DEFUNIAK SPRINGS, Fla. — Fusiform Rust (Cronarium quercuum f sp. fusiforme) is by far one of the worst diseases affecting southern pine species. A fungal rust disease with a complex life cycle and ability to infect trees at all stages of development; this disease is something you do not want plaguing your pine forest or plantation. The disease causes large galls in stems and branches of all pine species in Florida. It causes significant economic loses through both tree loss and through degrading stem quality for timber. In seedlings and young saplings, it can cause mortality. The rust galls cause weak points in the trunk and can cause a loss of timber value along with increased risk of wind damage and breakage. Of the major pine species, loblolly and slash pine are the most vulnerable to rust infections, while longleaf pine is somewhat more resistant. Some regions across the state have higher rates of this disease than others, with the Panhandle being an area that has a moderate to high incidence of rust. This means that good cultural practices to avoid rust and management to lower the risk of infection are a must. Otherwise, you can have severe issues especially under conditions where fusiform rust becomes prevalent in a stand.
Fusiform rust is a very complex fungal pathogen with multiple different fungal phases and spore types. Its lifecycle is spent in two different hosts: pines and oaks. The lifecycle and disease phases can be roughly divided into two segments: those spent in the leaves of oaks and those spent in the stems of pines. During the oak phase, the spores infect the leaves and form orange to yellow dots on the foliage. New succulent growth is preferred. Live oak and red oak species are the most infected, while white oaks are less impacted. Water, laurel, southern red, turkey, and bluejack oaks are some of the most susceptible hosts. No noticeable damage is done to the oak aside from leaf spots and occasionally some leaf drop. Once it moves into the pine phase of its life cycle it causes noticeable stem and branch galls that are large and damaging. It can also cause severe growth deformities such as a bushy form in young trees and witches brooms in branches. Initial infection occurs on young succulent shoots in pine and progresses over years to a perennial gall. The galls are football or spindle shaped and in the spring they produce a bright orange powdery substance full of spores. From here the fungus goes on to infect more oak leaves and complete another round of life cycle.
Pines are most vulnerable to severe infections when they are young and have the highest amount of new growth each year. The damage from an infection is permanent and the gall remains present on the tree, often weeping sap and causing a potential point for other issues. Due to changes in forest composition and structure over the years, fusiform rust is much more of an issue than it was many years ago. Fire is less common in forests today, which has increased the oak component in many areas compared to the early 1900’s. Plantations of loblolly and slash are particularly susceptible, and these have come to dominate forest management in the Southeast. Significant research and funding have been directed to developing the best ways to mitigate fusiform rust in pine stands. When followed these recommendations can greatly reduce rust infection and damage.
Good forest management and cultural practices are the best methods for mitigating issues with fusiform rust. This is especially true early in stand establishment, as the infection risk is highest in younger stands. Here are some of the best methods to reduce fusiform rust problems in a forest stand.
- Plant genetically improved seedlings that are rust resistant. Pine breeding programs have produced slash and loblolly pine that are resistant to rust. When reforesting in areas with a potential for rust, coordinate with your forester and nursery to get a superior rust resistant line. Planting longleaf pine is another option, as it naturally has more resistance than loblolly or slash.
- Utilize vegetation management techniques that reduce issues with oak competition. Prescribed fire and selective herbicides offer excellent options to address excessive oak competition in pine stands. Since oak is the secondary host of fusiform rust, excessive oak composition and regeneration in a stand results in high rust potential. Eliminating all oaks is not feasible or desirable as oaks do have some benefit. The goal should be to manage oak in such a way that the species composition and vegetation stage is at the desirable level.
- Reduce or delay intensive pine management options like early fertilization when pines are younger. High growth rates in young pines results in large amounts of succulent tissue were rust infections can often form. By delaying some practices until later in stand development the most severe rust impacts such as large stem galls can be avoided.
- For stands that rust infection has already caused damage and loss of timber quality, the best option is removal. Using targeted thinning practices that remove rust infected trees early in the life of a stand can reduce loss and improve stand value. Trees with main stem rust infections will always have a lower growth rate and quality. By removing these trees, the stand growth and quality at end of rotation can be significantly improved.
Fusiform rust remains one of the most damaging diseases in pine management, but good options are available to avoid significant damage. When severe rust infections occur, the loss in value over the life of the stand can be significant. Advances in pine breeding have produced genetically resistant seedlings that are the best option for preventing rust infection. Following good cultural practices like vegetation management, prescribed fire, and thinning can also mitigate the issues fusiform rust causes. Unfortunately, once rust infections occur the damage is permanent, which is why good seedling selection and proper management early in stand establishment are essential. As a stand continues to grow, selective thinning practices to remove rust infected trees can improve stand value. If you notice what appears to be fusiform rust in your stand, work with your consulting forester to develop a plan to address it. You can also contact your county forester or local extension office for assistance with fusiform rust problems.
For more information on this subject, use the following Alabama Extension publication link:
–Ian Stone, UF/IFAS