White Pine Blister Rust

Natural Area Pests: Introduction

The fungal pathogen, Cronartium ribicola, is the causal agent of white pine blister rust (WPBR). While its origin is still debated, many researchers suspect that it is from central Asia. The pathogen was introduced from Europe into British Columbia around 1910. By the 1930s, the disease had entered northern California and reached the southern Sierra’s by the 1960s. WPBR is considered one of the most destructive forest pathogens of five-needle pines, causing lethal infections in both young and mature trees. Since its introduction in North America, it has had significant economic impacts on the timber industry and has negatively affected sensitive ecological systems.

Identification and Biology

Rust fungi, like the one responsible for WPBR, are basidiomycetes that have complex life cycles. They are considered obligate plant pathogens, meaning they cannot survive without a living host, and they often require two different host species to complete their lifecycles. Rust fungi that require two hosts have an aecial host, where the fungus produces aeciospores, and a telial host, where it produces teliospores. Aeciospores are dispersal spores and can travel far distances, while teliospores can better survive harsh environmental conditions.

In the case of WPBR, the primary aecial hosts are five-needle white pines. Although much of the monitoring and research in California has focused on sugar pines (Pinus lambertiana) and whitebark pine (P. albicaulis), the fungus can infect all six of the native five-needle white pine species in California: sugar pine (P. lambertiana), limber pine, (P. flexilis) foxtail pine (P. balfouriana), whitebark pine (P. albicaulis), western white pine (P. monticola), and the bristlecone pine (P. longaeva). Spores infect white pines through the needle stoma, the natural openings of the pine needles. The first visible symptoms of infection are yellow needle spots, which often go undetected.

As the fungus grows into the plant tissue, sections of the branch begin to die, visible by branch flagging, which are often the first noticeable symptoms on pine hosts. Cankers that have moved into the bole (trunk) of the tree may have a swollen, elongated appearance. Insects or rodents are often attracted to the cankers due to the accessible high concentrations of nutrients. After 1 to 4 years of growth on white pines, the canker will produce fruiting bodies with aeciospores (bright orange spores) in spring to early summer. These aeciospores can travel long distances, and some estimates suggest that they can travel on wind currents for hundreds of miles to infect the alternate hosts.

The most common alternate hosts, or telial hosts, include the genus Ribes, including gooseberries, currants, and the wildflower genera, Castilleja and Pedicularis. On the underside of Ribes leaves, yellow-brown rust pustules are formed (uredinia), which produce urediniospores that can continue to infect other Ribes leaves. Red-orange telia columns then produce thin-walled basidiospores, and signs of these fruiting bodies can be found on the alternate hosts throughout summer and fall. Basidiospores travel short distances, typically less than 1000 feet, to infect white pine hosts. Typically, the fungus takes 3 to 6 years to complete its life cycle. It thrives in cool, wet conditions and infections on pine trees often occur following a rainfall event.

Damage

Cronartium ribicola infections on the Ribes hosts may cause premature leaf drop but generally do not cause death of the alternate hosts. However, the damage is often substantial for five-needle pines. The basidiospores can infect five-needle pines of all ages and sizes but damage to young trees is often more substantial. This is in part due to the distance of initial infection to the main stem or bole, which is typically shorter in small trees. In contrast, larger trees may be able to suppress the infection in their limbs before it reaches the main stem. In some instances, however, high numbers of cankers can kill mature trees over time. Additionally, infections can make the trees more susceptible to other issues like bark beetles or other diseases.

In California, the occurrence of WPBR varies greatly from region to region, with more disease occurring in northern forests. To date, WPBR has not been found on white pines south of the Sierra Nevada, likely due to climatic conditions that are not favorable to WPBR. California stands of sugar pine, whitebark pine, and western white pine have suffered significant losses from WPBR. Timber production of western white pine and sugar pine has also been significantly impacted by WPBR disease. In the early 20th century, western white pines used to be the primary species planted in the Pacific Northwest. By the 1950’s, however, white pine timber production dropped drastically due to a combination of WPBR disease and bark beetle infestation. As of 2006, they are planted in roughly 5% of their original planting range. While there hasn’t been a comprehensive economic impact study, WPBR has significantly impacted the lumber industry in the United States.

Lastly, an important ecological concern is the impact WPBR is having on sensitive high elevation ecosystems, which is predicted to increase under climate change. The white pine species at high elevations, including whitebark pine, limber pine, bristlecone pine, and foxtail pine, play important ecological roles, supporting wildlife and water cycling. Loss of these pines could affect local bird and herbivore populations and the carnivores that feed on them. Without mature pines to sustain the shallow soils, these areas might topographically change as well.

Management

Historically, control focused on eradicating the alternate Ribes hosts to disrupt the life cycle of WPBR. This strategy is somewhat effective at protecting young or high valued trees but is not a cost-effective approach for preventing WPBR spread across larger areas. This is in part due to the difficulty of traversing mountainous terrain and the fact that Ribes roots are difficult to remove completely. Many eradication programs were discontinued in the 1960s, and the focus of management has since shifted to breeding WPBR-resistance in tree populations.

Prevention

To help reduce the likelihood of WPBR infections in pine plantings, consider site location and the local plant community, including alternate host species. If planting white pine species, choose areas with preferable micro-climates, including areas with lower relative humidity (e.g., farther from streams, moist meadows, and other water bodies). In areas that are more suitable for WPBR spread, like areas with higher humidity, consider planting non-host pines species. Other California native pines that are not hosts for WPBR include lodgepole pine (Pinus contorta), ponderosa pine (P. ponderosa), Jeffrey pine (P. jeffreyi), gray pine (P. sabiniana), and others in Pinus subgenus Pinus.

