Regeneration and stand management activities can alter the impact of pine stem rusts. As obligate parasites, pine stem rusts can be managed by applying various silvicultural treatments. These treatments include the selective removal of live host trees during spacing, removing infections (pruning infected branches and excising cankers), or through stand density management. However, the negative impact to severely infected stands can be increased through premature stand entry and by spacing to low stand densities. A longer-term approach is to develop rust-resistant seedlings through an intensive tree improvement program.
The major pine stem rusts covered in this guidebook are:
This guidebook is designed to provide the user with a reference for identifying, detecting, assessing, and treating pine stem rusts in managed forest stands. This guidebook is intended to be of use primarily in the preparation of silviculture and stand management prescriptions.
Table 1. Pine stem rusts: distribution, alternate hosts, and host susceptibility
Figure 1. Western gall rust may result in stem breakage.
Figure 2. The orange spores found in comandra blister rust cankers dramatically increase its detectability in late spring.
Figure 3. Comandra blister rust is difficult to detect in the summer.
Figure 4. Alternate host of comandra blister rust, bastard toad-flax (Geocaulon lividum).
Figure 5. Stalactiform branch infection (note canker length).
Figure 6. Squirrels commonly feed on bark infected by stem rusts. However, since they do not remove the leading edge of the canker, spread continues.
Figure 7. Sweet fern blister rust canker on lodgepole pine sapling. Note the swollen woody ridges which are diagnostic for this disease.
Figure 8. White pine blister rust on white pine showing stem canker and flagging of infected branch.
Table 2. Signs of the pine stem rusts of B.C.
The last three rusts will be referred to collectively as the Cronartium rusts.
Sweet fern blister rust only infects very young seedlings in close proximity to its alternate host (which is relatively uncommon and restricted to wetlands). Thus, the overall infection risk to pine by sweet fern blister rust in B.C. is quite low. For this reason it is not dealt with in detail in this guidebook.
The life cycles of these four rusts are well described in Ziller (1974). The Cronartium rusts require alternate hosts in order to complete their life cycles. Only the spores from an infected alternate host may infect a lodgepole pine. The spores produced by the rust on an infected lodgepole pine may only infect an alternate host and cannot directly infect another lodgepole pine. The cankers caused by a rust infection continue to release spores each year in late spring until the infected branch or stem dies.
Western gall rust does not require an alternate host. Spores released from a gall rust infection may directly infect the expanding candles of the same or another pine tree. Gall rust spores can travel hundreds of kilometres making sanitation difficult, if not impossible. In the case of the Cronartium rusts, spores from the pine hosts may also travel several hundreds of kilometres. However, spores from the alternate hosts are relatively fragile and may only spread up to several kilometres. A severe rust infection in a stand usually places nearby stands at a higher risk of becoming infected.
Infection by pine stem rusts tends to occur in "wave years." These are years of high infection that occur at irregular intervals. Many stands are not exposed to wave years of rust infection when they are young and most susceptible. In these stands damage due to rusts is minor. In the stands that do experience wave years, and in certain areas where risk is high, damage can be considerable. Young stands severely infected with any one or a combination of these rusts may fail to reach minimum stocking levels.
The main impact of hard pine rusts occurs primarily in young stands, although mature trees may be infected. Stem rusts cause mortality when the rust mycelium girdles and kills the stem. The impacts of rusts are believed to be the greatest on the most vigorous trees. Since rusts are obligate parasites, if conditions are good for tree growth they are also good for rust growth. Lower branches are the most susceptible infection sites and infection rates decline rapidly following crown closure, when lower branches are killed due to suppression. Crown closure also reduces the light available to the alternate host species.
In heavily stocked stands stem rusts can act as natural thinning agents. However, the distribution of stem rusts throughout young stands is often somewhat aggregated. Thus, mortality due to rusts may result in the stocking of some areas dropping below recommended minimum levels while other areas in the same stand remain overstocked. Hence, the need for proper stratification prior to treatment is essential.
Western gall rust is not considered an efficient thinning agent. Trees with stem galls will not likely become crop trees. However, they do not die quickly. These trees continue to compete with surrounding trees for resources while contributing little to stand productivity. Stalactiform blister rust often infects pole-size trees well after spacing has occurred. It also causes mortality, but this often takes a long time. Stalactiform infected trees often do not reach rotation but still occupy valuable growing space. Those trees that do reach rotation suffer a serious degradation in wood quality.
