An Integrated Approach Toward Reducing Losses from Fusiform Rust in Merchantable Slash and Loblolly Pine Plantations
The primary objective of this study was to evaluate the selective thinning of trees with rust galls as a means of reducing losses to the fusiform rust (Cronartium quercuum (Berk.) Miyabe ex Shirai f. sp. fusiforme) disease in merchantable slash (Pinus elliottii Engelm. var. elliottii) and loblolly (P. taeda L.) pine plantations. Additional objectives were to assess the post-thinning occurrence and impacts of southern pine beetles (Dendroctonus frontalis Zimm.) and annosum root rot (Heterobasidion annosum Fr.). Nineteen rust-infected plantations were selected in Alabama, Georgia, and South Carolina to represent a wide range of stand and rust conditions. Thinnings were based primarily on the removal of trees with severe and moderate stem girdling caused by fusiform rust galls. Approximately 750 acres were thinned operationally to salvage potential mortality; 2,250 nonthinned acres served as controls. Study plots were surveyed annually for 10 years to determine the amount and causes of mortality. Stand growth and development were evaluated at the end of 5- and 10-year periods after treatment. Removal of rust-infected stems greatly improved the quality of trees in the residual stands. The periodic diameter growth of individual dominant and codominant slash and loblolly pine trees was affected by the extent of stem girdling. Trees with severe stem girdling (250 percent) grew significantly less than gall-free trees or trees with small or moderate stem girdling. Trends in periodic stand growth and total volume production were similar for slash and loblolly pine. Standing volume at the end of 10 years was greater in nonthinned portions of the plantations than in thinned portions. However, periodic stand growth and ingrowth of rust-free sawtimber (29 inches in diameter at breast height) were greater in thinned portions of the plantation. Rust-associated mortality was the primary factor that reduced volume growth and production in slash pine plantations. Losses from fusiform rust and the southern pine beetle had a severe impact on total production in the loblolly plantations. Thinning significantly reduced losses from both of these forest pests. Management implications can be incorporated into conventional thinning procedures for regulating stand density, increasing growth, and improving stand health.