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  • Author or Editor: Kim E. Hummer x
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Stem and bud tissues of promocanes from more than 260 Rubus genotypes were evaluated for mid-winter cold hardiness after laboratory freezing in January 1990. T50 values were calculated for cane samples of red, yellow, black and purple raspberry, and blackberry cultivars, hybrids and species. Red raspberries exhibited the hardiest stem tissue, although several purple raspberries (Rubus sp. cvs. Brandywine, Royalty) survived as low as -33 C. Fall fruiting red raspberries, such as R. idaeus L. cvs. Zeva Remontante, Indian Summer, St. Regis, and Fallred, survived from -23 to -25 C. Summer-bearing cultivars, Canby and Puyallup, survived to -30 C. Stems of several black raspberries (R. occidentalis L. cvs. New Logan, Bristol) survived to -27 C. Stems of the hardiest blackberry cultivars, (R. sp. cvs. Black Satin, Smoothstem) survived to -22 C. In most genotypes the region of the bud at the axis of the stem was less hardy than tissues within the bud scales. Buds tissue was 2 to 10 C less hardy than stem tissue. Field plants were also visually rated for cold injury following record low temperatures occurring in 1989, 1990, and 1991.

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Open-pollinated hop seeds (Humulus lupulus var. lupuloides E. Small, H. l. var. pubescensE. Small, H.l. var. neomexicanusNelson and Cockerell, and H.l. var. lupulusL.) were collected from the United States, Canada, and Kazakhstan and screened for resistance to hop powdery mildew (Podosphaeramacularis Braun & Takamatus). A total of 2108 seedlings were repeatedly inoculated with high levels of P. macularis. Under greenhouse conditions, most seedlings exhibited a compatible disease response typical of susceptible hosts with sporulating colonies covering both leaf surfaces. Three genotypes from hop seed native to Emba, Kazakhstan, remained uninfected in greenhouse assays. Further experimentation of those genotypes in growth chambers at 18 °C revealed that they demonstrated an atypical disease response. Six days after inoculation, necrotic lesions on the adaxial leaf surfaces were visually apparent. Microscopic examination showed areas of collapsed epidermal cells, collapsed hyphae, and golden-brown discolorations extending out from the center of the infection. These symptoms contrasted those of a hypersensitive response in which fungal growth is restricted by the collapse of epidermal cells in advance of the fungus. The infection frequency of Kazakhstani genotypes was intermediate compared to H.l. cultivars Symphony (susceptible) and Nugget (resistant) after exposure to three preinoculation temperature regimes. These genotypes could represent a new source of polygenic resistance to hop powdery mildew, and could broaden hop germplasm available to global breeding programs.

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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.

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Asian germplasm has significantly contributed to berry crops in America in several ways. The American wild octoploid species [Fragaria chiloensis (L.) Mill. and F. virginiana Mill.], and subsequently, the cultivated strawberry (F. ×ananassa Duch. ex Rozier), have benefitted from Asian heritage in the evolutionary time scale. Second, breeders have combined Asian germplasm in crosses for improved fruit cultivars. Third, Asian temperate fruit species have been collected from wild stands in their native ranges, imported, and in some cases improved and are now cultivated in the West or throughout the world. The objectives of this article were to 1) describe evolutionary contributions of Asian species to the American strawberry genome; 2) present examples of breeding Asian species (Rubus L. subgenus Idaeobatus) into cultivated raspberries; and 3) give examples of two Asian fruit species that have been recently introduced and cultivated or that could be developed for cultivation in the United States.

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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.

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The genetic control of flowering habit in many species of Fragaria has not been well studied. Identification of flowering traits and patterns for these taxa could be used in the quest for perpetual flowering (PF) genes and for the octoploids, broaden the genepool of available PF parents for breeding programs. As such, clones from the Fragaria germplasm collection housed at the USDA-ARS National Clonal Germplasm Repository in Corvallis, OR, were evaluated to describe flowering habits in various taxa and identify PF clones. Flower presence was recorded monthly for 962 clones of 36 taxa from the first of May through October in 2015 and 2016 to determine flowering habit and pairwise comparisons between taxa were examined using Pearson’s Chi-squared test. Taxa with the largest percent of PF accessions were F. vesca subsp. vesca f. semperflorens, F. vesca subsp. vesca f. alba, F. vesca subsp. americana, and F. virginiana subsp. glauca. These taxa had similar flowering habits to each other but were significantly different (α = 0.05) from most other taxa in which the seasonal flowering (SF) trait was predominant. Fifteen clones that demonstrated the PF phenotype in both 2015 and 2016 were identified. Differing genetic controls have been observed for flowering habit in F. ×ananassa and F. vesca. Additional studies are needed to determine genetic control of flowering in other Fragaria taxa.

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Identifying and evaluating genetic diversity of culinary rhubarb (Rheum ×rhababarum) cultivars using morphological characteristics is challenging given the existence of synonyms and nomenclatural inconsistencies. Some cultivars with similar names are morphologically different, and seedlings may grow and become associated with the parental name. Morphological traits of one cultivar may vary when measured under different environmental conditions. Molecular markers are consistent for unique genotypes across environments and provide genetic fingerprints to assist in resolving identity issues. Microsatellite repeats, also called simple sequence repeats (SSRs), are commonly used for fingerprinting fruit and nut crops, but only 10 SSRs have previously been reported in rhubarb. The objectives of this study were to use short-read DNA sequences to develop new di-nucleotide-containing SSR markers for rhubarb and to determine if the markers were useful for cultivar identification. A total of 97 new SSR primer pairs were designed from the short-read DNA sequences. The amplification success rate of these SSRs was 77%, whereas polymorphism of those reached 76% in a test panel of four or eight rhubarb individuals. From the 57 potentially polymorphic primer pairs obtained, 25 SSRs were evaluated in 58 Rheum accessions preserved in the U.S. Department of Agriculture, National Plant Germplasm System. The primer pairs generated 314 fragments with an average of 12.6 fragments per pair. The clustering of many accessions in well-supported groups supported previous findings based on amplified fragment length polymorphisms (AFLPs). Cluster analysis, using the proportion of shared allele distance among the 25 SSRs, distinguished each of the 58 accessions including individuals that had similar names or the same name. Accessions that grouped in well-supported clusters previously belonged to similar clusters with high bootstrap support based on AFLP. In summary, our technique of mining short-read sequencing data was successful in identifying 97 di-nucleotide-containing SSR sequences. Of those tested, the 25 most polymorphic and easy-to-score primer pairs proved useful in fingerprinting rhubarb cultivars. We recommend the use of short-read sequencing for the development of SSR markers in the identification of horticultural crops.

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