Cooperative Extension Service
The University of Georgia College of Agricultural and Environmental Sciences
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Edward A. Brown II, Former Extension Plant Pathologist
Kim D. Coder, Professor Silvics/Ecology, Warnell School of Forest Resources
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, cronartium 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.
Losses to this disease amount to millions of dollars a year from the death of infected trees and from its degrading effect on products cut from the tree. Trees with stem cankers are unsuitable for poles or pilings. Most waste will be incurred in cutting lumber and other products from them.
It is difficult to place a dollar value on pines in the home landscape lost to this disease every year. Its importance here is its destruction of beauty and design.
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. 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 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. Most pine infections occur in April and early May in Georgia.
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| Hairlike structures on the underside of oak leaves produce the spores that infect pines. | Fusiform canker on young loblolly pine stem. |
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.
| Life Cycle of the Fusiform
Rust Fungus (Cronartium quericum f. sp. fusiforme) |
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You can take certain precautions that will reduce or minimize damage by fusiform. By observing these precautions, you can keep infected trees to a mini-mum.
Plant Stock -- In nurseries, fusiform infection is prevented by spraying with fungicides such as Ferbam, Ziram or Bayleton from the time of seed germination until early summer. In spite of these intense precautions, infected trees occasionally may be shipped. These seedlings can be recognized by the small, spindle-shaped swellings on their stems and should be discarded at the planting site.
Planting -- If yours is an area where fusiform rust is severe, you might consider planting shortleaf or longleaf pine, which are more resistant. Research has shown that fusiform infection is greater in sparsely stocked than in dense stands. It is also greater in rapidly growing stands than in those with a lower rate of growth. Plantings, therefore, should be at closer spacings on better sites where rapid growth is expected. Closer spacing will encourage early, natural pruning of lower branches. This will minimize the spread of branch cankers into the stems.
In addition, closer spacings allow for the removal of more diseased trees while full stocking is maintained. Poor sites capable of supporting fewer stems per acre and where growth will be slower can be planted on a wider spacing. This will still allow for removal of infected trees without leading to under-stocking.
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| A fully developed limb canker. | Brooming effect caused by fusiform infection. |
Young Stands -- Early fertilization and cultivation, which stimulate early spring growth, favor high rust infection.
Early thinning in young stands will minimize losses from infected trees. Consider removing trees with stem cankers. Do not over-thin stands to remove all infected trees. The loss in total growth from over-thinning will usually be more than the possible loss from fusiform infection. In addition, opening the stand too much by over-thinning is likely to increase the number of new rust infections and hardwood competition.
Pruning out limb cankers within 15 inches of the stem will save many trees, since infection will spread this distance. This practice, however, can usually be justified only in high-value stands or over small areas.
Older Stands -- Studies have shown that loblolly pine sawtimber trees with as much as half their trunk circumference infected by fusiform canker can be left in well-stocked stands with little risk of loss from wind breakage. In such stands, the location of cankers and their influence on products to be cut from the tree are more important to the market than the risk involved in leaving them. Again, it is important that adequate stocking be maintained.
Homeowners -- If fusiform rust is severe in your area, you might consider selecting shade trees other than slash and loblolly pine. If either slash or loblolly pine is an important part of your landscape, protecting them with a fungicide spray program until they are at least 5 years old is one possibility. Protection and preventing
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| Main stem infected by an old limb canker. |
infection is very important, since we are unable to recommend a successful treatment once pine is infected. Apply fungicides from the first of May to mid-June. Refer to the Georgia Pest Management Handbook for up-to-date recommendations.
Many homeowners have so-called tree surgeons knock on their doors in early spring when the orange spores show up on pine cankers. They insist they should be
allowed to cut out stem and branch cankers to save the tree. Some even guarantee their work for a year. Such a guarantee is worthless; the orange spores will not appear until next spring even if nothing is done.
Keep these points in mind: The surgical 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 weaken-ing 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.
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| Wind damage to fusiform-infected pine. | Old infection on loblolly pine stem. |
Trade names are used only for information. The Cooperative Extension Service, the University of Georgia College of Agricultural and Environmental Sciences, does not guarantee or warrant published standards on any product mentioned; neither does the use of a trade or brand name imply approval of any product to the exclusion of others that may also be suitable.
The authors wish to express appreciation to H.R. Powers, Principal Plant Pathologist; Fred Matthews, Research Plant Pathologist; and S.J. Rowan, Research Plant Pathologist, at the USDA Forest Service, Forestry Sciences Laboratory, Athens, Georgia, for their contributions through research and their valuable assistance in the preparation of the original manuscript for this publication.
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