Forest Pests: Insects, Diseases & Other Damage Agents

Anderson, R. L.; Mistretta, P. A. Management strategies for reducing losses caused by fusiform rust, annosus root rot, and littleleaf disease. Agric. Handb. 597. Washington, DC: U.S. Department of Agriculture, Forest Service, 1982. 30 p.

Fusiform Rust

Significance

Stem and branch galls caused by the fusiform rust fungus (Cronartium quercuum (Berk.) Miy. Ex Shirai f. sp. fusiforme) appear on all growth stages of infected pines, from seedlings to mature trees. Economic loss results from the death of trees, breakage of the stem at the gall, or reduction of sawtimber quality in trees not killed outright. On about 13 million acres of slash and loblolly pine in the South, at least 10 percent of the stems are infected. Annual loss is estimated to exceed $100 million.

Symptoms

Diagnostic characteristics vary with different stages of fungal or gall development. The fungus has five spore forms in its life cycle: the pycniospore and aeciospore occur on pine, while the urediospore, teliospore, and basiospore occur on alternate hosts, such as water oak (Quercus nigra L.) and willow oak (Q. phellos L.). The fungus does not spread directly from pine to pine. The basidiospores, produced on the oak host during cool, humid weather in the spring, are carried by the wind and infect pines primarily through tender stem tissue.

Figure 2. - Slight swelling and epidermal discoloration from fusiform rust on young, tender shoots of loblolly pine. F-703140.	Figure 3. - Spindle-shaped swellings on branches and main stem are symptomatic of fusiform rust. F-703141.
Figure 4. - Main stem infection of pine caused by the fusiform rust fungus. F-703142.	Figure 5. - Main stem breakage at fusiform cankers, caused by wind. F-703143.

Year-round, spindle-shaped swellings occur on the branches or main stem (fig. 3). Main stem infections on older trees can be somewhat depressed (fig. 4), and the trees sometimes break at the gall (fig. 5).

Hazard Rating

To estimate hazard for new stands, examine the existing stand or adjacent stands. If 50 percent or more of the trees have galls on or within 12 inches (30 cm) of the main stem, the site is rated high hazard; 25 to 50 percent of stems infected is moderate hazard, and less than 25 percent of stems infected is low hazard.

Figure 6. - Fusiform rust infection zones for loblolly pine plantations (prepared by H. R. Powers).

Figure 7. - Fusiform rust infection zones for slash pine plantations (prepared by H. R. Powers).

Figures 6 and 7 are fusiform rust hazard maps, which depict the levels of rust infections for loblolly and slash pines. Using these maps, resource managers can geographically locate the stand and make an estimate of fusiform rust hazard. This geographic hazard estimate must be modified on site since specific areas within a hazard zone may be different from the average for the area.

Table 1. – Adequate, marginal, and inadequate stocking for disease-free southern yellow pine sapling and merchantable stands¹

¹Consideration of the number of disease-free stems, stocking level, and the average height and diameter of the plantation is essential in making management decisions. This table should be of assistance in deciding when disease, coupled with other factors, has reduced stocking to an unacceptable level. (Desired levels of stocking can vary with specific management needs and objectives). ²Plantations should be surveyed for fusiform damage 3 to 5 years after planting.

It is desirable to evaluate each stand 3 to 5 years after planting to determine if enough rust-free trees are present (Table 1) to warrant maintaining the stand until rotation.

When other factors influencing infection are not limiting, intensive site preparation may result in increased rust hazard by increasing the amount of susceptible juvenile tissue.

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