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
Todd A. Burnes, Robert A. Blanchette, Jason A. Smith, and James J. Luby
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.
Kim E. Hummer and Stan Pluta
In the late 1800s a European disease called white pine blister rust, Cronartia ribicola Fisher, was introduced into the United States. By 1937 this disease had naturalized and was firmly established in native Ribes across the country. White pine blister rust causes economic damage to white pines and infects leaves of some Ribes late in the summer after harvest. Ribes serve as obligate alternate hosts for this disease. Our objective was to determine which Ribes species were susceptible to white pine blister rust under field conditions in Corvallis, Ore., where inoculum is naturally present. In 1995 and 1996, 57 Ribes taxa from North and South America, Europe, and Asia, were evaluated in mid-August and mid-September for presence of white pine blister rust. Susceptibility was determined by the rust infection of the abaxial leaf surfaces. Rust infection was rated on a scale from 1, no infection observed, to 9, severe infection covering almost the entire surface of at least three or more leaves. Data from 1995 indicated that 22 Ribes taxa were susceptible to white pine blister rust, while 35 others had no infection. The 1996 data will be reported. Species without infection may offer resistance genes to breeders who wish to develop rust-resistant commercial fruit cultivars.
Recent interest in expanding commercial currant and gooseberry (Ribes L.) plantings in the United States has put pressure on the states with Ribes restrictions to review their regulations. A meeting on 9 January 1998 initiated discussion between the state agriculture regulatory agencies, forest pathologists, and horticulturists. Since then a white pine blister rust (WPBR), Cronartium ribicola J.C. Fischer) World Wide Web (Web) site (McKay, 1998) and list serve have been activated to facilitate communication. Vermont is a state that has no regulations on the books at this time. Connecticut and New York also have mentioned that infection rates are low. Maine retains a Ribes reduction program, and Massachusetts is strictly enforcing their regulations. The following summarizes the general consensus among the majority of regulating states: 1) It is desirable to find a way for both white pines (Pinus L.) and commercial Ribes plantings to coexist. 2) More research is needed to survey existing Ribes and pines, the potential impact of commercial plantings versus the impact of existing Ribes, and the potential impact of escape /volunteer seedlings from immune Ribes cultivars. 3) There is interest in permitting immune Ribes cultivars to be planted. 4) There is interest in having consistency in regulations from state to state.
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.
Geral I. McDonald
Frequency of infection, main effects and interactions among four geographic sources of white pine blister rust (WPBR) (Cronartium ribicola J.C. Fisch.) and 10 Ribes-sites (Ribes L. specie × site combinations) was investigated using leaf disk assay. Two clones, one of R. hudsonianum (Dougl.) Jancz. and one of R. viscosissimum Prush., were not infected by any sources of WPBR. One clone of R. viscosissimum that was not infected by two sources of WPBR was susceptible to the other sources. Highly significant WPBR sourc × Ribes-site analysis of variance interaction for incubation period and infection efficiency precluded testing main effects. Profile plots of incubation period interaction means showed orderly interaction by all WPBR sources and plots of infection efficiency showed that aggressively virulent WPBR from Oregon (Champion Mine) ranked near or at the bottom for infection efficiency for all Ribes-sites. Meanwhile, aggressively virulent WPBR from Idaho (Merry Creek) ranked near the bottom for infection efficiency when inoculated onto Ribes obtained from the Cascade Mountains but switched to the highest ranking when inoculated onto Ribes obtained from Idaho. Geographic interaction of white pine blister rust and Ribes for incubation period and infection efficiency may help to explain geographic patterns recently observed in WPBR molecular markers.
R.S. Hunt and G.D. Jensen
For the white pine blister rust disease (WPBR), reports conflict concerning the time of year the pathogen, Cronartium ribicola J.C. Fisch., infects western white pine (Pinus monticola D. Don) and what needle age increments are most susceptible. To determine timing of infection, western white pine seedling were placed under infected currants (Ribes nigrum L.) for 1-week periods from May to November. Needles became spotted and stems cankered after exposure to diseased currants from early summer until leaf drop in November. To determine what foliage age increment was most susceptible, 5-year-old seedlings were placed in a disease garden, and older trees were inoculated in situ. All age increments of pine foliage were susceptible to infection. For young seedlings, all age increments were about equally susceptible, but on some older seedlings and trees, the current year's foliage appeared more resistant than older foliage.
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'.
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.
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.