In Vitro Colonization and Resistance of Loblolly Pine Embryos Infected with the Fusiform Rust Fungus

Abstract

Cronartium quercuum (Berk.) Miyabe ex Shirai f. sp. fusiforme is a macrocyclic rust, requiring both pine and oak or chestnut species to complete its complex lifecycle (Fig. 1). Previously known as Cronartium fusiforme Hedgc. and Hunt ex Cumm. (3), this fungus is the incitant of fusiform rust disease, so named because of the distinctive spindle-shaped (fusiform) galls that often form on the stems and branches of infected pines (6). Infrequently encountered until the advent of pine plantation management in the 1930s, fusiform rust incidence has been increasing yearly (5). Some pine plantations have become 100% infected within 3 to 4 years after establishment (6). Losses occur due to seedling death, excessive branching, and as a consequence of trunk gall formation. Trunk galls decrease wood quality and increase the risk of wind and fire damage (7). Fusiform rust is now considered the most destructive disease of southern conifer forests (8). Although a range of resistance has been noted among various pine species, loblolly pine (Pinus taeda L.), the species most frequently planted in the South, is highly susceptible to this disease (9).