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D.L. Hopkins and J.W. Harris

Screening for resistance to Elsinoe ampelina (de Bary) Shear, causal agent of grape anthracnose, in grapevine seedlings is commonly conducted by natural infection over 3 to 4 years in the vineyard. The objective of this research was to develop a greenhouse screening method for selecting grapevine seedlings with resistance to anthracnose. Spores of E. ampelina were obtained from 3- to 4-week-old cultures on potato dextrose agar. Inoculum concentrations ranging from 1.3 × 103 to 1.3 × 107 E. ampelina conidia per mL were evaluated and 106 conidia/mL was optimum. The time of incubation of seedlings in a moist chamber after inoculation varied from 24 to 120 hours with 24 to 72 hours resulting in good symptom development. Temperatures in the moist chamber from 16 to 32 °C were evaluated and the most consistent results were obtained at 20 to 28 °C. The most effective method for selecting anthracnose-resistant grape seedlings in the two-to-three true-leaf stage was misting the seedlings with a suspension containing 106 conidia/mL in water and placing the inoculated seedlings in a moist chamber at 24 °C for 48 hours, followed by 8 days on a greenhouse bench. Resistant seedlings from the greenhouse screening (those with <10 foliar lesions) were transplanted into the vineyard and found to be resistant to anthracnose infection under rainy, humid conditions. This greenhouse procedure for selecting grapevine cultivars and breeding lines with resistance to anthracnose is accurate, economical, and labor-saving.

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D.L. Hopkins and C.M. Thompson

Prevention of the introduction of bacterial fruit blotch of watermelon, caused by Acidovorax avenae subsp. citrulli, into the transplant house or field is the most effective control strategy. Watermelon seedlots currently are screened for A. avenae subsp. citrulli, but other cucurbits, often grown in the same transplant house or field, generally are not as carefully monitored. In 1997 and 1999 field tests, cultivars of watermelon, muskmelon, honeydew melons, acorn squash, butternut squash, yellow squash, zucchini squash, cucumber, and pumpkin were evaluated for foliar and fruit susceptibility to bacterial fruit blotch and for seed transmission of A. avenae subsp. citrulli. The bacterium was introduced into the field on infected watermelon transplants or by misting a bacterial suspension onto fruit of the cucurbits. Foliar and fruit symptoms were more extensive in the watermelon, muskmelon, and honeydew melons than in the other cucurbits. In greenhouse grow-out assays, seed transmission of A. avenae subsp. citrulli was detected in every cucurbit in at least one of the two seasons, even though there were no fruit symptoms in some of them. Thus, any cucurbit crop plant should be considered a potential source for the introduction of A. avenae subsp. citrulli into the transplant house or field.

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D.L. Hopkins, C.M. Thompson, and G.W. Elmstrom

Seedlings of 22 watermelon [Citrullus lanatus (Thunb.) Matsum. and Nakai] cultivars and two plant introductions were screened in the greenhouse for resistance to the fruit blotch bacterium. There were significant differences in disease severity among cultivars, but no cultivar was immune to the bacterium. In field tests, fruit of 18 commercial cultivars were inoculated individually or became infected naturally from diseased foliage. Cultivars with relatively resistant fruit included `Sugar Baby', `Jubilation', `Mirage', `Calsweet', `Crimson Sweet', `Royal Sweet', and `Sangria'. The more susceptible cultivars generally had a light-colored rind. Cultivar level of resistance to bacterial fruit blotch may not be sufficient under conditions conducive to severe disease development.

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J.A. Mortensen, J.W. Harris, and D.L. Hopkins

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J.A. Mortensen, J.W. Harris, and D.L. Hopkins

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J.A. Mortensen, J.W. Harris, D.L. Hopkins, and P.C. Andersen

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D. J. Gray, Z. T. Li, D. L. Hopkins, M. Dutt, S. A. Dhekney, M. M. Van Aman, J. Tattersall, and K. T. Kelley

Pierce's disease (PD), caused by the xylem-limited bacterium Xylella fastidiosa, is endemic to the coastal plain of the southeastern United States. Although native southern grapevines are tolerant to X. fastidiosa, all varieties of Vitisvinifera grown in the region will succumb to PD. Genetic transformation to add disease resistance genes, while not disturbing desirable phenotypic characters, holds promise for expanding the southeastern U.S. grape industry by allowing use of established fruit and wine varieties. We utilize embryogenic cell cultures and Agrobacterium strain EHA105 to refine transformation systems for Vitis species and hybrids. V. vinifera`Thompson Seedless' is employed as a model variety to test various transgenes for disease resistance, since as many as 150 independent transgenic plant lines routinely are produced from 1 g of embryogenic culture material. Transgenic plants are stringently screened for PD resistance in greenhouses by mechanical inoculation with X. fastidiosa. Transgenic plants are compared with both susceptible and resistant control plants by assessing typical PD symptom development and by assaying bacterial populations in xylem sap over time. Using these procedures, nine putative PD resistance genes have been inserted into grapevine and over 900 unique transgenic lines have been evaluated. A range of susceptible-to-resistant responses has been catalogued. Thus far, the best construct for PD resistance contains a grape codon-optimized hybrid lytic peptide gene in a high-performance bi-directional 35S promoter complex. Certain transgenic plant lines containing this construct exhibit better resistance than that of resistant control vines.