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J.A. Pattison, W.F. Wilcox, and C.A. Weber

A hydroponic method was developed and tested for screening red raspberry genotypes for resistance to Phytophthora fragariae var. rubi, the most common causal agent of Phytophthora root rot in raspberry. Plants of `Titan' and `Encore' exhibited typical disease symptoms, with the latter developing significantly smaller stem lesions and fewer petiole lesions. The resistant cultivar, `Latham', regenerated healthy root tissue from the crown and older-order roots after initial infection and necrosis of young roots and exhibited no other symptoms beyond minor leaf chlorosis. This component of the resistance reaction has not been documented previously. A segregating F1 population from the cross of `Latham' × `Titan' had a survival rate of 56% with 42% classified as resistant, exhibited minimal symptoms, and produced varying amounts of healthy root tissue. This screening method allows multiple observations of all plant tissues, including roots, under repeatable and definable growth chamber conditions. It should be useful for classifying the phenotype of individuals in segregating red raspberry populations to investigate the inheritance of Phytophthora root rot resistance using molecular markers.

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K.M. Haymes, W.E. Van de Weg, P. Arens, J.L. Maas, B. Vosman, and A.P.M. Den Nijs

Two dominant sequence characterized amplified region (SCAR) markers (linked at 3.0 cM, coupling phase) were constructed for the strawberry (Fragaria ×ananassa Duch.) gene Rpf1. This gene confers resistance to red stele root rot, caused by the soil-born fungus Phytophthora fragariae Hickman var. fragariae. The SCAR markers were developed originally from the sequence of RAPD OPO-16C(438) that is linked in repulsion phase to the Rpf1 allele. This SCAR primer set produced multiple bands in the resistant test progeny and in some of the susceptible progeny; therefore, new SCARs were developed based on the sequence differences among these bands. These new SCARs were linked in coupling phase to the Rpf allele and mapped to the same location as the original RAPD OPO-16C(438). The SCAR markers, as well as some additional RAPD markers known to be linked to Rpf1, were shown to be highly conserved in linkage to the gene based on examination of 133 European and North American Fragaria L. sp. cultivars and breeding selections. These flanking RAPD and SCAR-PCR markers can be used in breeding programs for the selection of red stele (Rpf1) resistance.

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Shahrokh Khanizadeh, Deborah Buszard, Odile Carrise, and Pierre O. Thibodeau

( Phytophthora fragariae ) et aux maladies foliaires (tache commune et tache pourpre). The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement

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G.J. Galletta, J.L. Maas, J.M. Enns, J.C. Scheerens, R. Rouse, and R.F. Heflebower Jr.

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G.J. Galletta, J.L. Maas, J.M. Enns, A.D. Draper, A. Dale, and H.J. Swartz

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Wendy K. Hoashi-Erhardt, Patrick P. Moore, Gwenyth E. Windom, and Peter R. Bristow

Root rot caused by the persistent soilborne oomycete Phytophthora fragariae var. rubi is a serious disease of red raspberry in the Pacific Northwest and worldwide. It can decrease the vigor and yield of raspberry plantings and require

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Hugh A. Daubeny

The indigenous North American red raspberry, Rubis strigosus has been neglected in breeding programs. Only four cultivars, `Cuthbert', `Latham', `Herbert' and `Ranere' provide most of the germplasm contained in present-day cultivars; no more than six individual wild genotypes of the species are represented by the four cultivars. In recent years, the B.C. breeding program has screened seedling populations of hitherto unexploited genotypes of the species from various locations in North America. Useful traits identified in selections from the populations include levels of resistance to 1) the North American aphid vector, Amphorophora agathonica, of the raspberry mosaic virus complex, 2) to several cane diseases and 3) to root rot caused by Phytophthora fragariae var rubi, as well as desirable fruit traits, such as bright, non-darkening red color and easy release. Selections with cultivar potential have now been identified in the second and third backcross generations from the species.

