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Blair Buckley III and Christopher A. Clark

Bacterial blight, incited by Xanthomonas axonopodis pv. vignicola, is a major disease of cowpea. Strong resistance has not been reported in commercial cultivars. Many cultivars released within the last ten years have not been previously screened. Thirty-eight cowpea cultivars, 60 breeding lines, and 25 PIs were screened for reaction to bacterial blight in tests conducted in a greenhouse. Inoculum was adjusted turbidimetrically (OD620nm = 0.1) to approximately 108 cfu/ml. Seedling plants were inoculated by either leaf infiltration or stem puncture methods. Ratings were on a 1-6 scale in which 1 = no symptoms, 2 = localized lesions at site of inoculation, 3 = lesions spreading slightly near site of inoculation, 4 = any symptoms on systemically infected leaves or stems, 5 = extensive wilting and/or stem collapse, 6 = dead plant. All cultivars were susceptible to the pathogen. Cream-type cultivars Bettergreen, Tender Cream, Zipper Cream, Carolina Cream, and Mississippi Cream were among the most susceptible. Breeding lines MN13, MN150, TX57069-11, TX 58048-2000, and TX 59069-11 produced hypersensitive reactions in response to leaf infiltration inoculation. However, the three TX lines were rated susceptible when inoculated by stem puncture. Eighteen PIs (including PI293467, PI293521, PI293525, PI293567, and PI293571) were highly resistant to bacterial blight.

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Frank A. Buffone, Don R. La Bonte, and Christopher A. Clark

DNA isolated from Fusarium lateritium Nees: Fr.-infected `Jewel' sweetpotato [Ipomoea batatas (L.) Lam.] plants was compared to F. lateritium-free `Jewel' plants for differences in random amplified polymorphic DNA (RAPD) marker products. Differences in RAPD marker products were detected. Amplified DNA isolations from F. lateritium-infected `Jewel' plants generated additional, unique DNA fragments not found in amplified DNA isolations of F. lateritium-free `Jewel' plants. These unique amplified DNA fragments were consistent with those obtained from amplified DNA isolations of the F. lateritium isolate, 91-27-2, used for inoculation. We found that F. lateritium DNA successfully competes with sweetpotato DNA in the polymerase chain reaction for priming sites in a 3: 1 ratio of sweetpotato DNA to F. lateritium DNA. Our results indicate the importance of avoiding plant material infested with pathogens to avoid spurious marker bands.

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Ramón A. Arancibia, Jeffrey L. Main, and Christopher A. Clark

Tip rot, or restricted end rot, is a new sweetpotato (Ipomoea batatas) disease/disorder in Mississippi with unknown etiology. Since pathogen isolations have been inconsistent, a relationship of this disorder with stress is proposed. This disease/disorder is manifested as a restricted lesion at or close to the proximal end of the storage root and appears after 2 to 4 weeks in storage. In many cases, the lesion necrosis expands internally. On-farm and research station studies with preharvest foliar applications of ethephon were conducted in Mississippi to determine the potential association of tip rot with ethephon-induced stress. In addition, the effects of ethephon rate and interval between application and harvest on tip rot were investigated. After 1 to 2 months in storage, tip rot incidence was observed mostly in storage roots from ethephon-treated plants. The increase in tip rot incidence was well correlated with ethephon rate. These results suggest that preharvest applications of ethephon trigger a response that results in the tip rot disorder.

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Mary Helen Ferguson, Christopher A. Clark, and Barbara J. Smith

Xylella fastidiosa Wells et al. causes disease in a number of plants in the southeastern United States, including southern highbush blueberry (Vaccinium corymbosum interspecific hybrids), but little was known concerning its potential impact in rabbiteye blueberry (Vaccinium virgatum Aiton syn. Vaccinium ashei Reade). In a naturally infected orchard in Louisiana, mean yields of X. fastidiosa–positive plants were 55% and 62% less than those of X. fastidiosa–negative plants in 2013 and 2014, respectively. Average berry weight was also lower in X. fastidiosa–positive plants. Within 3 years of testing positive for X. fastidiosa, four of nine X. fastidiosa–positive plants appeared dead. However, plants that were X. fastidiosa–negative in 2013 remained so until 2015, indicating that the bacterium did not spread rapidly in this established orchard during this time. Other factors, including soil chemistry variables, Phytophthora cinnamomi, ring nematode, and ringspot symptoms, were also investigated to determine if one of these might predispose plants to infection with X. fastidiosa or be partly responsible for observed yield loss. In most cases, interactions were not found, but associations with soil Cu and Zn suggest a need for further research on whether these elements predispose rabbiteye blueberry to X. fastidiosa infection and thereby contribute to yield losses. Researchers, extension workers, and growers should be aware of X. fastidiosa as a potential yield- and survival-impacting factor in rabbiteye blueberry.

