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Stanislaw Pluta and Agata Broniarek-Niemiec

Field resistance to white pine blister rust (WPBR) (Cronartium ribicola J. C. Fischer) was investigated on 53 black currant (Ribes nigrum L.) genotypes (cultivars and breeding selections) in 1998 and 1999. Uredia did not form on the black currant `Titania' and 17 advanced selections during field evaluations made at the Experimental Orchard at Dabrowice, near Skierniewice, Poland.

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M.E. Ostry

White pine blister rust (Cronartium ribicola J.C. Fisch.) (WPBR) was discovered on Ribes L. in New York in 1906, although it was accidentally introduced from Europe on pine (Pinus L.) seedlings. The spread of this destructive fungus has changed the forests in North America. After decades of reduced planting because of the concern over the impact of WPBR, white pine (Pinus strobus L.) is now being restored in the lake states of Minnesota, Wisconsin and Michigan. Although the potential for growing white pine is high on many sites, the disappearance of a seed source because of logging and fires means that reestablishment of white pine to these areas will require active management. A series of plantings have been established on three national forests in Minnesota and Michigan to evaluate various silvicultural treatments intended to minimize the incidence of WPBR and to compare the performance of seedlings selected for disease resistance to nonselected planting stock.

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Bohun B. Kinloch Jr.

After a century since introduction to North America from Europe, white pine blister rust, caused by Cronartium ribicola J.C. Fisch., is recognized as one of the catastrophic plant disease epidemics in history. It has not yet stabilized and continues to spread and intensify. Its nine native white pine hosts comprise major timber producers, important watershed protectors, keystone ecological species, and the oldest trees on earth. All are highly susceptible and some have been damaged severely in parts of their native range, as well as where they have been planted as exotics. Resistance, the most promising approach to control, requires understanding of genetic interactions between hosts and pathogen, a quest that has been ongoing for half a century. Unlike other hosts of spectacular exotic diseases, such as chestnut blight [caused by Cryphonectria parasitica (Murrill) M.E. Barr] and dutch elm disease [caused by Ophiostoma ulmi (Buisman) Nannf.], white pines (Pinus L.) exhibit a surprising number of resistance mechanisms to blister rust, if at only low frequencies. There are three main kinds:

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Paul J. Zambino

Artificially inoculated single-leaf cuttings and small plants consistently differentiated european black currant (Ribes nigrum L.) cultivars susceptible to white pine blister rust (WPBR; Cronartium ribicola J.C. Fisch.) from immune cultivars carrying the Cr resistance gene. Black currant cultivars Consort, Crusader, and Titania showed no signs of infection with any of 21 strains of WPBR, suggesting that strains able to overcome immunity conferred by the Cr resistance gene, if they exist, are uncommon in North America. However, in red currant (Ribes rubrum L.), two sources of material presumed to represent the immune cultivar Viking showed no resistance to infection. All rust strains infected and sporulated as if the cultivar were fully susceptible, casting doubt on the true identity of available sources of `Viking'.

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Ed Mashburn

In North America for many years the commonly held solution to white pine blister rust (Cronartium ribicola J.C. Fischer) (WPBR) was to eradicate all currants and gooseberries (Ribes L.). That approach was tried to no avail. Can currants and gooseberries be successfully grown in North America? You bet they can! Vast areas of the United States and Canada are ideal for Ribes production. Black currants (Ribes nigrum L.) are a processed fruit and production may compare to that of grain. Many of the areas that presently grow other berries could easily grow Ribes. The main barriers for production in North America are state restrictions and the availability of up-to-date information and data for growers, processors, legislators and the consuming public. I suggest that this conference and the people herein form that task group and initiate the cooperative dialogue and set forth a process to approach the WPBR problem in a holistic manner.

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D.R. Bergdahl and H.B. Teillon

White pine blister rust (WPBR) (Cronartium ribicola J. C. Fischer) has been present in Vermont and other northeastern states since the early 1900s. The fungus is commonly observed on currants and gooseberries (Ribes L.) every year, but incidence varies on eastern white pine (Pinus strobus L.). Our general impression has been that Vermont has had a relatively low level of infection on eastern white pines; however, we recently found rust incidence in Christmas tree plantings in northern Vermont to range from 10 to 42% (average 20%) based on 721 trees surveyed. Also, in pole-sized stands in southern Vermont, incidence ranged from 12 to 46% (average 32%) and 76% of these trees had main stem infections. In the southern survey, 98% of wild ribes plants had varied amounts of both urediniospores and teliospores. These preliminary survey data suggest that incidence of WPBR may be more significant than previously thought and therefore, additional survey work is needed. We screened cultivars of Ribes for susceptibility to WPBR. Eighteen cultivars were inoculated in the field with a mass collection of aeciospores of C. ribicola. The percentages of leaf area infected ranged from 0 to 49 for the urediniospore stage and from 0 to 55 for teliospores. The gooseberry (Ribes uva-crispa L.) `Welcome' had the highest percentage of leaf area with urediniospores, while black currants (R. nigrum L.) `Coronet,' `Consort,' and `Crusader' had no visible infection. Presently, Vermont has no WPBR regulations. However, previous federal laws did restrict black currant cultivation. Little is known about the genetic diversity of WPBR or its potential for change. Caution must be used when considering any cultivation of Ribes for the purpose of producing fruit because our valued white pine resources could be negatively impacted.

