Rust Diseases


Forest Health Guide for Georgia Foresters
Written by Terry Price, Georgia Forestry Commission
Adapted for the web by the Bugwood Network

Fusiform Rust

Fusiform rust, Cronarium quercuum f sp. fusiforme, is the most damaging disease of slash and loblolly pines throughout the southeastern United States. This disease causes stem and branch cankers (galls) to form on infected trees (Figure 129). The majority of infections occur prior to age 5. In early spring active galls produce yellow-orange spores (aeciospores) that are windblown to young, tender oak leaves. Water, willow and laurel oaks are the species most affected.

The fungus goes through three additional spore stages on the oak leaves. The fourth and final stage results in basidiospores being formed. The basidiospores are windblown to new pine needles and/or succulent green bark areas of young trees thus completing the disease life cycle.

Figure 129
photo by Terry Price

The amount of infection in planted and natural pine stands varies considerably across the South. Factors such as individual tree resistance, site characteristics and rust virulence all interact to influence infection levels.

When regenerating sites to pine, land managers must take steps to deal with fusiform rust. Below are some practices that will help minimize rust problems and protect economic investments.

Artificial Regeneration

Use rust resistant seedlings (improved) that are available from state and privately owned nurseries. These seedlings are the result of many years of valuable research that hopefully will result in increased seedling resistance to the disease. Further protection from rust is provided by timed applications of a fungicide to seedlings in the nursery. Rare cases have been documented due to equipment breakdowns, where seedlings are not sprayed while in the nursery bed. Seedlings that become infected with rust while in the nursery will oftentimes develop galls at or near the ground during the first growing season. Landowners should realize this and be sure to question the nursery from which they are buying seedlings from to confirm they have been treated with an appropriate fungicide.

In high hazard regions, increase planting densities to allow for anticipated losses. This will, of course, increase the number of thinnings required throughout the life of the stand.

Consider site preparation techniques that reduce oak trees unless they conflict with other management objectives (prescribed burning, herbicides, and intensive mechanical preparation). Use seed from a resistant source. Seed should be treated with an approved bird and rodent repellent.

Natural Regeneration

If seed tree or shelterwood methods are used, be sure to select disease-free seed trees.

Managing Established Stands

Evaluate plantations at age 3-5 to determine degree of rust infection. Plantations that are showing 50% or more infections should be seriously considered for clearcutting. If the decision is made not to clearcut, a sanitation cut should be planned as soon as it is economical.

After age five, it is usually uneconomical to clearcut and start over. However, rotation ages will need to be adjusted in order to maximize on the establishment costs.

In older stands with less than 50% stem infections, remove as many stem-cankered trees as possible with each thinning. Try to maintain a basal area of 75-85-sq. ft. per acre or at least 200 trees per acre. When thinning plantations, be sure to consider annosum root rot.

Eastern Gall Rust

(Cronartium quercuum f.sp. virginianae and echinatae)

Eastern gall rust attacks many hard pines, particularly Virginia, spruce, sand and shortleaf pines. Infections produce globose galls on main stems and branches. In spring the galls become covered with large blisters that rupture and release spores in the form of a pale, orange-colored powder (Figure 130).

The rust fungus must pass through a stage of growth on the leaves of various oak species before it can reinfect pines. The life cycle is very similar to that of fusiform rust. Weather and available hosts species greatly influence the extent of infection. Trees are seldom killed. Disfigured and malformed trees may result from infection.

Figure 130
photo by Robert L. Anderson

Unlike fusiform galls, eastern gall rust galls seldom spread from branch to stem and usually remain localized at the point of infection. Shade and yard trees may be protected by pruning infected branches. Under forest conditions stems with galls should be removed during thinnings.

Red Cedar Rusts

Eastern red cedar is host to species of Gymnosporangium rusts that infect the foliage, twigs and branches. The red cedars in Georgia are affected by both needle and twig rusts. These rusts while causing some concern on the part of the landowners are not of economic importance on the cedars.

During wet weather in the spring, the gall rusts produce bright orange gelatinous finger-like masses of spores (Figure 131A). The twig and branch rusts produce tongue-like spore masses on the branch swellings (Figure 131B).

Red cedar should not be planted near apple orchards as apple, hawthorn and crabapple are alternate hosts to the rusts and some species may be severely defoliated. No chemical control is recommended for cedar rust under forest conditions. The homeowner may wish to prevent or reduce infections on crabapple, hawthorn or apple trees by spraying with recommended fungicides.

Figure 131A
photo by Terry Price

Figure 131B
photo by Terry Price

Pine Needle Rusts

The foliage of the two and three needle hard pines in Georgia are subject to several needle rusts in the genus Coleosporium. Most of these pathogenic fungi have herbaceous alternate hosts. The alternate hosts are generally golden rod and aster.

The most noticeable symptoms of needle rusts are the bright orange aeciospores on the pine needles in the spring when weather conditions are favorable for the development of the disease (Figure 132) The entire tree may have a yellow to orange appearance. Often heavily infected needles are shed giving the tree a thin looking crown. Young trees may be heavily defoliated and growth stunted, but older trees are seldom damaged.

No chemical controls are recommended for this disease either under forest or shade tree conditions. Elimination of the alternate host in the vicinity of the pines should reduce the incidence of infection.

Figure 132
photo by Robert L. Anderson

Southern Cone Rust

Southern cone rust, Cronartium strobilinum, attacks seed-bearing slash and longleaf pines in the South. The disease is usually not considered serious under forest conditions, but may periodically become severe in localized areas. Damage could become an economic consideration in seed orchards and seed production areas. The disease has various species of oak as the alternate host.

New conelets are infected in winter from spores produced on the leaves of the evergreen oaks, i.e., live oak and low growing runner oaks. The disease becomes evident on the new conelet shortly after pollination. The infected conelet increases in size very rapidly so that by April or early May it is several times larger than uninfected cones. A reddish color is also generally observed. In late spring, the diseased cones become orange-yellow in color due to the abundant spores produced by the

Figure 133
photo from USDA Forest Service Archives

fruiting body of the fungus (Figure 133). The diseased cones usually abort and fall to the ground.

Past surveys have shown losses up to 20% of slash pine cones in an average year. The incidence of rust on cones will depend upon air temperatures and moisture at the time of spore production on the oak leaves.

For best control, slash and longleaf pine seed orchards should be established in areas with the lowest number of evergreen oaks. Chemical control should be directed mainly to seed orchards. Losses can be reduced by spraying with fungicides; however, spraying should start as soon as the female flowers emerge and continue on a five-day schedule until pollination has ceased. This schedule would ordinarily require five or six sprayings per season.

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