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Brent L. Black, Stan C. Hokanson and Kim S. Lewers

In the perennial strawberry production system, removal of the harvested crop represents a loss of nitrogen (N) that may be influenced by cultivar. Eight strawberry (Fragaria ×ananassa Duch.) cultivars and eight numbered selections grown in advanced matted row culture were compared over three seasons for removal of N in the harvested crop. Replicated plots were established in 1999, 2000, and 2001 and fruited the following year. `Allstar', `Cavendish', `Earliglow', `Honeoye', `Jewel', `Northeaster', `Ovation', and `Latestar' and selections B37, B51, B244-89, B683, B753, B781, B793, and B817 were compared for yield and fruit N concentration. Harvest removal of N (HRN) was calculated from total season yield and fruit N concentration at peak harvest. There were significant differences in HRN among genotypes, ranging from 1.80 to 2.96 g N per meter of row for numbered selections B781 and B37, respectively. Among cultivars, HRN ranged from 2.01 to 3.56 g·m–1 for `Ovation' and `Jewel', respectively. The amount of HRN was largely determined by yield, however, there were also significant genotype differences in fruit N concentration, ranging from 0.608 to 0.938 mg N per gram fresh weight for B244-89 and `Jewel', respectively. These differences indicate that N losses in the harvested crop are genotype dependent.

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J.F. Hancock, R. Grumet and S.C. Hokanson

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K.S. Lewers, J.L. Maas, S.C. Hokanson, C. Gouin and J.S. Hartung

Bacterial angular leafspot disease (Xanthomonas fragariae Kennedy and King) of strawberry (Fragaria species and F. ×ananassa Duch. cultivars) has become increasingly important to strawberry fruit and plant production. Strawberry cultivars and species vary in susceptibility to infection. However, little is known regarding epidemiology of the disease and resistance to infection. Two octoploid genotypes, a native F. virginiana (US 4808, tested as SG-89) and a F. virginiana (SG 26) × F. ×ananassa (`Earliglow') hybrid (US 4809, tested as 80-4-38), previously were found to be highly resistant to two differentially pathogenic strains of X. fragariae representing two of four genotypic strain groups. Our objective was to determine the number of genes involved with resistance for these two strawberry genotypes, whether strawberry resistance is conferred by dominant or recessive alleles, and whether or not the heritability is high enough for breeders to reliably make selections of resistant individuals in breeding populations. About 120 F1 seedlings from crosses of susceptible `Sweet Charlie' with each of the two resistant genotypes were clonally propagated and challenged with each of four X. fragariae strains. These strains were selected to represent four genotypes of X. fragariae defined by repetitive element based PCR: ATCC 33239, Xf-3, Xf-6, and Xf-1425. Plants were quantitatively rated on a scale of 0 (resistant) to 5 (susceptible) in replicated evaluations. High estimates for broad sense heritability support the conversion of the quantitative disease scores to qualitative scores and the classification of genotypes as resistant or susceptible. The qualitative ratings were used to estimate the number of genes involved with resistance. Some segregation ratios fit a 7S:1R ratio, and others fit a 15S:1R ratio, indicating that three or four unlinked loci could explain the inheritance of resistance in these populations. The high estimates for broad sense heritability show that resistant progeny can be selected with confidence, though large populations will be needed to identify enough resistant progeny from which to select for other important traits.

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J.L. Maas, C. Gouin-Behe, J.S Hartung and S.C. Hokanson