As with many plant diseases, appropriate horticultural care can help reduce disease incidence. For nursery stock, it may be helpful to avoid watering the needles to prevent WPBR infection.

Resistance

Some genetic resistance has been found in natural populations of white pines species, and these trees have been bred and outplanted in restoration projects in the Sierra Nevada. This is a slow process, however, as genetic variability of stands must be maintained while also introducing the genetic resistance. Major gene resistance has been identified in sugar pine, western white pine, and limber pine (). However, researchers believe that partial resistance or quantitative resistance may be present in all species. Resistance is continually being investigated by United States Department of Agriculture Forest Service, Canadian Forest Service, and other research institutes.

More options are available for resistant Ribes species, in part due to currant and gooseberry production. While cultivation of these crops is uncommon in California, there are some cultivars of Ribes species that are resistant to WPBR.

Black currant (R. nigrum)—Because black currant is a highly efficient host of WPBR, cultivation of black currant was federally outlawed in the early 1900s. This ban was lifted in the 1960’s around the time the eradication efforts ended. While some states still have restrictions on cultivation of black currant, the state of California does not. Resistant black currant cultivars include Consort, Coronet, Crusader, Doch Siberyachki (Daughter of Siberia), Titania, and Willoughby.

Red/white currant (R. rubrum)—Red and white cultivars are the same species and only differ in fruit color and taste. R. rubrum is less susceptible to WPBR than black currants, but they can still become infected. Red currant varieties that are resistant include Rondom, Rolan, Red Lake, and Viking. Resistant white currant varieties include White Imperial and 1301.

Gooseberry (R. uva-crispa x R. hirtellum)—Most gooseberries cultivated in the United States are hybrids of the European gooseberry (R. uva-crispa) and the native swamp gooseberry (R. hirtellum). The following cultivars are considered resistant to WPBR: Achillies, Captivator, Columbus, Downing, Jeanne, and Sabine.

There are no Ribes species native to California known to be resistant to WPBR.

It is important to note that resistance does not mean immune. Under favorable conditions and high disease pressure, many of these cultivars may still become infected.

Physical Control

Eradication

While large scale eradication efforts have been ineffective at controlling WPBR at the landscape level, removing Ribes species ideally within 1,000 feet of high valued white pines may help reduce infections of isolated trees. Ribes species, however, are difficult to eradicate because they can resprout from their roots and their seeds can survive for long periods in the soil. Mechanical disturbance and fire disturbance can also encourage Ribes growth, which may increase white pine infection rates in disturbance areas.

Pruning

Infections on white pines begin in the needles and small branches and spread toward the main trunk of the tree. If trees exhibit a small canker infection in the outer branch tissue, it is possible to control the infection by removing the infected branches, which can prolong the life of the tree. Pruning is not 100% effective, however, because WPBR infection often extends past the visible canker and may already be in the bole of the tree. Once the infection has moved into the main stem it is often too late to save the tree, unless the infection is located at the top of a large pine. In such cases, a large pine may be topped, removing the infection similar to pruning branches.

Chemical Control

Fungicides are not recommended in the treatment of WPBR on pine trees and have not been found to be effective in managing this disease due in part to the complex, multi-year life cycle of WPBR.

Generally, fungicide application is not advised for treatment of WPBR in Ribes; however, for Ribes species near high valued white pines, fungicide spray may be considered. In such cases, myclobutanil (Rally 40WP) may be used before bloom, at bloom, and two weeks post bloom to effectively control WPBR on currants and gooseberries. Always follow label instructions.

A White Pine Blister Rust ID Guide is available online.

References

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Dudney, J, Willing, CE, Das, AJ, Latimer, AM, Nesmith, JCB, Battles, JJ. 2021. Nonlinear shifts in infectious rust disease due to climate change. Nature Communications 12:5102.

Frank, KL, Geils, BW, Kalkstein, LS, Jr. Thistle, HW. 2008. Synoptic climatology of the long-distance dispersal of white pine blister rust II. Combination of surface and upper level conditions. International Journal of Biometeorology, 52(7):653–66. [Accessed February 4, 2026]

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Kimmey JW, and Willis WW. Spread of White Pine Blister Rust from Ribes to Sugar Pine in California and Oregon. U.S. Department of Agriculture, Forest Service Technical Bulletin 1251.

Koetter, R, and Grabowski, M. 2019. White Pine Blister Rust. University of Minnesota Extension. Accessed 23 Sept. 2025. [Accessed February 4, 2026]

Luffeman, M. 2000. Canadian breeding program for white pine blister resistance in black currants. HortTechnology, 10(3), 555–556. [Accessed February 4, 2026]

Maloney PE. 2011. Incidence and distribution of white pine blister rust in the high-elevation forests of California. Forest Pathology, 41(4): 308–316

Munck, IA, Tanguay, P, Weimer, J, Villani, SM, and Cox, KD. 2015. Impact of White Blister Rust on Resistant Cultivated Ribes and Neighboring Eastern White Pine in New Hampshire. Plant Disease, 99(10): 1374–82

Sniezko RA and Liu J. 2022. Genetic resistance to white pine blister rust, restoration options and potential use of biotechnology. Forest Ecology and Management, 520(2022): 120168.

Sniezko, Richard A. 2025. Gooseberry and Currant (Ribes spp.) Blister Rust. Pacific Northwest Plant Disease Handbook. [Accessed February 4, 2026]

Zambino, PJ. 2010. Biology and pathology of Ribes and their implications for management of white pine blister rust. Forest Pathology 40(3–4), 264–291. [Accessed February 4, 2026]

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