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Heather L. Wallace, Don R. La Bonte, and Christopher A. Clark

Virus infections and genetic mutations have been implicated in the decline of sweetpotato yield and quality. Virus-tested mericlones were derived from 12 infected clones of `Beauregard' sweetpotato by meristem-tip culture. Field studies were conducted to evaluate yield differences between the virus-tested and the virus-infected plants of each respective clone. After a 90-day growing period, the storage roots were harvested, weighed, and analyzed with a colorimeter to gauge color of skin and flesh. Yield was 7% to 130% greater in virus-tested mericlones compared to their respective virus-infected clone. Data also show these 12 virus-tested mericlones vary in yield by up to 118%. This suggests genetic differences between clones greatly affect yield. The virus-tested mericlones also show a more desirable darker-red hue for skin and flesh than the virus-infected clones. The incorporation of virus-tested material into foundation seed programs could potentially increase yield and quality with little added expense to growers, thereby netting a higher return on their crop.

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Don R. La Bonte, Arthur Q. Villordon, Christopher A. Clark, Paul W. Wilson, and C. Scott Stoddard

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Don R. La Bonte, Paul W. Wilson, Arthur Q. Villordon, and Christopher A. Clark

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David G. Clark, Christopher Dervinis, James E. Barrett, and Terril A. Nell

Experiments were conducted to determine if the seedling hypocotyl elongation and petal abscission assays could be used to identify differences in ethylene sensitivity among seedling geranium (Pelargonium ×hortorum) cultivars. When seedlings of six geranium cultivars were germinated and grown in the dark in the presence of the ethylene biosynthetic precursor 1-aminocyclopropane-1-carboxylic acid (ACC) at various concentrations, they exhibited the triple response (measured as reduced hypocotyl length). While seedlings from all six cultivars were sensitive to ACC, `Scarlet Elite' seedlings were most sensitive, and `Multibloom Lavender', `Elite White' and `Ringo 2000 Salmon' seedlings were the least sensitive when germinated and grown on 20 mm [2022 mg·L-1 (ppm)] ACC. Florets representing three developmental stages of each of the six cultivars were exposed to 1 μL·L-1 of exogenous ethylene for 0, 30, or 60 min to determine if differences in cultivar sensitivity could be determined for petal abscission. Of the six cultivars tested, `Ringo 2000 Salmon', `Multibloom Lavender' and `Elite White' were the least ethylene sensitive. Florets were also self-pollinated to test for cultivar differences in ethylene synthesis and subsequent petal abscission. Ethylene production and petal abscission were both promoted in self-pollinated florets compared to nonpollinated florets. `Ringo 2000 Salmon', `Multibloom Lavender' and `Elite White' florets produced similar amounts of ethylene as all other cultivars, but abscised fewer petals after pollination. Our results indicate that the seedling hypocotyls elongation assay may be used to identify geranium cultivars with reduced sensitivity to ethylene. The data also suggest that genetic variability exists among geraniums for both ethylene sensitivity and biosynthesis.

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Christopher A. Clark, Tara P. Smith, Donald M. Ferrin, and Arthur Q. Villordon

Because sweetpotato (Ipomoea batatas) is vegetatively propagated, viruses and mutations can accumulate readily, which can lead to cultivar decline. Sweetpotato foundation seed programs in the United States maintain the integrity of commercial seed stock by providing virus-tested (VT) foundation seed to commercial producers. A survey was conducted in Louisiana from 2007 to 2009 to examine the performance and quality of the foundation seed after it had been integrated into commercial sweetpotato operations. G1 seed [grown 1 year after virus therapy in the foundation seed production field at the Sweet Potato Research Station, Louisiana State University Agricultural Center (LSU AgCenter), at Chase, LA] was used as a reference to compare the yield and virus incidence of growers' generation 2 (G2) and generation 3 (G3) seed roots (grown in the growers' seed production fields 1 or 2 years following the year of foundation seed production). Although yields of plants grown from G2 and G3 seed were 86.3% and 86.1% for U.S. No. 1 and 83.3% and 86.0% for total marketable, respectively, compared with the yields from G1 seed, they were not significantly different. Yield and virus incidence data suggest that seed quality may vary from year to year and from location to location. Results from this study suggest that producers are realizing yield benefits by incorporating VT foundation seed into their production schemes, but further benefits could be attained if ways to reduce re-infection with viruses can be found.

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Don R. La Bonte, Christopher A. Clark, Tara P. Smith, and Arthur Q. Villordon