Pine stem rusts do not infect pruning wounds or branch stubs. Gall rust spores infect the newly expanding candles of branches and stems through the epidermis, whereas the Cronartium rusts infect branches and stems through live needles.
White pine blister rust affects trees of all ages; the greatest impact occurs in young trees. Rust spores infect new needles, eventually causing a canker to develop in the bark of the host. Mortality occurs when a blister rust canker girdles the main stem. White pine blister rust has largely prevented western white pine from being considered a preferred or acceptable species for regeneration in B.C. Without intervention (branch pruning) only a few young white pine trees survive to maturity. Western white pine was formerly one of the most productive tree species in western North America. A considerable amount of research has been directed towards developing resistant western white pine trees. The identification of trees exhibiting natural resistance mechanisms, such as bark reactions, has led to genetic screening programs in the US and Canada. Early field test results of resistant stock are promising, especially on low to moderate hazard sites.
Two other soft pines, white bark and limber, have also been severely affected by white pine blister rust. Their continued survival as species is threatened by this disease.
The following five steps integrate the evaluation of stand, site, and disease conditions for plans and prescriptions:
These steps are detailed in the following sections.
Figure 9. Diagram of hard pine stem rusts management process.
Appendix 1 of this guide lists high rust-hazard ecosystems. This chart should be consulted as a first step in evaluating the risk of damage presented by stem rusts. Silviculture prescriptions require an assessment only where pine is to be a crop tree in the future stand. Stand management prescriptions for lodgepole pine in high hazard ecosystems should always include an estimate of rust incidence. This estimate may be derived from a walkthrough survey.
A less reliable measure of hazard (probability of pest occurrence) is detailed by biogeoclimatic subzone and variant in the provincial forest health charts provided in Appendix 6 of the Establishment to Free Growing Guidebook.
Evidence of stem rusts is uncommon in mature stands. Therefore it may prove difficult to determine future stand risk from an assessment of rust incidence during the prescription walkthrough. A more reliable method involves examination of young stands within a 1- to 2-km radius for rust incidence. However, since collecting this information will add to the cost of a silviculture prescription, and may not prove warranted, it is recommended that local knowledge or silviculture survey records be used where possible.
In most cases, collection of rust data during stocking surveys prior to spacing for maximum density should allow for treatment adjustments using the information in the section "Minimal treatment levels."
Stand management prescription
A walkthrough should be done before any stand management activity is conducted. Walkthroughs should be scheduled for late spring and early summer when cankers are most visible. If rust incidence is below the minimum threshold, no further evaluation is necessary and treatment may proceed without alteration (a useful contract clause would be to remove all lethally infected stems). If rust incidence is obviously above the maximum threshold level, no further assessment is necessary and spacing or pruning should be delayed. If the incidence level is between these two thresholds, or if the surveyor is uncertain, a more detailed survey should be scheduled.
Stem rust management thresholds for pre-stand tending activities are detailed in the section "Minimal treatment levels."
Maintaining post-treatment stocking at higher than optimum levels during stand management to compensate for rust mortality must be evaluated when rust incidence levels fall between minimum and maximum thresholds. The loss in growth incurred by higher stocking levels must be carefully weighed against the insurance provided by leaving additional stems.
Assessment and stratification procedures are more fully explained in the next section.
The procedure is simple. It involves inspecting all pine in each silviculture or inventory plot and recording all unacceptable damage, by damage agent code. The data are then summarized by plot and by disease strata indicating the incidence of each rust on the host species. Based on this incidence and other factors, such as susceptible species composition, management recommendations should be made in accordance with the thresholds and strategies detailed in the next section of this guide.
Where possible, stratify areas into distinct disease treatment strata based on rust incidence. This finer resolution will allow appropriate management tactics to be employed only where required.
If rust is a possible concern, conduct pre-stand tending surveys in late spring when rust cankers are highly visible. This will help ensure good survey results.
The rationale behind the recommendations is that treatment for stem rusts should focus on stand density management. Since rusts behave as natural thinning agents, they will continue to thin a stand after a spacing treatment. An environment that disfavors rusts and compensates for serious stem loss should be built into the stand management prescription. In general, this environment is present in dense naturally regenerated stands that have rapid self-pruning of lower live limbs. A high stand density provides a buffer against rust-caused tree mortality.
Addressing such mortality is not believed to be critical until the losses due to rust are expected to reach 20 per cent or more. A general rule of thumb is that estimated losses from rusts will be twice the current observed incidence (current observed incidence does not include past mortality).
