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Bill Starr: Rust in the most unexpected of places

Rust is one of those words that usually conjures up some kind of negative thought. You definitely don’t want to be told that your car has rust on it, or your boat trailer has rust on it, but what about your pine trees? Wait a minute you say; pine trees don’t rust. Well, they don’t rust but they can get a fungal disease called rust — fusiform rust to be exact.
My daughter and I were walking in the woods near my house the other day and we noticed several pine trees with a strange orange powder on them. This strange orange powder is actually the spores from a fungus called fusiform rust. Most Georgians have seen the spindle-shaped swelling on branches and stems of loblolly and slash pine trees. These swellings are especially noticeable in early spring when they’re covered with blisters filled with orange powder (spores).
The fungus Cronartium quericum f. sp. fusiforme causes this disease. It is commonly called southern fusiform rust, canker rust, fusiform canker, or simply fusiform rust. The disease is most damaging on slash and loblolly pines. It occurs to a lesser degree on longleaf and is not a problem on shortleaf pines. Fusiform rust spreads by means of fungus spores (the fine orange powder you see on fusiform swellings in early spring). These spores do not infect pines directly. The fungus requires two living host trees, pine and oak, to complete its life cycle. The fungus cannot spread from pine to pine. Young oak leaves are infected in the spring by wind-borne spores produced on pine trees. The fungus does minimal damage to the oak leaves but later in the spring, wind-borne spores produced on the young oak leaves infect new pine growth. The fungus can stunt, deform, or kill pine trees. In many cases, the fungus weakens the tree, causing branches to be easily broken off or the whole tree blown over by wind. They are carried by the wind to oak leaves, where spores capable of infecting pines are produced.
Water, willow, laurel, southern red, and several other oaks are susceptible hosts on which this transformation can take place. The rust causes a faint leaf spotting on the oaks but does little or no harm. Brown, hair-like bristles develop on the underside of infected oak leaves. These bristles contain microscopic spores usually released in late spring and carried by the wind to new pine needles and branch tips. Upon germination, the fungus begins its growth and infection of the pine. This entire cycle takes place within a few weeks each spring. Usually, the first visible sign of fusiform infection is a small, elongated swelling on one side of or completely encircling a stem or branch. These swellings grow throughout the season but produce orange spores only in early spring. Some cankers will not produce spores for three or four years and may appear dead, but they will begin producing spores again the next year. A portion of the old canker occasionally will die. This causes a flat or depressed canker, which sometimes gives off large amounts of resin.
Some people believe that fusiform may spread through the system of the tree, causing the formation of new cankers. This is not true. Each canker is the result of a separate infection and damages only that part of the tree. Branch cankers, however, often grow down into the trunk and develop into a damaging stem canker.
So, what do you do if you find trees with rust on them? Usually, once the rust is discovered no treatment is recommended. Keep these points in mind: The removal of fusiform cankers from the stem or trunk is generally not recommended. It is difficult to remove all the infected tissue around large cankers without weakening the tree severely and making it susceptible to wind breakage. Small cankers can be successfully removed, but they rarely penetrate deep enough to weaken the tree substantially.
Prune limbs that are cankered within 15 inches of the trunk. Limb cankers more than 15 inches from the trunk are of no importance, since infection will not spread this distance. Rust on pine trees has been prevalent already this year. Just be happy that rust on your vehicle isn’t.

-Bill Starr is the Sumter County Extension coordinator and ANR agent, University of Georgia Cooperative Extension Service. Contact him at 229-924-4476.