Insect and Disease Forest Health Protection and State Forestry Organizations Management Series 14.2 WEB 2013 White Pine Blister Rust John Schwandt, General Ecology and Management Holly Kearns, and James Byler Cronartium ribicola J.C.Fisch. in Rabh Forest Pathologists Forest Health Protection Conifer Hosts - All native five- This fungal disease was USDA Forest Service needled pines accidentally introduced from Alternate hosts - primarily Ribes Europe in early 1900s. Since then, Topics spp.(currants and gooseberries) it has devastated western white possibly Pedicularis spp. and pine forests in the northern and Introduction 1 Castilleja spp. central Rocky Mountains. Life Cycle 2 Identification 4 Introduction Management 6 Management of western white pine needled pines native to North Planting 7 (Pinus monticola) (WWP) has been America are susceptible (Hoff et al. Pruning 8 confounded by the introduction of 1980), but the Great Basin white pine blister rust (WPBR), Bristlecone pine (Pinus longaeva) is the Thinning 11 caused by the fungus Cronartium only pine host that has not yet been Prioritizing Stands 12 ribicola. This disease was introduced infected in its natural range (Nevada Excising Cankers 12 into the west coast of North America and Utah). The disease continues to Ribes Management 13 in the early 1900s on infected eastern spread and intensify throughout the WPBR Hazard 13 white pine seedlings grown in France range of its hosts and poses a threat (Hunt 2009). One documented to five-needled pines in Mexico. Monitoring 14 introduction was at Point Gray near Future 16 Vancouver, B.C. (Mielke 1943) but it In the Inland Northwest, WPBR is Literature Cited 17 is possible there were other now distributed throughout nearly all Tree Improvement Efforts 20 introductions along the West Coast of the ranges of all five-needled pine (Hunt 2009). All nine of the five- hosts in Idaho, Montana, and Rust Status Form 24 Pruning Priority Guide 25 Figure 1. Decline in western white pine forest type on Idaho Panhandle National Forests in Northern Idaho from 1930 to 1980. Key Points WPBR is caused by an introduced fungus with a complex life cycle WPBR attacks 5- needled pines of all sizes and ages Impacts can be reduced by a combination of treatments Periodic monitoring is critical. 14.2 White Pine Blister Rust Management Pa ge 2 western Wyoming. This includes million acres of western white pine whitebark pine (Pinus albicaulis) and cover type in the Inland Northwest Historic Facts limber pine (Pinus flexilis) as well as still carries a major component of western white pine. Western white western white pine (Fins et al. From 1925 to 1934 pine played a critical role in 2001). This loss of western white the average annual stabilizing ecological processes and pine has resulted in a major shift in harvest of western was also the most valuable timber historic forest succession pathways white pine from the species, dominating the timber (Fig. 1) (Samman et al. 2003). Inland Northwest was industry between 1900 and 1965 Western white pine has largely been 430 million board feet; (Fins et al. 2001). Unfortunately, replaced by Douglas-fir (Pseudotsuga enough to build western white pine proved to be menziesii) and grand fir (Abies 43,000 homes per highly susceptible to WPBR with grandis), which are both much more mortality rates of 90 percent or susceptible to a variety of insects year. more in what were once vigorous, and diseases. well-stocked stands. Today, only 5 Five million acres of to 10 percent of the original 5 the Inland Northwest were once considered Life Cycle white pine cover type. Cronartium ribicola is an obligate par- wort (Pedicularis spp.) and Indian asite, which means it can only sur- paintbrush (Castilleja spp.) can also vive on living host tissue. It re- be infected, but their importance in quires two different hosts to com- spread and intensification of WPBR plete its complex life cycle (Fig. 2). has yet to be determined One host is a five-needled pine and (McDonald et al. 2006). the other is a shrub or herbaceous On white pine hosts, infections host referred to as the alternate must occur through the needles by host. Currants and gooseberries Rust Biology spores that come from an alternate WPBR requires 2 (Ribes spp.) are the primary alter- host usually late in the fall during hosts to complete nate hosts. Some species of louse- its life cycle Figure 2. Life cycle of white pine blister rust WPBR does not go from pine to pine WPBR cankers may be active even if: Not sporulating every year The branch has no foliage (and the canker is near the bole) Trees with severe stem cankers may still have good growth for many years 14.2 White Pine Blister Rust Management Pa ge 3 periods of high relative humidity. many miles to infect leaves of the Over 50% of the infections occur alternate hosts (Buchanan and on one-year old foliage, 25-30% on Kimmey 1938). 