Due to a variety of biological and logistic factors, stand tending treatments usually do not alter the current incidence of rust in a stand. For example, if 15 per cent rust incidence is detected in a pre-stand tending survey for a pre-free growing stand, then 15 per cent of the residual stems will remain infected (they were missed during spacing) with a further 15 per cent of the stand to be lost to new infections that occur following spacing. In this example, post-treatment stocking density should be 30 per cent higher than target stocking. This would help compensate for losses to both current and future infection.
As indicated in Table 3, stands between age 15 and 25 require fewer additional stems to compensate for future mortality. This is due to several factors. One is that the period of highest susceptibility to rusts mainly occurs before age 15. As stands age, new infections decrease and existing lower branch infections that have not reached the bole die from shade pruning. Also, as the live crown lifts it becomes easier to spot cankers. Therefore, for older stands, higher rust incidence thresholds are acceptable when considering stand management activities. Removing infected trees can improve stand productivity by eliminating trees that are occupying valuable growing space.
Table 3. Disease incidence and treatment levels by activity
There are no restrictions on stand management regimes. In all cases eradicate infected stems during spacing operations in order to minimize impact of early mortality.
Alternate disease treatment level
Review this option carefully. This treatment prescribes post-treatment stocking densities that will compensate for losses due to infections that remain after spacing and for new infections. The importance of this option increases with higher incidence levels. Once incidence reaches 20 per cent, the importance of leaving extra stems is paramount. Significant mortality can be expected at this level, often leading to stands becoming not satisfactorily restocked (NSR) shortly after spacing if target stocking levels are not adjusted.
This treatment regime takes into account that most spacing is done prior to the time when the risk of infection decreases. New infections will occur after spacing for some time. Opening up the lower crown to sunlight slows natural pruning of the lower, often rust infected, branches.
Timing silviculture operations to coincide with the peak rust sporulation period allows for dramatically improved detection and subsequent removal of infected trees. This period occurs from mid-May to mid-June and varies by latitude. Optimal timing provides better disease control and results in lower additional stocking requirements.
Intensive disease treatment level
Delay activities for 3 to 5 years in order to allow natural thinning to take its course and for disease levels to stabilize. Resurvey after this period to determine infection levels and whether or not spacing is still required. If the infection level falls into a lower treatment level, proceed with spacing under those recommendations listed above. If disease levels are still at the intensive disease level, delay spacing for another 3 to 5 years. If after this time spacing is no longer feasible, cancel treatment.
Another option to consider is double entry spacing. The primary objective of the first entry is to remove infected stems and to thin to an intermediate density (perhaps 1000–2000 stems above target). The intention of the second entry (perhaps combined with pruning), in 3 to 5 years, is to allow rust mortality to continue without subjecting the stand to severe losses. The second entry may not be necessary given a careful evaluation of stand stocking after the waiting period. The determining criterion for whether such a prescription has merit is whether sufficient well-spaced, uninfected trees will remain after the two thinnings. This will probably not be achieved in most heavily infected stands.
This approach involves prescribing a mix of ecologically suitable host and non-host species for regenerating the stand. This would limit the use of Pl and Py as acceptable crop trees, if feasible, especially in high hazard ecosytems. As always, the species or species mix that best fulfils the management objectives for the site should be prescribed.
This approach involves favoring non-host species during spacing to maintain future stand stocking levels (any non-pine species would be preferred over Pl or Py). As always, the species or species mix that best fulfils the management objectives for the site should be prescribed.
This option involves the eradication of all lethally infected stems during treatment. While all stems with lethal infections should be cut, consideration may be given to retaining stems with non-lethal infections if they are required to maintain stand stocking. Stand density management is the primary purpose of this treatment. This entails removing trees that will likely die prior to harvest allowing the residual trees to grow with less competition.
It is recognized that even with well-trained crews and optimal conditions, many infections will be missed. As a result, efforts to "sanitize" will never be 100 per cent successful. The goal of rust management is to reduce losses, not to sanitize.
Increase target stocking
This option involves increasing the prescribed target stocking of the post-treatment stand in order to compensate for future mortality. The adjusted prescribed target stocking level is determined by the rust incidence within the strata (see Table 3).
Definition: a non-lethal canker is one located more than 15 cm from the stem and, therefore, prunable. A lethal canker is one located at or within 15 cm of the stem (not prunable) or one within 60 cm of the stem if no pruning is planned.