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K.E. Maloney, W.F. Wilcox, and J.C. Sanford

`Titan' red raspberry (Rubis idaeus L.), highly susceptible to root rot caused by Phytophthora fragariae Hickman var. rubi Wilcox & Duncan (syns. P. erythroseptica Pethyb., “highly pathogenic” P. megasperma Drechs.), was planted in June 1990 in a silt loam naturally infested with the pathogen. Raked beds (0.36 m high) dramatically reduced disease incidence and severity relative to flatbed treatments. In contrast, metalaxyl at 372 mg·m-1 of row provided little benefit when applied to flat beds and provided consistently moderate but statistically insignificant effects when applied to raised beds. Relative to the flat bed system, primocane vigor was increased in 1992 by 16%, 190%, and 224% in the flat bed plus metalaxyl, raised bed, and raised bed plus metalaxyl treatments, respectively; total yields were increased by 7%, 231%, and 272% with these same respective treatment. The results indicate that raised-bed planting systems can provide substantial control of phytophthora root rot of red raspberries even when highly susceptible varieties are grown on otherwise marginal sites. Metalaxyl appears more effective as a supplement rather than substitute for raised beds under such conditions. Chemical name used: N- (2,6-dimethylphenyl) -N- (methoxyacetyl)alanine methyl ester (metalaxyl).

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Kevin Maloney, Marvin Pritts, Wayne Wilcox, and Mary Jo Kelly

Various soil amendments and cultural practices were examined in both a phytophthora-infested (Phytophthora fragariae var. rubi) (+PFR) and uninfested field (–PFR) planted to `Heritage' red raspberries. Although plants in the +PFR field did not exhibit typical disease symptoms due to unseasonably dry weather, their growth was less than those in the –PFR field. After 2 years, plants in the +PFR site had the highest yields in plots treated with phosphorous acid or amended with gypsum, whereas compost-amended plots had the lowest yields in both +PFR and –PFR sites. A second field study confirmed the positive effect of gypsum on growth and yield of raspberries in an infested site. In a third study, `Titan' raspberries grown under greenhouse conditions in pots containing unamended soil from the infested site, then flooded, exhibited severe disease symptoms; however, pasteurization of the soil, treatment with phosphorous acid and metalaxyl fungicide, or gypsum amendment mostly prevented symptoms from developing. These three studies suggest that a preplant soil amendment containing certain readily available forms of calcium, such as found in gypsum, can help suppress phytophthora root rot and increase survival, growth and yield of raspberries in sites where the pathogen is present.

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F.A. Hammerschlag

As part of a program to develop transgenic highbush blueberry (Vaccinium corymbosum L.) and strawberry (Fragaria ×ananassa Duchesne) cultivars with increased levels of disease resistance, we have investigated the feasibility of introducing genes for the antimicrobial peptides cecropin B and MB39, α-thionin DB4 (DB4) and γ-thionin RsAFP1 (RsAFP1) by testing the effects of these peptides on several important pathogens of these two crop species. A thin-layer plate bioassay was conducted with these peptides and the pathogens Botrytis cinerea (Pers.ex. Fr.), Botryosphaeria dothidea (Mouq.ex. Fr.) Ces & de Not., Colletotrichum acutatum Simmonds, C. gloeosporioides (Penz.) Penz.et Sacc., C. fragariae Brooks, Monilinia vaccinii-corymbosi Reade (Honey), Phytophthora fragariae Hickman and Xanthomonas fragariae Kennedy and King. The minimum lethal concentration (μm) for cecropin ranged from 0.02 for X. fragariae strains 10 and 128 to 72.8 for C. gloeosporioides isolate Akp1. For DB4, the minimum inhibitory concentration (μm) ranged from 0.03 for X. fragariae strain 6 to 87.2 for B. cinerea isolate cc. For RsAFP1, the minimum inhibitory concentration (μm) ranged from 0.13 for X. fragariae strain 6 to 61.4 for M. vaccinii-corymbosi isolate 9423-x-45. These results indicate that introducing genes for either cecropin, DB4 or RsAFP1 into strawberry may be useful for controlling bacterial angular leaf spot disease caused by X. fragariae.