the disease itself (e.g., infected trees dying between surveys).

The actual impact of rust on forest stands is extremely difficult to assess. Rust may kill young trees, thus reducing stocking, but if sufficient numbers of healthy trees remain, no reduction in yield at harvest may occur. However, if stocking is severely reduced, yield reductions will occur. Volume loss can also accrue from trees that are deformed or degraded by infection but that will survive until harvest. Unfortunately, there is no direct way of assessing these losses from FIA data. In this analysis we used >10-percent infection as a minimum because levels lower than that were unlikely to affect stand management or yields.

Because only 2 survey cycles including rust data were available for all 11 Southern States, no regional, long-term trends in rust infection (if they exist) could be detected. Data from 3 survey cycles were available for four States and were compared. Data from 3 survey cycles will be available from all 11 Southern States in the future. At that time, a similar analysis can be performed to examine longer term trends.

Changes in rust levels reported here are the result of applying the methods described to the current and immediate past surveys in each State. Data on the number of acres reported here should not be directly compared to past reports on rust where different methods were used.

Landowners concerned with fusiform rust have a number of management options they can implement--the use of resistant planting stock, reduction of local oak populations, adjustments in planting density, and species/site matching. Also, managers may apply thinning strategies to reduce the effects of rust during midrotation and to capture volume that would otherwise be lost. Information on fusiform rust and management techniques are available for land managers interested in this disease (Anderson and others 1980, Powers and others 1993).

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