Bacterial angular leafspot disease (BALD) of strawberry, caused by Xanthomonas fragariae Kennedy & King, has dramatically affected commercial fruit and plant production throughout the world. Leaf lesions may kill leaves, while lesions on sepals make fruit unmarketable. The bacterium can kill stolon-tip plantlets that are being rooted for transplanting. Since plants become systemically infected, there is no adequate chemical control for BALD under conditions that favor development and spread of the disease. Strawberry is the only host and no cultivars or advanced selections have proven resistant to this disease. We screened 23 Fragaria ×ananassa, 13 F. chiloensis, 56 F. virginiana, and 2 F. vesca genotypes for resistance to two pathogenic isolates of X. fragariae (ATCC-33239, the original strain from Minnesota and Xf-3 from North Carolina). Leaves were inoculated by forcing bacterial suspensions into leaves under pressure with a syringe barrel and plunger. Plants were incubated in a moisture chamber for 3 days, followed by 1 week under mist and then placed on a greenhouse bench. Experiments were done twice for obviously susceptible reactions and three and four times for questionable and resistant reactions, respectively. Only two genotypes were found to show a resistant reaction: 80-4-38 (`Earliglow' (F. virginiana clone SG-26 from Georgia) and F. virginiana clone SG-89 (=Luby MS 7-7 from Minnesota). Each of these genotypes exhibited typical hypersensitive responses by walling-off inoculation areas. All other genotypes exhibited typical BALD symptoms 5 weeks after inoculation with both isolates.

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K.S. Lewers, S.M.N. Styan, S.C. Hokanson and N.V. Bassil

Although simple sequence repeat (SSR) markers have been developed for species in the closely related genera Fragaria L. (strawberry) and Rubus L. (raspberry and blackberry), the number of SSRs available is insufficient for genetic mapping. Our objective was to use and compare multiple approaches for developing additional SSRs for Fragaria and Rubus. The approaches included: the development of SSRs from GenBank sequences from species of varied relatedness to Fragaria and Rubus and identified with two different data-mining methods (BLAST and SSRIT); the evaluation of some previously published SSRs designed from related species; and the development of SSRs from a genomic library made from F. ×ananassa Duschene ex Rozier `Earliglow'. When an SSR was developed from a known gene sequence, the location of the repeat in the gene was determined to evaluate the effect on amplification and polymorphism detection. Cross-generic amplification between closely related Fragaria and Rubus as well as transference from species of varied relatedness to Fragaria and Rubus also was evaluated and indicated limited transference within the subfamily Rosoideae. However, development of SSRs for Fragaria and Rubus from Rosa L. (rose) and Rosaceae genera outside Rosoideae was not efficient enough to be practical for new map development. SSRIT was superior to BLAST for identifying GenBank sequences containing repeats. SSRs developed from repeats found in either the 5′UTR (80% polymorphic) or 3′UTR (85% polymorphic) were most likely to detect polymorphisms, compared with those developed from coding regions (30%). SSRs developed from the genomic library were only slightly superior to GenBank-derived SSRs in their ability to detect polymorphisms.

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H.S. Aldwinckle, P.L. Forsline, H.L. Gustafson and S.C. Hokanson

Resistance to apple scab (Venturia inaequalis) in apple cultivar breeding has been derived mainly from the Vf gene from Malus floribunda 821, which introgresses horticulturally unfavorable characters. M. sieversii, now thought to be the primary progenitor of M. × domestica, grows wild in many diverse habitats in Central Asia and can have fruit quality comparable to commercial cultivars. Since 1989, four major collections of M. sieversii have been made in Central Asia, where scab is endemic. Some seed collections have been made from trees with superior fruit, that were not infected with scab. Over a 6-year period, 3000 seedlings from 220 wild M. sieversii trees representing 10 diverse ecosystems in Kazakstan, Uzbekistan, Kyrgyzstan, and Tajikistan have been inoculated with conidia of five races and two wild types of V. inaequalis. Suspensions (270,000 conidia/ml) were applied to 4- to 8-leaved seedlings, which were incubated for 48 h at 19°C with constant leaf wetness. Symptoms for three resistant reactions were assessed 2 to 4 weeks after inoculation: A = chlorosis with crinkling (Vf type reaction); B = stellate necrotic lesions (Vr type reaction), and N = large necrotic areas (uncharacterized resistant reaction). Results indicated that nearly 20% of the seedlings showed one or more of the resistant reactions. The range of resistance within seedling populations from each of the 220 single-tree sources ranged from 0% to 75%. Significant differences existed among seedlings from each of the ecosystems. Most resistance reactions appeared to be similar to those observed for Vr from “Russian seedling.” Resistant selections with superior horticultural traits may constitute a genepool for increased efficiency of breeding scab-resistant cvs. This genepool may also be useful to address the breakdown of resistance to V. inaequalis race 6.

