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Brent L. Black, John M. Enns, and Stan C. Hokanson

Anticipating the phaseout of methyl bromide, the USDA-ARS small fruit breeding program at Beltsville, Md., discontinued soil fumigation in strawberry breeding and selection trials in the mid 1990s. To address resulting weed and pathogen pests, a modified or advanced matted row system was developed. This system uses matted row-type culture, established on raised beds with subsurface drip irrigation and organic mulch. The mulch is the residue of a killed cover crop that fixes some nitrogen and provides an economical, biodegradable mulch for suppressing weeds and reducing erosion. Since 1996, the small fruit breeding program has conducted replicated performance trials on both advanced matted row and a regional adaptation of annual hill plasticulture. Both of these systems were managed without methyl bromide fumigation or fungicide application. Data from these trials were used to compare advanced matted row and plasticulture for yield, fruit quality and harvest season. Yield for the two systems was genotype dependent, and the advanced matted row system had later production and slightly lower fruit quality.

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David C. Zlesak, Vance M. Whitaker, Steve George, and Stan C. Hokanson

Regional, replicated cultivar trials of landscape roses are an ongoing component of the Earth-Kind® program, which was started at Texas A&M University in the 1990s to support environmental landscape stewardship. The rose trials within the Earth-Kind program identify and promote the most regionally adapted rose cultivars and are conducted without fertilizers or pesticides and greatly reduced irrigation. Black spot (caused by Diplocarpon rosae Wolf) is the most serious disease of outdoor-grown roses worldwide as a result of the potential for rapid leaf yellowing and defoliation. Earth-Kind designated cultivars for the south–central United States and roses under trial in other regions or considered for future Earth-Kind trials (n = 73 roses) and two susceptible control cultivars were challenged with North American Races 3, 8, and 9 of D. rosae, which were previously characterized at the University of Minnesota. Young expanded leaves were inoculated using detached leaf assays. Lesion length (LL) was measured for susceptible reactions and cultivar ploidy was determined using root tip squashes. Diploid, triploid, and tetraploid cultivars (n = 20, 30, and 23, respectively) were identified, and race-specific resistances and partial resistances were also identified. Race-specific resistance was generally more prevalent in newer rose cultivars and rose cultivars more recently included in Earth-Kind trials. Nine cultivars were resistant to all three races (Brite Eyes™, ‘Grouse’, Home Run®, Knock Out®, Paprika™, Peachy Cream™, Pink Knock Out®, Rainbow Knock Out®, and Yellow Submarine™). Blushing Knock Out®, a sport of Knock Out®, was susceptible to Race 8. Partial resistance rank for LL was generally consistent across races for roses susceptible to multiple races. The application of these data includes: characterizing the minimum resistance level needed for roses to warrant inclusion in Earth-Kind field trials, the identification of additional race-specific resistance genes, identifying resistance-breaking isolates of D. rosae, understanding race composition in field trials based on infection patterns of key cultivars, selection of parents for resistance breeding efforts, and continued comparisons between LL and growing bodies of Earth-Kind field resistance data.

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John L. Maas, John M. Enns, Stan C. Hokanson, and Richard L. Hellmich

Larvae of several insects injure and kill strawberry (Fragaria ×ananassa Duchesne) plants by burrowing into and hollowing out plant crowns. Occasionally, these infestations are serious enough to cause heavy economic losses to fruit producers and nursery plant growers. In 1997 in Beltsville, Md., we observed wilting and dying mature plants and unrooted runner plants in two experimental strawberry plantings. Injury by larvae was extensive; large cavities occurred in crowns, and the central pith tissues were removed from stolons and leaf petioles. Often, insect frass was seen at entrance holes. Larvae removed from hollowed-out parts of injured plants were identified as the European corn borer (Ostrinia nubilalis Hübner) in their fifth instar stage. Their presence in this instance also was associated with a cover crop of millet [Setaria italica (L.) P. Beauv., `German Strain R'] planted between the strawberry rows for weed suppression. This is the first published report of the European corn borer attacking strawberry. Although this insect may occur only sporadically in strawberry plantings, it may become important in the future. Growers and other professionals should become aware of this new strawberry pest and recognize that its management in strawberry will be different from management of other crown-boring insects.

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Stan C. Hokanson, Steve McNamara, Kathy Zuzek, Nancy Rose, and Harold Pellett

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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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Stan C. Hokanson, Fumiomi Takeda, John M. Enns, and Brent L. Black

