Pine blister rust resistance screening in Ribes germplasm.

A preliminary study at the National Clonal Germplasm Repository in Corvallis, Oregon, determined that 110 field-grown Ribes L. genotypes were resistant to natural infection from white pine blister rust (Cronartium ribicola C. J. Fischer). Uredinia did not form under conditions of natural infection on these black currant (R. nigrum L.), red currant (R. rubrum L., R. sativum L.) or gooseberry (R. uva-crispa L., R. oxyacanthoides L.) genotypes for 3 to 5 years. The objective of this study was to determine if uredia would develop on these resistant genotypes after artificial inoculation. Ribes nigrum cv. Ben Alder was chosen as a susceptible control. In August 2000, uredinial spores were gathered from infected black currants leaves in the Corvallis field planting. An agar-water suspension (30,000 spores ml) was prepared and applied to leaves of four branches of intact plants for each clone. Three weeks after inoculation, 68 of the tested clones developed infection. The abaxial leaf surfaces of the blackcurrants ‘Ben Alder’ and ‘Pilot Alexandr Mamkin’ and R. × nidigrolaria Bauer cv. Jostiki, were covered with uredinia. Fewer uredinia developed on R. × nidigrolaria Bauer cv. Josta, 26 black, 11 red, 5 white currants and 12 gooseberries. No uredinia developed on 42 clones, including 12 black, 5 red, 1 white currant, or 24 gooseberries. Blackcurrant cultivars with the Cr gene for white pine blister rust immunity, i.e., Consort, Coronet, Crusader, or Titania, remained uninfected. The broad range of species and geographical origins of the highly resistant, uninfected clones suggest that several additional genetic mechanisms for rust resistance may exist in Ribes. Further studies will seek to identify additional rust-immune genotypes. INTRODUCTION White pine blister rust (Cronartium ribicola J. C. Fischer) is an Asian disease which was introduced through Europe into the United States at the end of the nineteenth century (Hummer, 2000). Cronartium ribicola is expanding its range in the United States and requires both Ribes (currants and gooseberries) and five-needled pines (Pinus L. section strobus) to complete its life cycle. This disease can cause early defoliation of Ribes. In Corvallis, Oregon, leaves of susceptible cultivated Ribes become infected with uredia in July, August or September. A number of states have regulations restricting Ribes cultivation because of the severe damage or death of pines caused by this rust (McKay, 2000). Agencies that prepare regulations for these states and growers who wish to produce Ribes are very interested in cultivars that are resistant or immune to rust. Most cultivars of blackcurrants (R. nigrum L.) are susceptible (Brennan, 1996). Ribes ussuriense Jancz. , a blackcurrant native to the Primorskij region in the Russian far east, carries a dominant gene, Cr, for rust immunity (Brennan, 1996). A. W. Hunter, a Canadian breeder, crossed this species with R. nigrum in the 1940s to obtain immune cultivars (Bergdahl and Teillon, 2000; Hunter, 1950; Hunter, 1955). Cultivated red currants can be susceptible or resistant. Ribes sativum L. is rust-susceptible; R. petraeum Wulf. and R. rubrum L. are rust-resistant (Bergdahl and Teillon, 2000; Hahn, 1943). Gooseberry (R. uva-crispa L., R. hirtellum) and jostaberries (R. × nidigrolaria Bauer) Proc. 8th IS on Rubus and Ribes Eds. R.M. Brennan et al. Acta Hort. 585, ISHS 2002 288 tend to be resistant (Bergdahl and Teillon, 2000; Darrow, 1937). The Corvallis Repository has been evaluating rust incidence after natural infection on about 300 genotypes of Ribes since 1995. More than 100 cultivars of blackcurrants, red and white currants, gooseberries and jostaberries (R. × nidigrolaria Bauer) were not observed to host the rust, i.e., did not develop uredia, during three years of field testing in Corvallis, Oregon (Hummer and. Finn, 2000). We wondered if these cultivars were rustimmune or just avoided natural infection; and whether these genotypes would produce uredia when subjected to high levels of inoculum under humidity conditions favoring rust infection. The objective of this study was to determine if these resistant genotypes would develop uredia after artificial inoculation. MATERIALS AND METHODS This study examined 110 elite Ribes genotypes which had not been observed to host rust uredia for three years in Corvallis, Oregon. These seven to eight-year-old plants were growing in the field of the National Clonal Germplasm Repository, Corvallis, Oregon. A uredinial spore suspension from severely naturally-infected leaves of susceptible blackcurrant cultivars in the Repository field, was prepared on August 22, 2000. About 26 infected leaves, each with the abaxial surface covered with rust uredia, were placed in 0.7% agar. The solution was agitated to dislodge uredinial spores and the leaves were removed. The resulting spore suspension contained 30,000 uredinial spores ml. On August 24, 2000, branches about 15 cm long, containing 5 or more fully mature, green leaves, were chosen from two plants of each genotype. The undersides of all leaves on each branch were inoculated with 2 ml of sprayed suspension from a hand mister. The branches were individually enclosed in polyethylene bags with twist-ties. The bags and ties were removed after 24 hours. On September 14, 2000, leaves were evaluated for presence or absence of uredia. On October 14, 2000, uredia were counted from four infected leaves, the two most infected leaves from each of the treated branches. RESULTS AND DISCUSSION Of the 110 rust-resistant genotypes that were artificially inoculated with uredinial spores, 68 genotypes were observed to produce at least one uredium on the four inoculated branches. The genotypes had low levels of infection with distinct, countable uredinial areas. These infections comprised a much smaller surface area than that of the susceptible cultivars. For example, almost the entire abaxial leaf surface of naturally infected or artificially inoculated ‘Ben Alder’ was covered with uredia. We suspect that the sporulation from the inoculated leaves of resistant cultivars was lower than that of naturally infected leaves of susceptible cultivars, although this measurement was not quantified. The jostaberry genotypes produced an average of 18 or less uredia per leaf. Jostaberry genotypes are noted for their resistance to natural infection (Brennan, 1996) so uredia formation on these clones was unexpected. No uredia were observed on any leaves of the inoculated branches of 42 genotypes (Table 1). These non-infected plants included 12 black, 5 red, and 1 white currant and 24 gooseberries. The blackcurrants ‘Docz Siberjoczk’ and ‘Lunnaja,’ have R. nigrum var. sibiricum W. Wolf in their pedigree. This taxon has been used in rust resistance breeding in Poland (Somorowski, 1964) and Russia (Volunez, 1966). The red currant R. rubrum cv. London Market produced no uredia in our study and is also known for rust resistance (Brennan, 1996). We conclude that these genotypes are highly resistant because no uredia were observed under natural infection (Hummer and Finn, 2000) or after artificial inoculation in the field (Table 1). Zambino (2000) demonstrated that some rust-resistant cultivars can become infected and produce uredia under controlled laboratory conditions (100% relative humidity in a growth chamber). Therefore, further artificial inoculation must be performed in the laboratory to determine if these cultivars can be considered “immune. ” The four blackcurrant cultivars with the Cr gene, ‘Crusader’, ‘Coronet’, ‘Consort’