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  • Author or Editor: Robert E. Davis x
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Phytophthora ramorum, while thought to be primarily an aboveground pathogen, can be introduced into soilless potting media in the nursery industry as sporangia or chlamydospores and remain undetected while disseminated geographically. Inoculum of this pathogen, both North American (A-2 mating type) and European (A-1 mating type) isolates, was used to infest potting media components or soil, using either sporangia, chlamydospores produced in vermiculite culture, or dry infected `Nova Zembla' rhododendron (Rhododendron sp.) leaf pieces. Vermiculite chlamydospore/oospore inoculum of P. citricola, P. cactorum, and P. citrophthora were included for comparison. Survival was determined monthly by leaf disc baiting or direct plating on selective medium. Results indicated that P. ramorum survived in most media components or soil for up to 6 months when introduced as sporangia, or up to 12 months as chlamydospores. However, it was not detected at all from infected rhododendron leaf pieces by either detection method. These results show that P. ramorum can survive in potting media if introduced as sporangia or chlamydospores, and accordingly the pathogen could be disseminated geographically without being detected visually.

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A collection of mycoplasma-like organisms (MLOs) was maintained in plant tissues micropropagated in vitro. MLO-infected plants included Chrysanthemum frutescens L. with chyrsanthemum yellows disease, Gladiolus sp. L. with “germ fins,” Hydrangea macrophilla (Thunb.) DC. with virescence, Rubus fruticosus L. with rubus stunt, and periwinkle [Catharanthus roseus (L.) G. Don] singly infected by the following MLOs: Italian periwinkle virescence, chrysanthemum yellows, North American aster yellows, Italian periwinkle stunt, American periwinkle little leaf. Shoots micropropagated in vitro exhibited symptoms of little-leaf and/or abnormal proliferation of axillary shoots resulting in “witches' broom” appearance that resembled symptoms in grafttransmitted greenhouse-grown or naturally infected field-collected plants. These symptoms, typical of infection by MLOs, were not observed in micropropagated healthy shoots of the same plant species, and, compared with the healthy ones, varied with MLO strain and host plant species. Dot hybridizations with a nonradioactive cloned DNA probe provided evidence for the presence of MLOs in propagated tissues through serial subcultures.

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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.

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Abstract

A controlled environment study was conducted to determine if ‘Russet Burbank’ potatoes (Solanum tuberosum L.) were more tolerant of NH4-N as established plants than as young plants. When grown on one N source for 86 days, root and shoot growth were best with NO3-N, intermediate with NH4 + NO3, and least with NH4-N. Changing the N source from NO3-N or NH4 + NO3 to NH4-N at tuber initiation (58 days after planting) reduced both shoot and root growth. Conversely, changing the N source from NH4-N to NH4 + NO3 improved growth. The highest tuber weights were obtained with continuous NH4 + NO3 or when NH4-N was applied at tuber initiation to plants previously grown on NO3-N alone. Nitrogen source also influenced absorption of other nutrients. Changing the N source once plants were established, however, did not have a consistent effect on mineral composition. We conclude that when NH4-N is the sole form of N available to the plant, it is detrimental to potato growth regardless of stage of development.

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Breeding and improvement of new bermudagrass (Cynodon spp.) cultivars with superior nematode tolerance are essential because sting nematode (Belonolaimus longicaudatus Rau) is a major limitation for use of bermudagrass in the sandy coastal soils of the southeastern United States. The screening of both African (Cynodon transvaalensis) and common (C. dactylon) bermudagrass is necessary to develop triploid hybrid cultivars. Five commercial cultivars and 46 germplasm accessions of bermudagrass were tested for nematode responses in two greenhouse trials in 2009. Turfgrass was grown in sand-filled plastic conetainers and inoculated with 50 sting nematodes per conetainer. Nematode and root samples were collected 90 d after nematode inoculation. Fifteen bermudagrass accessions did not have measurable root loss from inoculation with sting nematode. Seven bermudagrass accessions, including ‘Celebration’, produced longer roots in sting nematode-infested soil than the standard ‘Tifway’. Differences in final nematode numbers were identified among the genotypes, and different relative responses were identified in variable ploidy levels and origins. This could aid a turfgrass breeding program by elucidating the genetic diversity available for breeding future bermudagrass cultivars for golf course cultivation.

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