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Xia Xu*, Zhongbo Ren, and Jiang Lu

Pierce's Disease (PD) is a major factor limiting grape production in the southeast United State. This disease is caused by a bacterium, Xylella fastidiosa Wells et al., which is transmitted to the xylem system of the grapevines primarily by glassy-winged sharpshooters (Homalodisca coagulata Say). Once it is in the xylem, the X. fastidiosa will use the xylem sap as a nutrient source to multiply, colonize, and eventually plug the xylem vessels and cause the PD in susceptible cultivars. On the other hand, symptoms of PD in tolerant cultivars do not appear until fruit maturation, and symptoms are rarely observed in PD resistant cultivars. In order to understand the correlation between X. fastidiosa and PD symptom development, a study was initialed to monitor X. fastidiosa in xylem of resistant, tolerant, and susceptible vines on a monthly basis. Presence of X. fastidiosa was detected directly from xylem sap of field-grown vines by medium culture and confirmed by polymerase chain reaction (PCR). Xylella fastidiosa was detectable throughout the growing season in PD susceptible cultivar `Chardonnay', PD tolerant Florida hybrid grape `Blanc du Bois', and muscadine cultivar `Carlos'. The bacteria were also appeared in the dormant vines with high density in cultivars `Chardonnay' and `Blanc du Bios'. Although X. fastidiosa was also found in dormant canes of `Carlos', the density decreased throughout the late fall and winter months, and they were hardly found before June. The results indicated that X. fastidiosa were carried over from previous season in cultivars `Chardonnay' and `Blanc du Bois', while in PD tolerant cultivar `Carlos', they were newly acquired from the sharpshooter feedings during the growing season.

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Mike Murray, Bob Beede, Bill Weir, and Jack Williams

Physiological effects on plant growth caused by the plant hormone ethylene have been noted for many years. More than 100 years ago, workers noted that illuminating gas or broken gas mains had deleterious effects on surrounding trees or plants. It was not until the 1960s that scientists documented that plant growth may be manipulated by applying ethylene. Some of the biological effects since noted include premature defoliation, fruit maturation ripening, induction of flowering, stimulation of sprouting or germination, and shortening of plant height. These effects are noted on a wide variety of agricultural crops, including vegetables, field crops, tree crops, and ornamentals. Ethylene is a gas and dissipates rapidly, and, thus, does not lend itself to field application. In the 1960s, the product ethephon [(2-chloroethyl)phosphonic acid] was developed. When taken up by the plant, ethephon is converted to ethylene in the cells and becomes available for physiological interactions. Because ethephon precipitates a wide variety of biological reactions, application technology becomes extremely important. Factors such as plant growth stage, plant stress status, plant foliage spray coverage, ethephon rates, and environmental conditions determine the responses obtained. An example is provided by processing tomatoes, where the desired response is to maximize fruit maturity enhancement and minimize premature defoliation—both ethylene responses. We have selected five agricultural applications of ethephon as examples of how plant growth may be altered. These are: increased boll opening in cotton; enhanced pistillate flower induction in hybrid squash seed; accelerated fruit maturity in processing tomatoes; enhanced hull splitting in walnuts; and reduced lodging in wheat. Each of these applications, and others, are common in California agriculture. Brevity necessitates providing only a summary of relevant applied research activities, which are not intended to be complete or thorough. Details on specific ethephon applications may be obtained from that particular researcher.