Tissue-culture derived mother plants were established in a greenhouse suspended-gutter, nutrient-film technique growing system to evaluate runner tip productivity in the system. Effects of cultivar (`Allstar', `Chandler', `Latestar', `Northeaster', and USDA selection B 27) and duration (0, 1, or 2 months) of cold storage at 1 °C on tip viability, rooting success, and performance in fruit production were determined. The average number and weight of runner tips produced in the gutter production system, the capacity of runner tips to form cohesively rooted plug plants, and the number and length of adventitious roots produced by runner tips varied significantly among the cultivars and the three storage durations (0, 1, or 2 months). In the field, plants produced from runner tips stored for 2 months produced more runners than plants produced from freshly harvested runner tips. Crown number differed among the cultivars, but was not affected by cold storage treatment. No treatment differences were noted for the fruit harvest parameters evaluated. The results suggest that the transplants derived from mother plants grown in a greenhouse-based soilless system can be useful for annual plasticulture strawberry production in colder climates. Although long periods of cold storage of runner tips resulted in lower tip-to-transplant conversion ratios, field performance of transplants was not adversely affected. Additional research is needed to improve greenhouse strawberry production practices for increasing runner output and storage conditions that maintain the integrity of cold-stored runner tips. Without these improvements it is unlikely that soilless runner tip production will become a widely accepted technique that would replace the field nursery tip production method currently used by commercial strawberry propagators.

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John L. Maas, John M. Enns, Stan C. Hokanson, and Richard L. Hellmich

Larvae of several insects injure and kill strawberry (Fragaria ×ananassa Duchesne) plants by burrowing into and hollowing out plant crowns. Occasionally, these infestations are serious enough to cause heavy economic losses to fruit producers and nursery plant growers. In 1997 in Beltsville, Md., we observed wilting and dying mature plants and unrooted runner plants in two experimental strawberry plantings. Injury by larvae was extensive; large cavities occurred in crowns, and the central pith tissues were removed from stolons and leaf petioles. Often, insect frass was seen at entrance holes. Larvae removed from hollowed-out parts of injured plants were identified as the European corn borer (Ostrinia nubilalis Hübner) in their fifth instar stage. Their presence in this instance also was associated with a cover crop of millet [Setaria italica (L.) P. Beauv., `German Strain R'] planted between the strawberry rows for weed suppression. This is the first published report of the European corn borer attacking strawberry. Although this insect may occur only sporadically in strawberry plantings, it may become important in the future. Growers and other professionals should become aware of this new strawberry pest and recognize that its management in strawberry will be different from management of other crown-boring insects.

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Stan C. Hokanson, Kelvin G. Grant, Elizabeth L. Ogden, and Lisa J. Rowland

Commercial strawberry plantings in the mid-Atlantic region are often quickly infected with one or more aphid-transmitted viruses, resulting in the loss of plant vigor, stunting, lowered yields, etc. To produce virus-free plant material for the strawberry industry and for cultivar development programs, heat therapy and/or meristem tip culture protocols are generally employed. One of the problems associated with meristem culturing is the potential for somaclonal mutations to occur in the meristem or surrounding proliferating tissue. As a result, distinct “bud lines” displaying functionally insignificant to distressingly high levels of phenotypic variation can arise from individual meristems. It would be desirable to differentiate these off-types by genetic fingerprinting to maintain trueness-to-type. Randomly amplified polymorphic DNA (RAPD) markers were evaluated for the potential to differentiate six pairs of strawberry bud lines that exhibit slight to fairly extreme levels of phenotypic variation. Reproducible RAPD marker profiles were generated using 10 primers in amplification reactions with genomic DNA obtained from multiple extractions. While five of the bud line pairs remained indistinguishable, three primers distinguished two variants of the Mohawk cultivar that are now in existence in the strawberry industry. Results suggest that typical somaclonal variation produced in the meristem culture process is of a magnitude that is not readily detectable with the RAPD protocol. The two Mohawk lines were probably produced by a higher magnitude mutation event than generally occurs or a cultivar mix-up.

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Stan C. Hokanson, Amy K. Szewc-McFadden, Warren F. Lamboy, and James R. McFerson

A diverse collection of 133 Malus species and hybrids from the USDA Plant Genetic Resources Unit's core subset collection was screened with five simple sequence repeat (SSR) primer pairs in order to determine genetic identities and overall levels of genetic variation. The number of amplification products (alleles) per locus (primer pair) in this collection ranged from 6 to 39, with some genotypes showing complex banding patterns of up to four products per locus, suggesting that duplication events may have occurred within the genome. Five primer sets unequivocally differentiated all but 10 pairs of genotypes in the collection, with seven of these 10 being pairs of the same species. Within three of the species holdings surveyed, M. honanensis, M. sargentii, and M. sikkimensis, no genetic variation was revealed with the SSR markers. The discrimination power for the combined loci in this collection was nearly one, which indicates that the likelihood of two genetically different accessions sharing the same alleles at all the loci included in this study would be nearly impossible. Coupled with results from a previous survey of M. × domestica accessions, this finding suggests that with five SSR primer pairs, the majority of the Malus holdings could be assigned a unique fingerprint identity. The average direct count heterozygosity over all loci was 0.620, ranging in value from 0.293 to 0.871 over individual loci. These heterozygosity counts will be compared with a survey of naturally occurring M. sieversii to determine whether current repository holdings are representative of the overall levels of diversity occurring in Malus. Information generated with this study, coupled with passport and horticultural data will inform curatorial decisions regarding deaccessioning of duplicate holdings and plans for future germplasm collections.