Fusiform Rust
Introduction
Fusiform rust is a very common and potentially serious disease, especially of slash and loblolly pines. The disease causes swellings (galls) on stems and/or branches which deform trees, reduce growth, and weaken wood making the trees more susceptible to breakage in high winds or bark beetle attack. Fusiform rust is also a problem in seedling nurseries; seedlings can be killed by the disease or the disease can be transported from the nursery to new plantings. The fusiform rust fungus requires an alternate host (oak) to complete its life cycle.
Identification
On pine, the most obvious symptom of infection is the formation of a spherical or spindle-shaped gall on a branch or main stem. Galls are caused by chemicals released by the fungus that trigger abnormal and excessive wood growth. Pitch canker is commonly found in association with fusiform rust galls, so the gall may be pitch-soaked and/or exude sap. Galls are often produced at the base of infected seedlings, though the swelling may be minor and often occurs at or below the soil line; these infections are frequently overlooked. Fusiform rust will frequently kill the lower needles of seedlings in nursery beds.
During cool spring months, bright orange spores are often produced on the gall surface. These aeciospores are blown off by the wind and serve to infect oak leaves. On oak, symptoms are limited to small leaf spots that may be chlorotic or necrotic. Often leaf spots are not noticed. Easier to observe are bright orange spores (urediospores) produced on the underside of the leaf.
Management
Avoid planting susceptible species in areas with a historically high incidence of fusiform rust. Relatively resistant loblolly pine seedlings are available, but disease may still occur and/or be less severe. Close spacing in pine plantations will allow for some mortality while maintaining adequate stocking levels, and encourage infected branches to break off before the fungus reaches the main stem. Branches with galls within 8 inches of the main stem should be pruned off (Fig. 4). Avoid practices that over-stimulate growth such as fertilization, as this has been shown to increase the incidence of rust.
