Limb Rust
Cronartium arizonicum Cummins

Host:  Ponderosa pine

Symptoms/Signs:  Dead and dying branches, often in the middle of the crown, are quite characteristic of this disease. Spore-producing sacs (aecia) are produced on live infected branches, and erupt through the bark in spring to midsummer. Trunks are not affected by this disease even though the fungus travels through it to get to the branches.

Biology:  Initial infection occurs on a needle-bearing twig. The fungus grows into the sapwood and eventually reaches the main stem. It grows up and down the stem, entering and killing individual branches. Disease from this pathogen results in a progressive invasion and killing of branches by the fungal mycelium that is perennial in the trunk, but does not injure it. From an initially infected branch, the parasite grows into the wood of the trunk and advances upward and downward up to 18-21 cm per year in each direction and then out into branches where it sporulates. Infected branches typically die the following year.

There are at least three varieties of limb rust in the western U.S., collectively known as Peridermium filamentosum. Cronartium arizonicum, which uses Indian paintbrush (Castilleja spp.) as an alternate host, is the most common variety in Arizona and New Mexico. Another variety that spreads directly from pine to pine has been detected on the Kaibab Plateau in northern Arizona.

Effects:  Although the disease can be very damaging to individual trees, its occurrence in an area is usually low. In Arizona, it has been observed throughout the ponderosa pine type, from the Coronado National Forest to the North Kaibab Ranger District.

Similar Diseases:  Many agents can cause branch mortality, but limb rust is distinctive because of the progressive mortality in the center of the crown.

References:  79, 92, 116