2-year old foliage, 10-15% on cur- rent year foliage and 5-10% on 3- year old foliage (Lachmund 1933). The following spring, small yellow needle spots form. The rust fungus grows through the branches about 2-inches per year toward the bole, killing tissue as it advances (Schwandt et al. 2013). Once it reaches the main stem, the fungus creates stem cankers that eventually Figure 4. Watery droplets of girdle the stem and cause top kill spermatia on WPBR canker. (Fig. 3) or mortality. All sizes of Figure 5. Characteristic blisters of aecia trees are attacked and small regen- erupting through the bark of a WPBR canker. The fungus builds up on the alter- nate host during the summer by producing urediniospores, which re- infect the same or adjacent alternate hosts. Usually in late summer the fungus produces teliospores on small hair-like projections on the underside of leaves of the alternate hosts (Fig. 6). During periods of high relative humidity, teliospores Figure 6. Hair-like telia on the produce thin-walled, delicate basidi- underside of the alternate host. ospores, which usually travel very short distances (a few hundred yards) to infect needles of white Figure 3. Top kill by multiple WPBR cankers. pines (Zambino 2010). On the al- ternate hosts, damage from WPBR eration can be killed rapidly. Gen- is usually confined to small leaf erally, the larger the tree is at the spots (Fig. 7), but premature defoli- time it becomes infected, the long- ation may occur if they are severely er it survives after infection. infected. About once every ten During the late summer, spermatia years conditions are highly favora- (pycniospores) are produced on the ble and a “wave year” of infections pine hosts near canker margins in occurs. It is unknown if this is due watery droplets (Fig. 4) which in- to: an early moist spring that would sects then carry from canker to favor early aecia production; and/or canker resulting in fertilization of a cool moist summer that would the rust fungus. The following favor spread and intensification on spring, cankers on white pine may the alternate hosts; and/or a pro- Figure 7. Small leaf spots on Ribes produce characteristic yellow- longed warm, moist fall that would caused by WPBR infections. orange blister-like aecia that erupt favor basidiospore production and through the bark (Fig. 5). Aecio- spread before frost causes Ribes to spores from these blisters can travel shed their leaves. 14.2 White Pine Blister Rust Management Pa ge 4 Symptoms Identification Dead branch “flags” There are several signs and symp- to recognize than others. It is also toms associated with WPBR can- important to look at many trees in a Swollen or kers that can help in making an ac- stand, since it is highly unlikely to discolored areas curate diagnosis. The most obvious find only a single infected tree in a Prolific pitching sign is the presence of aecial blisters stand. erupting through the bark near the Rodent chewing canker margin (Fig. 5). These blis- Once the fungus girdles the branch Roughened, ters and their papery remnants that or stem, it effectively kills the tis- cracked bark may remain most of the summer are sues beyond the canker resulting in a positive diagnosis that a tree is in- distinctive “flags” (Fig. 8) or top- Insect boring fected with WPBR. Unfortunately, activity aecia are not produced until at least 3-5 years after foliage infection, and may not be produced every year. A study that monitored nearly 200 individual cankers annually for 10 years, found individual cankers only sporulated an average of 1.2 times even though cankers were continu- ing to expand every year (Schwandt et al. 2013). During this period the total number of sporulating cankers per year varied from less than 10 to over 700 on 48 monitored trees. Figure 8. Distinctive branch flag caused by Another positive sign is the pres- WPBR. ence of the watery spermatial drop- lets that may form during the sum- mer near the canker margin (Fig. 4). kill (Fig. 3). Most bole cankers are Figure 9. Typical diamond shaped bole canker after light These are much less obvious than diamond shaped with a distinct yel- scrubbing with water. aecia, but if present are indicative of low/orange margin that is best seen WPBR infection. Production of when moistened or lightly scrubbed spermatia is also sporadic, which with water (Figs. 9 and 10). Large may help explain the inconsistent stem cankers often produce copi- production of aecia since it is ous amounts of pitch (Fig. 11) that thought that aecia are only pro- tend to “glue” bark to older dead duced on cankers that produce trees that have sloughed off much spermatia the prior year (Baxter of their bark.