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Kim D. Bowman

Microcitrus is one of five genera that are partially sexually cross-compatible with the genus Citrus. The genus Microcitrus contains seven species with characteristics that may be valuable for breeding citrus scions and rootstocks, including zygotic embryony, short juvenile period, short fruit maturation time, and resistance to nematodes and Phytophthora. However, relatively few F1 hybrids between Microcitrus and Citrus have been reported, and most of these have been pollen- and ovule-sterile. Some of these intergeneric hybrids have also been highly susceptible to cold damage. To create a genetic bridge for recombination of useful traits from Microcitrus into Citrus, two selections of Citrus ichangensis (an exceptionally cold hardy species with zygotic embryony and short juvenile period) were hybridized with Microcitrus warburgiana and two selections of Microcitrus inodora. Seed were collected from these crosses and germinated in a warm greenhouse. A total of 94 M. inodora xC. ichangensis hybrids and 34 M. warburgiana xC. ichangensis hybrids) were obtained and transplanted to 4-gallon pots in a screenhouse. At 33 months after planting the seed, 42% of the M. inodora × C. ichangensis hybrids and 67% of the M. warburgiana × C. ichangensis hybrids had flowered. Pollen germination tests on agar plates indicated that several hybrids produced large quantities of viable pollen. Numerous crosses were completed using some of these F1 hybrids as pollen and seed parents. Several F1 hybrids were confirmed to be highly fertile by recovery of healthy F2 and backcross hybrids with Microcitrus sp., Citrus sp., Poncirus trifoliata, and other Microcitrus (C. ichangensis) selections.

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John Jifon*

Use of plastic mulch to increase rhizosphere temperatures is a common practice in spring production of vegetable crops. However, supraoptimal soil temperatures during the fruit maturation period in early summer can impair root function and reduce produce quality. The effects of colored plastic mulch on rhizosphere temperature and `Primo' muskmelon root respiration were investigated in the field during Fall (Aug.-Nov. 2002) and Spring (Mar.-May 2003) seasons. Rhizosphere temperatures (measured at 0.1 m below the soil surface with thermo-couples) and respiration under four plastic mulches (black, silver, white, and clear), and a bare ground control were studied. The soil warming properties of the different mulches differed between Spring and Fall. Bare ground rhizosphere temperatures declined from ≈33 to 21°C in the Fall and increased from 14 to 26 °C in Spring. In both studies, black and clear plastic mulches had the highest rhizosphere warming effects (3-8 °C) compared to bare ground. In the Fall, average midday soil temperatures under the white and silver mulches were 2-3 °C cooler than the bare ground treatment. Canopy establishment was accelerated by plastic mulches in Spring but not in Fall. Root + soil respiration was positively correlated with measured rhizosphere temperatures (r = 0.69), with the highest respiration rates recorded under the clear and black plastic mulches. More than 80% of fruits from the clear plastic treatment were deformed and unmarketable. The number of marketable fruit was similar among the black, white and silver mulch treatments and significantly greater (32% in Spring & 12% in Fall) than in the bare ground treatments.

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Md. Shahidul Islam, S. Khan, and T. Matsui

Seasonal fluctuations of carbohydrate levels and compositions and the activities of related enzymes of three cultivated tomato (Lycopersicon esculentum Mill. cv. Lady First, Momotaro, and Minicarol) cultivars were examined at 45-days interval with seven different sowing in the relatively warm climate of Japan. Fruits picked on early winter to spring seasons had higher sugar concentrations compared to hot season. Fructose and glucose in nearly equal amounts were the predominant sugar in all the seasons. Sucrose was present in trace quantities, but cherry cultivar Minicarol accumulated higher levels than the other two large-fruited types.

Acid invertase (EC was highest at red stage during December to April, while fruit matured during May to August had lowest activity. The activity levels of soluble invertase were predominant compared to cell wall-bound fraction. The sucrose synthase (EC showed highest activity in rapidly growing fruits followed by a very low activity with fruit maturation. Sucrose synthase showed the higher activity during November to February, and almost low activity during all the experimental periods. The sucrose phosphate synthase (EC also showed higher activity during October to February, but the activity levels did not change drastically throughout the fruit development. The results substantiate the conclusion that, in all the planting seasons, acid invertase is a principal enzyme in the process of tomato fruit ripening and during early stage of tomato fruit development, sucrose synthase is the dominant enzyme, which, in turn, plays a part in regulating the translocation of sucrose into the fruit.

