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Plant viruses are extremely difficult to manage in vegetable crops, particularly those viruses transmitted by aphids (Aphididae) in a nonpersistent manner. Viruses in the genus Potyvirus [e.g., papaya ringspot virus (PRSV), watermelon mosaic virus

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Alstroemeria (Alstroemeria spp.) is cultivated for cut flowers. Traditional propagation methods are by division of rhizomes from mature plants, so that viruses occurring in the crop can be multiplied and cause a decrease in the quality and production. The objective of this work was to obtain Alstroemeria cv. Rosario plants free of Alstromeria Mosaic Potyvirus (AlMV) by in vitro culture of shoots and thermotherapy. The best percentage of explants without contamination was obtained when adding the disinfectant PPM (1%) to the medium Murashige-Skoog (MS) while the best induction of buds was obtained when using explants of 1.5 cm. in length. In vitro multiplication of shoots was best in treatments with 2iP (isopentenyl adenine), BA (benzyladenine), and zeatin (4.4, 6.1, and 6.6 buds per explant, respectively). Rhizogenesis was observed in rhizomes growing in MS with 4.9 μM AIB (indole butyric acid) and 1.5 g·L-1 of sugar. Sixty-seven percent of plants growing in vitro did not react to AlMV antiserum and did not show particles and viral inclusions. Thermotherapy treatments of 45, 50, and 55 °C during different periods of time produced from 25% to 87.5% of plants that did not react to AlMV antiserum and did not show virus particles or cytoplasmic inclusions.

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Zucchini yellow fleck virus (ZYFV) is a potyvirus that occurs in cucurbits grown in some Mediterranean countries. `Marketer' cucumber responded to ZYFV infection with a severe mosaic, stunting, and leaf and fruit deformation. A high level of resistance to this virus was found in a single plant selection of `Taichung Mou Gua' (TMG) cucumber from Taiwan. In F2 and backcross populations involving TMG × `Marketer', the resistance to ZYFV was determined to be conferred by a single recessive gene, to which the symbol zyf is assigned.

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Plants transgenic for potyvirus coat protein (cp) genes have been shown to be resistant to viruses homologous and heterologous to the cp source virus. We have produced plum plants transgenic for the papaya ringspot virus (PRSV) cp gene. PRSV is a potyvirus related to plum pox virus (PPV). PRSVcp transgenic plants have been inoculated with PPV under containment conditions at the USDA Foreign Diseases-Weed Science Research Facility, Frederick, MD, and evaluated for two years. At least one plant is apparently resistant or tolerant to PPV based on symptomology, ELISA and RT-PCR assays. This suggests the potential utility of cp-mediated virus protection in tree fruits. To further test this potential, both short and long-term studies are in progress to evaluate resistance and cp expression in various organs, throughout the year and over the commercial life of individual trees. Plum plants have also been transformed with the PPVcp gene. Studies are underway to evaluate the protection derived from this cp gene.

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genetic uniformity exists in cucumber for potyvirus resistance that could potentially be overcome by the viruses. Table 2. Three amino acid variants across the eight exons of the proposed candidate gene vacuolar protein sorting–associated protein 4-like

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the latter was characterized using the coat protein (CP) and not the P1 protein. The CP region of the genome is the most widely used for Potyvirus ( Adams et al., 2005 ; Shukla and Ward, 1988 ). The ZYMV-PR CP gene fragment showed a high percentage

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PRSV-W, previously known as watermelon mosaic virus-1 (WMV-1), is an important potyvirus causing significant economic damage to cucurbit crops ( Bateson et al., 2002 ). Occurrence of PRSV-W in cucurbit fields coincides with increased aphid

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strains may be controlled by genes on different linkage regions. Resistance to different virus was observed in a pepper genotype possessing two genes that confer resistance to different strains of the potato potyvirus Y ( Dogimont et al., 1996 ). If the

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Quechua farmers have cultivated mashua (Tropaeolum tuberosum Ruiz & Pavon) and other tuber crops for thousands of years. The practice of trading seed tubers may have contributed to dispersal of viral diseases, such as the tropaeolum mosaic virus (TropMV). We surveyed 17 accessions of mashua collected from Quechua farmers in the provinces of Cuzco and Ayacucho, Peru. Most cross-reacted with the TropMV antibody and showed viral disease symptoms. Significant differences were observed between accessions from Cuzco and Ayacucho, with respect to virus infection and tuber yield under greenhouse conditions. Of the accessions from Cuzco, 87% displayed viral symptoms, while only 22% from Ayacucho showed symptoms. Fewer tubers from Cuzco generated mature plants. In turn, those mature plants produced lower tuber yields. The practice of trading seed tubers may be advantageous for promoting crop diversity but can be harmful when diseased seed tubers are being traded. A program to generate and distribute virus-free seed tubers among Andean farmers would contribute to higher crop yields while preserving local customs and crop diversity.

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Abstract

In field studies, appearance of foliar symptoms in yellow squash (Cucurbita pepo L., cv Sundance) provided a better indication of time of watermelon mosaic virus 2 (WMV-2) inoculation than appearance of symptoms in fruit. Although the longest period of time between inoculation and foliar symptom appearance occurred with the last inoculation date (25 Sept.), differences in this period did not differ statistically among inoculation dates. Dates of fruit symptom appearance were more variable and were more related to plant developmental stage than to time of inoculation. The length of disease-free fruiting period and the number of disease-free fruit were reduced severely for plants receiving one of the first four weekly WMV-2 inoculations. Subsequent inoculations failed to reduce squash yield significantly.

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