Ornamental & Tree Diseases
Southern Fusiform Rust
Southern Fusiform Rust is a fungus-caused (Cronartiium quercuum f. sp. fusiforme) disease found only in the southeastern portion of the United States, from Maryland south to Florida and west to Texas and southern Arkansas. It is a major obstacle in seed orchard, nursery, and plantation management. Considerable genetic variation exists between and within the southern pine species in the way they react to the fusiform rust fungus. As species, loblolly and slash pine are susceptible; pitch, pond, and longleaf pines are moderately resistant, and shortleaf pine is immune.
Impact of fusiform rust on southern forest resources takes several forms. Seedlings are killed within 3 to 4 years, with losses in some nurseries exceeding 80 percent. Infections that occur on the main stem during the first 5 years normally result in death. Infections that occur after 5 years weaken the stem, resulting in wind breakage at the canker. Older infected trees are also lost as a result of the development of open cankers, which provide openings for wood decay fungi and insects. Losses also occur in the form of reduced growth.
The fusiform rust fungus is macrocyclic and heteroecious, that is, it produces five spore types and requires two different hosts (pine and oak) to complete its life cycle. A life cycle takes approximately 2 years to complete. Black, white, water, willow, laurel, black-jack, southern red and turkey oaks are affected but not seriously damaged.
The most diagnostic symptom produced by infected pines of Southern Fusiform Rust is the formation of perennial fusiform or spindle-shaped swellings on branches and main stems. Other useful signs include the bright-yellow aeciospores that collect on the surface of fusiform-shaped galls. Symptoms on oaks are relatively inconspicuous and require close examination of the lower leaf surfaces to observe yellow uredial pustules and black hair-like clusters of telia.
Susceptible pines are infected in early spring by wind-disseminated fungus spores (basidiospores) produced on oaks. Basidiospores germinate, produce germ tubes that penetrate pine needles, cotyledons, and/or tender stems. The first indication of infection on needles appears as purple spots 4 to 10 weeks after inoculation. Spots later turn yellowgreen. Growth of the fungus into woody tissues (branches, main stem of seedlings, etc) causes the pine to produce characteristic fusiform-shaped swellings (galls) within 3 to 4 months after infection.
Pycnial Stage. Cup-shaped pycnia develop under epidermal layers in galled tissues 1 to 2 years after infection. Pycnia produce pycniospores (spermatia) in small, orange, viscid droplets from September to February.
Aecial Stage. The following year, aecia develop in galled tissues. Large numbers of aeciospores are produced, imparting a yellow color to infected stems. Aeciospores are wind-disseminated during February, March, and April, and infect leaves of susceptible oaks.
Uredial Stage. Approximately 2 weeks after infection of oak leaves by aeciospores, uredia appear as pale-green spots on lower leaf surfaces. Uredia rupture lower epidermal layers and release orange urediospores that reinfect oak hosts.
Telial Stage. Telia and teliospores may appear concurrently with urediospores or later (February to June). Hair-like, black telia made up of chains of teliospores, serve as overwintering structures.
Basidial Stage. Teliospores germinate and produce wind-borne basidiospores from early April through June.
Management recommendations include:
1. Exclude the pathogen by eliminating oaks growing near nurseries.
2. Avoid the pathogen by locating nurseries in low-rust-hazard areas.
3. Eradicate or reduce the pathogen by culling infected seedlings.
4. Plant only rust-resistant pine seedlings.
5. In nurseries, protect susceptible seedlings with a triadimefon (Bayleton T/O, Strike 25W) or myclobutanil (Systhane WSP) fungicide. Be sure to follow label directions.
6. Wood-rotting fungi and insects invade large galls or large open wounds on pine. These will weaken the tree. Remove trees with larger galls or open wounds near buildings because of the possibility of damage from wind breakage.