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M. Clayton, W.V. Biasi, S.M. Southwick, and E.J. Mitcham

ReTain™, a commercial derivative of aminoethoxyvinylglycine, was applied as a single application at 124 g·ha-1 a.i. to `Bartlett' pear (Pyrus communis L.) trees 28, 21, 14, or 7 days prior to initial commercial harvest and at 62 g·ha-1 a.i. in combination with naphthaleneacetic acid (NAA) at 92 g·ha-1 a.i. 14 days prior to initial commercial harvest. Maturity and quality of treated fruits at harvest and following storage were compared with those of nontreated pears in 1996 and 1997. Ethylene production by mature green pears at harvest was not significantly affected by ReTain™ treatments, although softening, loss of chlorophyll, and starch clearance were usually inhibited by the 14- or 7-day treatment. ReTain™ suppressed ethylene production, softening and loss of chlorophyll in ripening pears and mature green pears cold-stored for 4 months, although loss of chlorophyll did not differ in the cold-stored fruit in 1997. ReTain™ had little effect on softening during a ripening period of 6 days after 4 months of cold storage. Application at 14 or 7 days prior to initial harvest appeared most effective, often with little difference between the two timings, and the 28- or 21-day treatment or combined ReTain™ and NAA treatment were seldom more effective. ReTain™ applied 14 or 7 days before initial harvest delayed fruit maturation by 4-10 days depending on the maturity index. The maturity or ripeness of pears from the combined ReTain™ and NAA, NAA only, and control treatments was often similar or differed only slightly. Premature ripening, prevalent in 1997, was dramatically suppressed in fruit treated with ReTain™. Ripening of both ReTain™- and non-ReTain™-treated fruit with ethylene reduced premature ripening by ≈50%.

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David L. Ehret, Brenda Frey, Tom Forge, Tom Helmer, and David R. Bryla

A 4-year study was conducted to establish the effects of drip irrigation configuration and rate on fruit yield and quality of young highbush blueberry plants (Vaccinium corymbosum L. ‘Duke’). Plants were grown in a silt loam soil on raised beds and were non-irrigated or irrigated using either one or two lines of suspended drip tape. Each line configuration had in-line emitters spaced every 0.3 or 0.45 m for a total of four drip configurations. Water was applied by each drip configuration at two rates, a moderate rate of 5 L/plant per irrigation event, and a heavy rate of 10 L/plant. The frequency of irrigation was guided by measurements of soil matric potential. Irrigation was applied each year, and plants were cropped beginning the second year after planting. Rainfall was above normal in the first 2 years of the study, and differences in soil moisture were most evident in the last 2 years, in which soil matric potential increased with irrigation volume. Neither the number of irrigation lines nor emitter spacing had an effect on yield or fruit quality. Yield was unaffected by irrigation rate until the fourth year after planting and was only higher when 5 L/plant was applied. The yield increase was the result of differences in fruit weight during the second of two harvests and was associated with delays in fruit maturation. Irrigation affected plant mineral concentrations but leaves and berries responded differently; affected minerals tended to decrease in leaves but increase in the fruit. Many irrigation-induced changes in fruit quality were evident 1 or 2 years before changes in yield. Higher irrigation volume increased fruit size and water content but reduced fruit firmness and soluble solids. Irrigation reduced fruit water loss during storage and thereby promoted longer shelf life. Irrigation also resulted in a change in anthocyanin composition in the fruit but did not affect antioxidants or total anthocyanin content.

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Jun Song, Weimin Deng, Randolph M. Beaudry, and Paul R. Armstrong

Trends in chlorophyll fluorescence for `Starking Delicious', `Golden Delicious' and `Law Rome' apple (Malus ×domestica Borkh.) fruit were examined during the harvest season, during refrigerated-air (RA) storage at 0 °C, following RA and controlled-atmosphere (CA) storage, and during a poststorage holding period at 22 °C. Fluorescence parameters of minimal fluorescence (Fo), maximal fluorescence (Fm), and quantum yield [(Fm-Fo)/Fm, otherwise denoted as Fv/Fm] were measured. During `Starking Delicious' fruit maturation and ripening, Fv/Fm declined with time, with the rate of decline increasing after the ethylene climacteric. During RA storage, all fluorescence parameters remained constant for approximately 2 weeks, then steadily declined with time for `Starking Delicious' fruit. Superficial scald was detected after Fv/Fm had declined from an initial value of 0.78 to ≈0.7. Fv/Fm was consistently higher for CA-stored fruits than for RA-stored fruits. We were able to resegregate combined populations of “high-quality” (CA) and “low-quality” (RA) `Law Rome' fruit with 75% accuracy using a threshold Fv/Fm value of 0.685, with only 5% RA-stored fruit incorrectly identified as being of high quality. During a poststorage holding period, Fo, Fm, and Fv/Fm correlated well with firmness for `Starking Delicious', but not for `Golden Delicious' fruit, which were already soft. Fo and Fm were linearly correlated with hue angle for 'Golden Delicious' fruit, decreasing as yellowness increased. The accuracy, speed of assessment, and light-based nature of fluorescence suggests that it may have some practical use as a criterion to assist in sorting apple or other chlorophyll-containing fruit or vegetables on commercial packing lines.

