The fungus Cronartium fusiforme is the cause of fusiform rust in pine trees. This disease is known to exist in the Southern U.S. states south from Maryland to Florida, continuing west to Texas and southern Arkansas. It takes its toll in forests and tree plantations. Fusiform rust attacks several Southern pines, however it is most severe in the slash and loblolly pine trees.
This fungal disease has killed and deformed pines (specifically the slash and loblolly pine) in the South, beginning in the late 1950s and early 1960s. The spread of the disease at this time was in epidemic proportions. Annual losses from fusiform rust are estimated at $35 million, and according to the U.S. Department of Agriculture, “Annual losses from this disease are estimated at 562 million board feet of sawtimber and 194 million cubic feet of growth stock. Stumpage losses are valued at $28 million annually.”
How It Occurs
Cronartium fusiforme produces five different spores; two of the spores are produced on the galls of the pine tree (on stems and branches). The other three types of spores are produced on the oak leaves (underside of the leaf). In the springtime the pine tree produces spores, which are then spread to oak leaves via the wind. Later in the spring, the wind spreads spores that are produced by the oak tree, which in turn infect the pine tree's new growth; the cycle is complete. The first five years of a tree’s life is when fusiform rust infection is likely to take place.
Cronartium fusiforme requires two different hosts in its life cycle. In the beginning of its life cycle it lives in the tissue of pine stems and branches. The rest of its life cycle is spent in the green leaves of certain species of oak trees. Black, water, willow and laurel oaks are the species that are most susceptible to fusiform rust. The effect of the disease on the oak trees is minimal as only the leaves are affected. But, the result of the infection in pine trees is deadly. This fungal disease cannot be spread from pine tree to pine tree, however.
Several factors bring about fusiform rust: rapid growth among young trees, the presence of host oak trees, well-drained soil, and a warm, moist climate. Nursery owners can reduce these conditional factors by planting rust-resistant seedlings, choosing a site with a minimal amount of oak trees, and delaying fertilization so no rapid growth takes place.
Treatment and Management
Management begins with site selection; to avoid fusiform rust, evaluate the site thoroughly before planting even begins so that the risk is kept at a minimum. Since this is a nursery, tree farm or tree plantation problem, the treatment and management of the disease is quite complex; however, the main objective of any treatment or management strategy is to reduce losses. On young tree farms when greater than 50 percent of young trees are infected, consider replanting with disease resistant seedlings. Other management options include pruning limb galls and sanitation/salvage thinning.