Introduction

Pitch canker¹ is major disease of planted pines in the South. Crown dieback, stem deformity, reduced growth rate and mortality are most severe on slash pine, but Virginia, shortleaf, loblolly, longleaf, south Florida slash, pitch, Table Mountain, sand and eastern white pines are also susceptible. Before 1974 and the start of the present epidemic in northern Florida, the disease occurred sporadically and caused variable amounts of damage. By 1976, the disease was present on an estimated 1.1 million acres (445,000 ha). In several, severely affected, east-central Florida counties, more than 51 percent of the planted slash pines were infected. The disease has recently intensified in western Florida and is frequently found in southeastern Georgia and on planted slash pine in South Carolina.

Elsewhere in the South, damage has most often been restricted to seed orchards of slash, longleaf, Virginia, shortleaf and loblolly pine. Cone production has been dramatically reduced in severely diseased orchards. The disease is most severe in orchards where the trees have been wounded by natural means or by cultural practices. Some clones in seed orchards are severely affected while others in the same orchard remain disease free.

In slash pine plantations, a large portion of the expected growth can be lost (figure 1). In pole-size stands in epidemic areas, mortality has been reported as high as 25 percent and infection levels have exceeded 90 percent (figure 2). Growth loss and stem deformity seem to be the primary losses in sapling-size trees (figure 3); infection in younger trees is relatively uncommon. Severely diseased pole- and sawtimber-sized trees often do not recover. Several thousand acres of slash pine plantations in Florida, and many valuable seed orchard trees southwide, have been prematurely cut because of this disease.

The destructive potential of pitch canker is sufficient to warrant increased attention to recognition of the disease and to consideration of management practices which may be useful in minimizing losses.
¹Caused by the fungus Fusarium moniliforme var. subglutinans.

Recognizing Pitch Canker

The fungus invades woody branch and main stem tissues and creates localized cankers. These cankers are annual on small-diameter shoots and perennial on larger shoots. Pitch flows freely (figure 4) from the diseased tissues, even on small-diameter shoots. The wood beneath the cankers becomes discolored and thoroughly resin-soaked (figure 5). These symptoms are characteristic of most pitch cankers and will help you distinguish this disease from other canker diseases and most insect injuries.

Cankers on Small Shoots

Cankers on small shoots result in crown dieback symptoms typical of the disease on slash pines in plantations. Cankers on small shoots also result in crown dieback of shortleaf and loblolly pines in seed orchards. Although the trees are infected mostly during the summer and fall, you are most likely to notice the symptoms in the late fall, during the winter or in the early spring. During that period the fully expanded needles turn bright reddish-brown (figure 6). Dead needles may persist on some diseased branches for more that a year, during which time the needles fade to a dull, grayish-brown color (figure 7).

Cankers on Larger Shoots

Cankers on larger branches or on the upper portion of the main stem often do not kill the shoot during the first season. If these cankers grow during the winter and spring they may girdle the larger shoots. New growth will then droop, turn brown, and die (figure 8).

Cankers on Older Growth

Cankers on the larger portions of the main stem and on large branches live longer and are usually accompanied by large amounts of resin flow, sometimes covering several feet of the stem (figure 9). Such cankers are relatively uncommon in infected slash pine plantations, but are more common on pines in seed orchards and urban plantings. The bark remains on the canker face, which may be slightly depressed. The wood beneath the canker is pitch soaked, often to the center of the stem. Large portions of the tree crown can be killed when such cankers eventually girdle the stem or branch. Do not confuse these symptoms with similar symptoms occurring at the base of sand pines affected by certain root and butt diseases.

Recognizing the Pathogen

In the Field

The fungus produces spores in small, cushion-shaped, salmon-pink colored structures (sporodochia) (figure 10). You are most likely to see sporodochia in the small depression where needles were formerly attached to the diseased branch. While not always present on diseased shoots, these structures can be found at any time of the year if you carefully examine branches whose remaining needles are still somewhat reddish-brown.

In the Laboratory

Because fruiting structures can be absent, and are usually inconspicuous, positive identification requires laboratory culturing and isolation.

There are many ways to isolate the pitch canker fungus from symptomatic tissues. One highly effective method is as follows:

• Collect disease samples from living branches that have both diseased and healthy tissue.
• With a sterilized knife, remove the outer bark, exposing the canker margin.
• Using aseptic techniques, remove ¼ inch (6mm) square wood chips from the canker margin, briefly dip the chips in 95 percent ethanol, and shake off the excess alcohol. Flame sterilize the outer surface of the chip, being careful not to overheat the sample.
• Transfer the surface-sterilized chips to potato dextrose agar or to a Fusarium-selective media as described by Nash and Snyder. Incubate plates for 5 to 10 days under normal laboratory lighting and temperature conditions.
• On the selective media, the pathogen appears as slow-growing, granular, white colonies (figure 11). Transfer a portion of the colonies to fresh potato-dextrose agar media or 2 percent water agar (supplemented with a washed, autoclaved, carnation leaf) for positive identification. The pitch canker fungus produces characteristic microconidia, macroconidia, and polyphialides (chlamydospores are absent) figure 12).

How the Pathogen Enters the Tree

Wounds are readily infected by the pitch canker fungus. Insects such as the deodar weevil2 (figure 13) and the subtropical pine tip moth3 create wounds which can be infected by airborne spores of the pathogen. In slash pine seed orchards, bole cankers often develop through injuries caused by mechanical shakers (figure 14) used in cone collection. Infections frequently occur at cone harvesting (figure 15). Weather-related injuries such as those from wind and hail may also serve as entry points. The fungus is also known to infect fusiform rust galls.

Managment Considerations

Specific control recommendations cannot be formulated until further research results become available. Until such guidelines can be developed, the following items are presented as considered that may be useful in the management of this disease in plantations and seed orchards. These management considerations are based on extensive field observations and the most current research information available.

In Plantations

Salvage. – The decision to salvage severely diseased plantations depends on the incidence (number of infected trees) and severity (amount of infection in each tree) of the disease in the stand, the expected future course of the disease, and the response of the infected trees. If stem-deformity, reduced growth rates, and lengthened rotations can be tolerated in sapling-size slash pines, moderately infected stands will normally recover. However, deformed stems may be more prone to breakage during windstorms. In severely infected pole-size slash pine stands, growth reduction and mortality losses may warrant salvage harvesting prior to scheduled rotation age. In addition, harvesting the infected trees should reduce the abundance of the pathogen in the area, thereby reducing the exposure of nearby stands.
²Pissodes nemorensis
³Phyacionia subtropic

Logging debris. – When possible, burn or broadcast logging debris. This practice will reduce the amount of the pathogen in the infected branches and thereby may minimize the exposure of nearby stands.

Thinning slash pines. - When economically feasible as a forestry practice, consider thinning of slash pine plantations with low disease incidence when they are located in high hazard areas (merchantable stands in outbreak areas). Stocking reduction in plantations with little disease will reduce competitive stress. This may, in turn, increase the resistance of the trees to future attacks or allow better recovery from previous infections. The effectiveness of this type of thinning has not been demonstrated, but removal of diseased trees during the thinning may limit future losses by reducing the level of airborne spores of the pathogen and by minimizing growth loss in the plantation. Removal of severely infected and low vigor trees may also remove some breeding sites (figure 16) for the deodar weevil, an insect that can vector the pitch canker fungus and can produce wounds that can be readily infected. Such thinnings should be done between December and May except during prolonged droughts or when bark beetles are a problem in the area. Treat stumps for annosus control when thinning stands on high hazard sites for annosus root rot.