A pawpaw (Asimina triloba) regional variety trial (PRVT) was established at the U.S. Department of Agriculture, Agricultural Research Service, National Clonal Germplasm Repository (NCGR), Corvallis, Ore., in Fall 1995. This orchard was a replicated planting of 28 commercially available varieties or advanced selections from the PawPaw Foundation (PPF; Frankfort, Ky.), with eight replicate trees of each selection grafted onto seedling rootstocks and planted in a randomized block design. Two years after planting, 32 trees had either failed to establish or had died after an initial healthy start. By July 1999, 25% of grafted trees had died due to a vascular wilt-like disease, and 2 years later mortality exceeded 50%. Grafted selections with the lowest symptom severity include 1-7-2, 2-54, 7-90, 8-58, 9-58, `Mitchell', `PA-Golden #1', `Taylor' and `Wilson'. Seedling guard trees were unaffected until July 2000, when six guard trees of 76 died and 10 more were declining. By July 2001, 14 guard trees were dead. No fungi were consistently isolated from declining trees. A number of bacteria were isolated from infected trees, but no specific pathogen has been confirmed as the causal agent. Polymerase chain reaction (PCR) tests for phytoplasmas and for the bacterium Xylella fastidiosa were also negative. Research is ongoing to determine if a bacterial pathogen was the cause of the pawpaw decline in the Oregon PRVT.
Powdery mildew (PM) occurs worldwide and is prevalent on susceptible cultivars wherever pears are grown, causing economic losses due to russeted fruit and an increased need for fungicides. A core subset of the Pyrus germplasm collection at the USDA National Clonal Germplasm Repository in Corvallis, Ore., was evaluated for resistance to Podosphaera leucotricha, the causal agent of PM, using greenhouse and field inoculations of potted trees. The core collection consists of about 200 cultivars and species selections, representing most of the genetic diversity of pears and includes 31 Asian cultivars (ASN), 122 European cultivars (EUR), 9 EUR × ASN hybrids and 46 pear species selections. Three trees of each core accession were grafted on seedling rootstocks. In 2001–02, trees were artificially inoculated in a greenhouse, grown under conditions conducive for PM, and evaluated for symptoms. The same trees were subsequently evaluated for PM symptoms from natural field infections during 2003 and 2004. In the greenhouse, 95% of EUR and 38% of ASN were infected with PM. Average PM incidence (percent of leaves infected) in the greenhouse (8% for ASN and 30% for EUR) was much higher than incidence in the field (2% for ASN and 5% for EUR) during 2003. Symptoms were also more severe in the greenhouse, with 46% of ASN and 83% of EUR with PM symptoms having a mean PM incidence of >10%. In the field, 42% and 22% of EUR and 23% and 13% of ASN were infected with P. leucotricha in 2003 and 2004, respectively. Field infection was very low during both years, with percentage leaves infected in ASN and species selections significantly different from EUR. In the field, 6% of ASN with PM symptoms had a mean PM incidence >10% during both years, while 15% and 2% of EUR accessions with PM symptoms had a mean PM incidence >10% in 2003 and 2004 respectively. These results should be very useful to pear breeding programs to develop improved PM resistant cultivars in the future, by using accessions with consistent low PM ratings.
Approximately 450 accessions representing 40 taxa from around the world, including 77 advanced breeder selections and 54 F1 hybrids are maintained at the repository in Corvallis. Most of the clones came from the collection of M. J. Murray of the A. M. Todd Company, Kalamazoo, MI. Others were received from the former USDA/ARS mint breeding program in Corvallis which ended in 1981. Data on origin, morphology, pedigree, oil analysis, fertility and chromosome number was provided with many of the accessions. We have confirmed the identity of the clones utilizing many of these features. We are actively seeking unrepresented germplasm. Chromosomes from pollen mother cells or root-tip cells are currently being counted. Nomenclature changes reflecting recent research have been made. Many clones have been indexed for viruses. Infected clones are treated with thermotherapy and meristem culture to produce virus-negative replacements. An in vitro backup collection is maintained in cold storage. Iniation of in vitro cultures has been complicated by internal bacterial contamination in some clones. Research to eliminate this problem is in progress. Cuttings and in vitro cultures of Mentha germplasm are available to researchers worldwide.
The rare plant, Fragaria multicipita Fern., was characterized by an unusual vegetative morphology that was long presumed to be suggestive of an ice front relict. While an additional species of Fragaria would be a potential source of genetic diversity for enhancing cultivated strawberry germplasm, evidence now indicates that such potential is not present in F. multicipita. Grafting of F. multicipita to F. chiloensis Duchesne resulted in transmission of a subgroup 16SrVI-B phytoplasma to, and the development of multicipital growth in, F. chiloensis. The results indicated that F. multicipita is a phytoplasma-diseased aberrant growth form of F. virginiana Duchesne and is an unfounded taxon. It is apparent that this plant population offers no unique potential for increasing genetic diversity in cultivated strawberry germplasm, but the phytoplasma may be capable of infecting commercial strawberry.
During Dec. 1997 and Jan. 1998, the gooseberry mite, Cecidophyopsis grossulariae Collinge, was observed to infest 48 currant and gooseberry (Ribes L.) cultivars in a field plantation in Corvallis, Ore. The mite was observed on 29 black currant, (Ribes nigrum L.), two red currant [Ribes rubrum L. and R. sativum (Rchbch.) Syme], 12 gooseberry [R. uvacrispa L., R. oxyacanthoides var. setosum (Lindley) Sinnot], and three R. ×nidigrolaria Bauer cultivars and the hybrid R. nigrum × R. pauciflorum Turcz. ex Pojark. A range of mite infestation levels was observed, with some cultivars not being infested, some with light infestation, having 1 to 100 adult mites per bud, and some heavily infested, with more than 100 mites per bud. On lightly infested buds, the mites were inside bud and leaf scales; in heavily infested buds, mites were also observed on floral primordia. Scales of infested buds were often loose and appeared more open than noninfested ones. Mite distribution varied by branch within a plant. Black currant cultivars with the heaviest infestation of C. grossulariae were of Scandinavian, Russian, Scottish, and Canadian origin. The Russian black currant cultivar Tunnaja was the most heavily infested with more than 1000 mites per bud. Floral primordia were damaged in heavily infested buds.
Edible european pears (Pyrus communis L. ssp. communis) are derived from wild relatives native to the Caucasus Mountain region and eastern Europe. Microsatellite markers (13 loci) were used to determine the relationships among 145 wild and cultivated individuals of P. communis maintained in the National Plant Germplasm System (NPGS). A Bayesian clustering method grouped the individual pear genotypes into 12 clusters. Pyrus communis ssp. caucasica (Fed.) Browicz, native to the Caucasus Mountains of Russia, Crimea, and Armenia, can be genetically differentiated from P. communis ssp. pyraster L. native to eastern European countries. The domesticated pears cluster closely together and are most closely related to a group of genotypes that are intermediate to the P. communis ssp. pyraster and the P. communis ssp. caucasica groups. Based on the high number of unique alleles and heterozygosity in each of the 12 clusters, we conclude that genetic diversity of wild P. communis is not fully represented at the NPGS. Additional diversity may be present in seed accessions stored in the NPGS and more pear diversity could be captured through supplementary collection trips to eastern Europe, the Caucasus Mountains, and the surrounding countries.