Rust Hazard Mapping

The rust infection recorded in the FIA data can also be used to map estimated hazard to rust. However, simple percentages of infected trees may not best represent hazard to new infections. To more realistically estimate the rust hazard, further restrictions were imposed on the FIA data sets. Slash and loblolly forest types were evaluated separately. For plots of natural origin, only those 5 to 15 years old were used--younger stands may not have expressed symptoms, and infected trees may have died in older stands--thus better expressing hazard for new plantings nearby. For plots of planted origin, only those with >30-percent infection were used--this potentially eliminates plots where the use of genetically resistant planting stock may have artificially lowered infection levels and thus confounded the estimation of hazard. These points were plotted on a map of the South by species. A grid was created from the point coverage (grid size was 1.86 miles or 3 km), and the infection percentage of each plot was used to calculate a weighted average for grid cells based on the inverse of the distance squared from each point. Grid cell values were interpolated from points within 99.4 miles (160 km) up to a maximum of 12 points. Grid cells were then classified as low hazard (0-9.9-percent infection), moderate hazard (10-30-percent infection) and high hazard (>30-percent infection). Grid cells were converted to polygons, and hazard rating was limited to counties with slash or loblolly occurrence as appropriate.

Maps were drawn to show slash and loblolly pine rust hazard zones (figs. 9, 10). These maps give a useful estimate of hazard to rust infection for both species over large areas of the South. They appear consistent with the ranges of the host species and our knowledge of where fusiform rust occurrence is most severe. Individual land management decisions, however, must be based on more localized rust hazard estimates made from nearby stands of host species. Survey methods for making such an estimate have been developed (Yandle and Roth 1971).

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Figure 9. -- Estimated fusiform rust hazard for slash pine based on infected Forest Inventory and Analysis plots.

Figure 10. -- Estimated fusiform rust hazard for loblolly pine based on infected Forest Inventory and Analysis plots.

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