Fusiform Rust

 Diseases of... Conifer Stem

Common Name:

FUSIFORM RUST

Fusiform rust gall.

Spindle-shaped (fusiform) gall of fusiform rust on the stem of an infected pine. Note bright yellow aecial pustules of the pathogen on the surface (aecial pustules occur only in the early spring).

Figure 15 - Life cycle of fusiform rust fungus

Pathogens: Fungus Cronartium quercuum f. sp. fusiforme (formerly, Cronartium fusiforme)
Common Hosts: Loblolly Pine Pinus taeda
Slash Pine Pinus elliottii
Other Pines Pinus spp.
Alternate Host: Oaks Quercus spp.
Significance: Fusiform rust is regarded as the most serious disease affecting pines in the southern United States. Estimates of timber losses to this disease in Florida alone have been placed as high as several million dollars annually. Fusiform rust attacks most of Florida's native pine species, but is generally inconsequential, except on loblolly and slash pines. The disease affects seedlings in commercial forest and ornamental nurseries as well as trees in field situations. Severe outbreaks of fusiform rust in commercial forest nurseries within the state in 1979 and 1980 resulted in losses of millions of seedlings valued at nearly $150,000. Infected seedlings are rarely killed in the nursery by fusiform rust infections. Instead, they are rendered useless essentially useless because they will often die within 2-to-3 years following outplanting. If they survive, they are generally deformed, grow poorly, and frequently break off at the point of infection several years later.
Recognition:

Fusiform rust infections typically result in definitive swellings called galls on infected branches and stems. Galls vary in appearance, but are most often spindle or fusiform in shape. On older trees, it is not uncommon for galls to appear somewhat depressed and canker-like on one or more sides of the stem. Witches brooms or marked proliferation of small branches are often associated with galled tissues. Stem galls are often associated with branches or branch stubs as a result of the rust fungus growing from infected branches into the main stems. Sometimes branches and stems are killed beyond the point of the galls. Stem breakage at galls is common. Pitch exudation is often associated with fusiform rust galls due to infestation by certain insects and/or infection by the pitch canker fungus. In the early spring, masses of showy, yellow-orange blisters called aecia (sing., aecium) appear on the surface of active galls. The aecia soon rupture and expose masses of powdery yellow-orange spores (aeciospores).

Fusiform rust infections on first year nursery seedlings appear as distinct knots or elongated swellings (galls) at or near the bases of seedlings. These galls are often accompanied by abnormal branch shoots (branchlets) which occur singly or in tufts. Occasionally, galls occur below the soil line in commercial forest nurseries and are therefore hidden from view. In general, seedling galls are not well developed until late in the growing season (fall or early winter). Latent gall development, in which galls do not develop until after seedlings are lifted and outplanted in forest plantations, is common.

Infection Biology:

Aeciospores produced on infected pines do not reinfect other pines. Instead, they are disseminated by wind and initiate infections on the tender young leaves of a variety of oaks. Oaks in the red or black oak group are particularly susceptible as alternate hosts, with water, willow, and laurel oaks heading the list. Later, in the spring and early summer, two different spore types (urediospores and teliospores) are produced on the undersurface of infected oak leaves. Urediospores are produced first and are often called repeaters, because they serve to initiate new infections on the leaves of susceptible oaks, thus repeating the infection cycle on the oaks.

Teliospores are produced later in red-brown, hair-like structures called telial columns, telial horns, or simply telia (sing., telium). Under conditions of warm temperatures and high relative humidities (60-80° F & 97-100% RH) the teliospores germinate, giving rise to minute spores called basidiospores or sporidia. Sporidia are wind-disseminated and thus initiate new pine infections on susceptible pine tissues including needles and succulent green bark. Sporidia are extremely delicate and can quickly lose their ability to cause infections if weather conditions are excessively dry. However, only a few hours of warm temperatures and adequate moisture on the surface of susceptible tissues are required for infection to occur.

Control:

Fusiform rust is readily controlled in seedling nurseries with the careful application of appropriately registered fungicides. Planting pines which are genetically resistant to fusiform rust infections is recommended in commercial forestry operations, especially in high hazard areas (see below, Figure 16). Avoid excessive site preparation when establishing pine plantations because this sometimes increases the incidence of infections, apparently by stimulating the growth of trees and resulting in either greater susceptibility, or larger target areas for infection (shoots of tender, succulent tissues). Delaying fertilization of young pine stands in high hazard areas until pines have grown beyond the size of critical vulnerability (about 8 years of age) is often advisable for similar reasons. Stem infections occurring after this period are usually located in the upper crown and are of little consequence. Reduction of oak populations where economy and management allow may be effective in reducing pine infections on a local scale. Salvage thinnings or complete harvests should be considered in severely diseased stands of marketable age. Destroy severely infected young stands and reestablish stands with either less susceptible species, or genetically resistant planting stock. Avoid outplanting infected nursery stock. This practice probably enhances the overall spread and intensification of the disease.

In urban areas, remove infected trees that represent stem breakage hazards. Prune infected branches, especially if galls are within 30 cm of the main stem. Branch galls more than 30 cm from the stem are unlikely to pose a significant threat to the stems. Branches with these types of infections are likely to die, killing the fungus at the same time, before the pathogen can grow into the stem. Careful surgery, removing the bark around galls encompassing less than 50% of the circumference of an infected stem, is an apparently effective treatment in certain cases.

Fig. 16

FIG 16- Incidence and distribution of fusiform rust in 8-12 year old slash pine plantations- 1973.

Bulletin No. 196-A | Printed October, 1983 | Contact the Forest Health Section