Fusiform rust, caused by the fungus Cronartium quercuum f.sp. fusiforme, is native to the southern United States. Fusiform rust is an economically important disease of pine in managed pine forests and occasionally in the home landscape. All southern pines can be damaged by the disease, although loblolly and slash pines are most commonly affected in Alabama. In the US, fusiform rust has a range that extends from Texas to Florida, then north to Maryland.

Fusiform Rust

The most obvious symptom is the elongated, swollen galls or cankers that develop on infected branches or the main stem (trunk) of pine. The tapered, spindle-shaped galls or swellings typically exhibit deep cracks (figure 1). In the early spring, fruiting galls produce orange-colored powdery spores in blister-like structures (figures 2 and 3). These brightly colored spores are spread by wind to leaves of the pathogen’s alternate host, oak trees. The fungus needs both pine and oak trees to complete its life cycle. The most commonly infected oak species in Alabama are water, laurel, willow, and southern red. Oaks in the white oak group are not often infected. Oak leaves become infected when the first flush expands in the early spring. The disease does little if any damage to oak leaves. In time, specialized spores are produced on oak that can then be blown back to susceptible pines from April through June, depending on prevailing temperatures. The pathogen infects pine at the growing tips completing the disease cycle. Within a few months, under favorable environmental conditions, a swelling or gall will start to develop at the site of infection on infected pine.

Fusiform rust infection of young pine seedlings is often deadly within a few years of infection. Rust infection of the main stem on trees less than five years old is typically lethal. Older trees that survive with a main-stem or branch infection may develop sunken cankers which cause weak, distorted tissue that can break in strong wind. Slash pine seems more susceptible to stem breakage than loblolly.

Fusiform rust is one of the most damaging fungal diseases in pine plantations, causing significant economic loss in the timber industry. In landscape settings, main-stem infections on larger pine trees may pose a risk to property due to breakage. Homeowners should consider pruning out these infected main stems if damage may occur when stems fall. On larger trees, branch infections can be left alone because they pose little threat of infecting the main stem.