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The Health of Southern Forests, USDA Forest Service, Forest Health Protection, Southern Region

Stressors of Pine Forests

Fusiform Rust

Fusiform rust is the most important pine disease in the South, where about 13.8 million acres of slash and loblolly pine show at least 10 percent of the trees infected (4.0 million acres slash, 9.8 million acres loblolly). This amounts to 29 percent of the host type (36 percent of slash acreage, 26 percent of loblolly acreage). Figure 14 shows areas with large stands of loblolly and slash pine.

Figure 14

Figure 14. Areas with large stands of loblolly and slash pine.

Before 1930, fusiform rust was found throughout the southern pine resource, but incidence of the disease was low. It increased steadily until it was well established by the 1970's. In the 1980's, the disease continued to increase, but at a slower rate of about 2 percent per year.

Shifts are now occurring where the rust is increasing in some areas, especially where slash and loblolly pine were planted outside their natural range, and along the leading edge of the current disease distribution. The overall trend suggests a slight decrease in infection. This is probably caused by changes in forest management practices and genetic resistance.

Infections that occur on the main stem within the first 5 years of a tree's life normally cause death. Spindle-shaped swellings or galls develop on the branches (Figure 15) or main stem, with main-stem infections on older trees somewhat depressed on one side. This results in wood quality loss, as well as frequent mortality from main-stem breakage at the site of the canker. Young trees with main-stem infections are often killed by the fungus. Losses in individual stands, as well as in nurseries, can exceed 80 percent. Loblolly and slash pines are the most susceptible species.

Figure 15
Figure 15. A fusiform rust gall on a branch.

By analysis of Forest Inventory and Analysis data, a hazard rating system for fusiform rust was developed for loblolly and slash pine. Three infection classes were established: low, 0 to 9 percent infected; medium, 10 to 30 percent; and high, 31 to 100 percent.

Figure 16 shows fusiform rust hazard for loblolly pine, and Figure 17 shows the hazard for slash pine. Maps were prepared using FIA plot data. All loblolly and slash pines 1 inch dbh and larger were examined for fusiform rust. Natural stands from 5 to 15 years old, and planted stands with at least 30 percent infection, were used to develop the maps.

As indicated by Figure 16, rust on loblolly pine is most common in the Piedmont and coastal plains of Alabama and Georgia and in northern Florida. In these areas, 16 percent of loblolly is classified as high hazard. Georgia, which has the most loblolly pine acreage (5,753,000 acres), has the second largest percentage of high-hazard sites for loblolly.

Figure 16

Figure 16. Fusiform rust hazard for loblolly pine.

Figure 17 shows that fusiform rust on slash pine is most common in north central Florida, southern Georgia, and areas outside the tree's natural range. About 50 percent of slash pine is classified as high hazard. Florida, with the most slash pine acreage (5,198,000 acres), has the lowest percentage of high-hazard sites for slash.

Figure 17

Figure 17. Fusiform rust hazard for slash pine.