WHITE PINE BLISTER RUST

Pathogen: Cronartium ribicola Fisch.

Identification: Infected pines exhibit yellow and red needle spots, spindle-shaped swellings, cankers with roughened bark, flagging of branches and tops and outright mortality, especially of sapling and pole-sized trees. Clear, sappy ooze (pycnia) and whitish to yellow-orange spore pustules (aecia) appear on the swollen portions of infected pine stems in spring and early summer. Infected Ribes show brown and reddish brown spore pustules (uredinia and telia) on the undersides of their leaves and may exhibit premature leaf fall.

infection

cankers

needle spots

Hosts in Southwest Oregon: Western white pine, sugar pine, and whitebark pine; alternate hosts are shrubs in the genus Ribes.

blisters

Effects: White pine blister rust is a branch and stem canker disease of five-needle pines. Damage includes mortality, topkill, branch dieback, and predisposition to attack by other agents, including bark beetles. The blister rust fungus is native to Asia but was introduced to British Columbia via Europe on a shipment of seedlings in 1910. It then quickly spread throughout most of the range of five-needle pines in the West. It has substantially decreased populations of five-needle pines throughout their ranges in the Pacific states except for those in southern California.

top kill

In many areas, the only natural hosts remaining in high hazard areas are large trees that have had their tops and numerous branches killed by the fungus. Trees in this condition are frequently predisposed to infestation by mountain pine beetle (Dendroctonus ponderosae). Its is a major killer of regenerating five needle pines and makes reestablishment of wild populations of these species on high hazard sites difficult or impossible.

Ecological Role: The pathogen is exotic; it has not coevolved with its hosts. Five needle pines have been all but eliminated in some areas and their numbers seriously reduced in others. Because of the importance of five-needle pines as elements of forest diversity and structure, and their special virtues such as resistance to laminated root rot, interest is high in managing white pine blister rust.

Life History: The life cycle of the fungus is complex and involves five different spore forms. It takes four to five years to complete. The fungus is an obligate parasite (its host must be alive for it to remain alive) and requires an alternate host in the genus Ribes, the gooseberries and currants. Yellow-orange pustules or blisters on pines produce aeciospores annually in the late spring and summer (depending upon elevation). These thick-walled, long living spores are fairly resistant to radiation and desiccation; thus they are capable of long distance dispersal by wind. Aeciospores land on and infect the leaves of Ribes. Ribes leaves support annual infections that produce urediniospores in midsummer. Urediniospores infect other Ribes leaves and build up the population of the rust in local areas.

infected Ribes

Basidiospores are produced on infected Ribes leaves in late summer and early fall and are disseminated via wind to pine needles where they land and germinate. These thin-walled, short-lived spores require cool temperatures and moist conditions for survival and germination; wet weather or low-lying clouds and fog in late summer and early fall increase the chance of successful movement from Ribes back to pine hosts.

Conducive Habitats: The distribution, frequency, and association of the hosts, as well as micro and macroclimatic conditions are important factors in the spread of the rust. Dense populations of Ribes increase the probability of having high production of basidiospores and infection of local pine populations. Certain Ribes species are more susceptible to infection than others. Large scale weather events that bring moist conditions to a region influence levels of infection and long distance travel of fragile spores. Topographic features combined with microclimate, such as saddles or mountain tops where late summer fog is frequent, locally influence infection.

Management Strategies: Currently, planting of five-needle pines with various levels of resistance to C. ribicola is accelerating on appropriate sites. Tree improvement programs to screen apparently resistant pines and to breed for higher levels of resistance have existed for some time. Because of C. ribicola's ability to mutate, special care is taken to maintain a variety of resistance mechanisms in the breeding programs. Other aspects of blister rust management include use of risk rating systems to match levels of resistance in planting stock to local site conditions and pruning projects aimed at removing the lower branches of planted five-needle pines. The aim of pruning is to prevent stem infections and alter the microclimates in plantations, making conditions less favorable for the pathogen.

Links:

Breeding resistance to white pine blister rust
- http://www.fs.fed.us/r6/dorena

Photos:

References:

Hansen, E.M., and Lewis, K.J.(editors). 1997.
Compendium of conifer diseases. APS Press. 101 p.

Kinloch, B.B., Jr., Marosy, M., and Huddleston, M.E. (editors). 1996.
Sugar pine: status, values, and roles in ecosystems: Proceedings of a Symposium presented by the California Sugar Pine Management Committee. University of California, Division of Agriculture and Natural Resources, Davis, California. Publication 3362. 225 p.

Scharpf, R. F., tech coord. 1993.
Diseases of Pacific Coast Conifers. USDA For. Serv. Ag. Hndbk. 521. 199 p.