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Christopher J. Clark and Douglas M. Burmeister

Development of browning induced in `Braeburn' apple (Malus ×domestica Borkh.) fruit by a damaging CO2 concentration was monitored weekly using magnetic resonance imaging (MRI) during a 4-week storage trial (0.5 °C, 2 kPa O2/7 kPa CO2). Discrete patches of high-intensity signal, distributed randomly throughout the fruit, were observed in multislice images of samples after 2 weeks of storage; these patches were eventually confirmed as being sites of browning reactions after dissection at the end of the trial. Subsequently (weeks 3 and 4), signal intensity at sites of incipient damage increased and patches enlarged and coalesced. After 2 weeks of storage, the extent of affected tissue, averaged across all image slices, was 1.5%, increasing to 15.9% and 21.3% after 3 and 4 weeks. The average rate at which tissue damage spread in individual slices was 0.81 (range: 0–3.70) cm2·d–1 between weeks 2 and 3, declining to 0.32 (range: 0–1.55) cm2·d–1 in the final week. Tissue damage induced under these conditions did not spread at the same rate at all locations within individual fruit, nor was it preferentially located toward the stem or calyx ends of the fruit.

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Christopher J. Clark, Annette C. Richardson, and Ken B. Marsh

Whole-fruit proton magnetic resonance (MR) imaging was performed on satsuma mandarin (Citrus unshiu Markovich cv. Miho Wase) during a 15-week period commencing 10 weeks after anthesis and continuing to maturity, and at 6 weeks after anthesis the following season. Images with long repetition times (>1600 ms) and short echo times (20 ms) provided the clearest details of anatomical changes in the peel (flavedo, albedo) and vascular system, while those with similar repetition times but longer echo times (120 ms) were best for viewing juice sac morphology within pulp segments. At 6 weeks after anthesis, images of fruits of slightly different physiological ages highlighted rapid changes in the vascular bundles and albedo tissue at this stage of development. Variation in the relaxation measurements, T1 and T2, was determined from quantitative MR images of the juice sacs in equatorial slices, and images of expressed juice from whole fruit. Seasonal measurements of T1 determined in situ (1760 ms) were significantly greater than those in juice (1413 ms). By contrast, there was no mean seasonal difference between in situ T2 measurements (360 ms) and those for juice (332 ms). No associations between trends in the MR data and total soluble solids, pH, titratable acidity, and sugar and organic acid composition of the juice were established. Cell structure is identified as a hindrance in the use of quantitative MR imaging for probing compositional changes in solution in serial imaging studies.

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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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Susan C. Miyasaka, Sharon Motomura-Wages, Ishakh Pulakkatu-Thodi, Michael J. Melzer, Christopher A. Clark, Don R. LaBonte, and Arthur Q. Villordon

Tissue-cultured, virus-tested (TC) plantlets of sweetpotato (Ipomoea batatas var. batatas) cultivars Okinawan, LA 08-21p, and Murasaki-29 were obtained from Louisiana State University Agricultural Center. The objectives of field trials conducted at the Kula Agricultural Park, Maui, HI, were to compare yield and pest resistance of 1) ‘Okinawan’ obtained from a commercial (C) field with TC ‘Okinawan’ and 2) TC Okinawan with the aforementioned TC cultivars. Trials were planted Oct. 2015 and Aug. 2016 and harvested 5 months later. Storage roots were graded according to State of Hawai’i standards, and marketable yields included Grades AA, A, and B. In addition, injuries due to sweetpotato weevil (Cylas formicarius elegantulus) or rough sweetpotato weevil (Blosyrus asellus) were estimated. In both trials, fresh and dry weights of marketable storage roots of TC ‘Okinawan’ were nearly twice those from commercial planting material. In both trials, marketable fresh weights differed among the three TC cultivars; however, significant interactions were found, indicating that yields of cultivars differed between years. In the first field trial, ‘LA 08-21p’ had fresh marketable yields 1.6 to 1.7 times greater than TC ‘Okinawan’ and Murasaki-29, respectively. In the second trial, fresh marketable yields of TC ‘Okinawan’ and ‘LA 08-21p’were similar and 1.7 to 1.5 times greater than that of ‘Murasaki-29’, respectively. In both trials, ‘LA 08-21p’ had greater sweetpotato weevil injury than did the other two cultivars. Interestingly, in the second year, TC ‘Okinawan’ had greater rough sweetpotato weevil injury than did the other cultivars. Our results indicate that tissue-cultured planting materials increased marketable yields of TC ‘Okinawan’ compared with C ‘Okinawan’ sweetpotato and that the other TC cultivars did not produce greater yields than TC Okinawan.

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Chad E. Finn and John R. Clark