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Richard A. Sniezko, Andrew Bower, and Jude Danielson

Seedlings from 12 sugar pine (Pinus lambertiana Dougl.) and 13 western white pine (Pinus monticola Dougl.) families were planted at Happy Camp, Calif., in 1996. Assessment in Summer 1999 indicated moderate levels of white pine blister rust (Cronartium ribicola Fischer) infection. This paper focused on the species differences and showed that sugar pine had a higher incidence of stem infection (active and inactive cankers) and more stem infections per tree than western white pine. An unexpected result was the very high percentage of infections that were bark reactions (completely inactivated infections), despite the fact that only some of the families of both species were selected for this mechanism. Assessments in subsequent years will track the future performance of the two species and of the individual families.

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Kim E. Hummer

The center of diversity for white pine blister rust (WPBR) (Cronartium ribicola J.C. Fischer) most likely stretches from central Siberia east of the Ural Mountains to Asia, possibly bounded by the Himalayas to the south. The alternate hosts for WPBR, Asian five-needled pines (Pinus L.) and Ribes L. native to that region have developed WPBR resistance. Because the dispersal of C. ribicola to Europe and North America occurred within the last several hundred years, the North American five-needled white pines, Pinus subsections, Strobus and Parya, had no previous selection pressure to develop resistance. Establishment of WPBR in North American resulted when plants were transported both ways across the Atlantic Ocean. In 1705, Lord Weymouth had white pine (P. strobis L.), also called weymouth pine in Europe, seed and seedlings brought to England. These trees were planted throughout eastern Europe. In the mid-1800s, WPBR outbreaks were reported in Ribes and then in white pines in eastern Europe. The pathogen may have been brought to Europe on an infected pine from Russia. In the late 1800s American nurserymen, unaware of the European rust incidence, imported many infected white pine seedlings from France and Germany for reforestation efforts. By 1914, rust-infected white pine nursery stock was imported into Connecticut, Indiana, Massachusetts, Minnesota, New Hampshire, Ohio, Pennsylvania, Vermont, and Wisconsin, and in the Canadian provinces of Ontario, Quebec, and British Columbia. The range of WPBR is established in eastern North America and the Pacific Northwest. New infection sites in Nevada, South Dakota, New Mexico and Colorado have been observed during the 1990s.

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John A. Muir and Richard S. Hunt

Introductions of white pine blister rust (WPBR, causal fungus: Cronartium ribicola J.C. Fischer) to eastern and western North America before 1915 caused such extensive damage that western white pine (Pinus monticola D. Don) was essentially abandoned as a manageable forest tree species for over 60 years. Recent results from WPBR resistance selection and breeding programs, and from field trials of tree spacing, pruning and bark excision treatments have supported efforts to increase establishment and to intensively manage western white pine. Western white pine is a desirable component in many forested areas because of its faster growth and much higher value compared to many other associated tree species. It also has a low susceptibility to armillaria root disease caused by Armillaria ostoyae (Romagnesi) Herink and laminated root rot, caused by Phellinus weirii (Murr.) Gilb. Some regulations, e.g., Forest Practices Code of British Columbia (BC) Act, require anyone who harvests timber on provincial forestland and uses western white pine for reforestation to either plant genetically resistant western white pine stock or prune susceptible young trees for protection. Risks of increased WPBR associated with increased commercial cultivation of gooseberries and currants (Ribes L.) have yet to be determined. However, major threats appear to include 1) increase in local amounts of spores for nearby infection of pines; and 2) possible introductions or development of new, virulent races of C. ribicola, particularly from eastern to Pacific northwestern North America. In view of these possible threats, we recommend that existing regulations and legislation should be amended, or possibly new measures enacted, to permit propagation and commercial cultivation only of varieties of Ribes that are immune or highly resistant to WPBR.

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Todd A. Burnes, Robert A. Blanchette, Jason A. Smith, and James J. Luby

can serve as alternate hosts for Cronartium ribicola , the causal agent of white pine blister rust (WPBR). This disease was introduced into North America over 100 years ago and has caused major mortality to native five-needle pines. Once the Ribes