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K.S. Lewers, W.W. Turechek, S.C. Hokanson, J.L. Maas, J.F. Hancock, S. Serçe and B.J. Smith

Anthracnose crown rot of cultivated strawberry (Fragaria ×ananassa Duchesne ex Rozier) has been a major disease problem in the strawberry producing regions of the southeastern United States since the early 1970s. Chemical controls are often inadequate, but use of resistant cultivars is seen as a credible option for managing this disease. Only a small portion of Fragaria L. germplasm has been screened for resistance to anthracnose crown rot. A core subset of the Fragaria collection maintained at the U.S. Department of Agriculture National Clonal Repository in Corvallis, OR, has been constructed to contain an elite group of native F. virginiana Mill. and F. chiloensis (L.) Mill. This collection, referred to as the “core collection,” has been characterized for many horticultural traits, including reactions to several common foliar diseases, resistance to black root rot (causal organisms unknown), and resistance to northern root-knot nematode (Meloidogyne hapla Chitwood) and root-lesion nematode [Pratylenchus penetrans (Cobb) Filipjev & Shuurmans Stekhoven]. Our objective was to evaluate the core collection for resistance to a selection of isolates of three Colletotrichum Corda species known to cause strawberry anthracnose, Colletotrichum fragariae A.N. Brooks, Colletotrichum gloeosporioides (Penz.) Penz. & Sacc. in Penz. [teleomorph Glomerella cingulata (Stoneman) Spauld. & H. Schrenk], and Colletotrichum acutatum J.H. Simmonds (teleomorph Glomerella acutata J.C. Guerber & J.C. Correll). No Fragaria subspecies or geomorph was more resistant than any other; rather, individual genotypes within these groups were identified as sources from which resistance can be obtained. Collecting germplasm in areas of intense disease pressure may not be as beneficial as one might assume, at least where anthracnose crown rot disease is concerned.

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John L. Maas, John S. Hartung, Cristina Gouin-Behe and Stan C. Hokanson

Bacterial angular leafspot disease (BALD) of strawberry (Fragaria sp. and F. ×ananassa Duchesne cultivars) has become increasingly destructive to strawberry fruit and plant production in Canada and the United States, as well as in other countries. The disease, caused by Xanthomonas fragariae Kennedy and King, was first documented in Minnesota in 1960, and has become of worldwide concern because of the economic impact of BALD in strawberry fruit and nursery-plant production and the lack of adequate disease control strategies. We tested 81 Fragaria genotypes, including representatives of F. ×ananassa, F. chiloensis (L.) Duchesne, F. virginiana Duchesne, and F. vesca L., for resistance to two pathogenic strains of X. fragariae. Two genotypes, a native F. virginiana from Minnesota and an F. virginiana × F. ×ananassa hybrid, were found to resist infection by both bacterial strains and may be potential sources of resistance to other strains of X. fragariae.

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J.F. Hancock, S.C. Hokanson, P.W. Callow, M. Sakin, K. Haghighi and J.A. Flore

Twenty-one western and 13 eastern strawberry [Fragaria × ananassa (Duch.)] cultivars were grown in a polyethylene-covered greenhouse (polyhouse) in deep beds at either 10 × 10 or 25 × 25 cm spacing. Runners were removed weekly from the closest-spaced plants (hills), and the more open-spaced plants were allowed to set four runners on each side of the mother plant before the runners were removed (matted rows). Temperatures were allowed to fluctuate normally in the polyhouse, except that winter temperatures were maintained above 0C. The average yield of eastern and western cultivars did not differ significantly in most comparisons, but the average fruit weight of the Californian cultivars was significantly higher than the eastern ones, and Californian cultivars allocated a higher proportion of their biomass to reproduction. Nonbearing plants of eastern and western cultivars produced similar numbers of runners per plant and daughters per runner. There was no significant relationship between CO2 assimilation rate and yield. Interbreeding eastern cultivars with the most productive western genotypes might result in increased yields, but only if the higher reproductive efforts of the western types can be captured and transferred.