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Olga Dichala, Ioannis Therios, Magdalene Koukourikou-Petridou, and Aristotelis Papadopoulos

A field experiment was conducted in a pomegranate (Punica granatum L.) orchard of the well-known cultivars Wonderful and Acco, located in the farm of Aristotle University. The trees were sprayed, every 15 days from flowering (April) to fruit maturation (September), with solutions containing 0, 25, 50, 100 μm Ni, and 100 μm Ni + 100 μm B prepared with Ni(NO3)2·6H2O and boric acid. Leaves and fully ripe fruits were initially sorted into cracked and uncracked ones, then further separated into peel and seeds, sampled, and analyzed. Nickel sprays were effective in controlling fruit splitting as well as Ca and Mg concentration of fruit peels. The correlation between cracking level and Ni concentration in solution was linear and negative. Cracking percentage with 50 μm Ni was lower in ‘Wonderful’, whereas no difference was recorded between the cultivars in the remaining treatments. Leaves had the smallest Ni concentration compared with fruit peel and seeds. Calcium concentration of pomegranate peels was higher than that of control peel at 50 μm Ni in ‘Wonderful’. Concerning ‘Acco’, the treatments 25 μm Ni, 50 μm Ni, and 100 μm Ni + 100 μm B reduced Ca concentration, compared with control. ‘Wonderful’ fruit peel contained more phenolics than ‘Acco’. The treatments 25, 50, and 100 μm Ni increased significantly the flavonoid concentration of fruit peels. The antioxidant capacity ferric-reducing antioxidant potential (FRAP) was linearly increased with Ni concentration in solution in ‘Wonderful’, whereas in ‘Acco’ it decreased at 25 and 50 μm Ni. Our data indicates that improving Ni nutrition of pomegranate can potentially reduce crop loss due to cracking and modified phenol and flavonoid concentration and FRAP value of fruit peel.

Open access

Rachel Leisso, Bridgid Jarrett, and Zachariah Miller

Haskap (Lonicera caerulea), also known as honeyberry, is a relatively new fruit crop in North America. To date, most academic activity and research in North America involving haskap has focused on cultivar development and health benefits, with relatively few field experiments providing information to guide field planning and harvest management for the recently released cultivars. In 2020, we documented preharvest fruit drop (PHFD) rates for 15 haskap cultivars planted in a randomized block design at our research center in western Montana with the aim of preliminarily determining whether certain cultivars may be prone to this phenomenon. Additionally, we evaluated two plant growth regulators (PGRs) to reduce PHFD in two cultivars previously observed to have high rates of PHFD. Results suggest cultivar-specific variations in PHFD near berry maturation. Because haskap harvest indices are not well-defined and may be cultivar-specific, we share our 1-year study results as preliminary information and as a call for further research. Cultivars Aurora, Boreal Blizzard, Borealis, Indigo Gem, Kapu, and Tana all had PHFD rates less than 12% of yield, where yield is the weight of berries lost to PHFD plus marketable yield and marketable yield is fruit remaining on the shrub at harvest. Cultivars Chito, Kawai, and Taka had the highest rates of PHFD, although marketable yields were still relatively high, especially for Kawai. We note that ease of fruit detachment is an important consideration in mechanical harvest, and this characteristic could be advantageous if managed appropriately. The PGRs evaluated (1-napthaleneacetic acid and aminoethoxyvinylglycine) did not influence PHFD rates; however, our study was limited by the sample size and by the lack of information regarding haskap abscission physiology. In summary, the haskap cultivars evaluated exhibited variable PHFD rates in the year of the study, and further research is needed to understand haskap fruit maturation, harvest indices, and abscission.