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Salah E. Youssef and Elizabeth J. Mitcham

Peaches and apricots were obtained at harvest. One-half were inoculated with the brown rot organism (Monilinia fructicola) and incubated overnight before immersion in 52C water for 2.5 and 2 minutes, respectively. Fruit were placed in storage at SC in air, 2% O2 and 15% CO2, or 17% O2 and 15% CO2 for 5 or 15 days before ripening at 20C. For peach, controlled atmosphere (CA) had no influence on decay while hot water significantly reduced decay incidence and severity. For apricot, after 15 days cold storage, both hot water and controlled atmosphere storage reduced decay incidence and severity. CA with 2% O2 and 15% CO2 controlled decay better than 17% O2 and 15% CO2. Growth and sporulation of Monilinia fructicola in air and CA was also evaluated in vitro. The combination of heat and CA controlled decay better than either treatment alone. The hot water treatment resulted in minor surface injury on peaches while apricots were not injured. Fruit were evaluated after storage for firmness, soluble solids, and titratable acidity. Accumulation of ethanol and acetaldehyde as a result of CA storage was monitored.

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Elizabeth J. Mitcham and Roy E. McDonald

Mature green mango (Mangifera indica L.) fruit were heated (100% RH) at 50C for 120, 180 or 240 min or 46C for 160, 220 or 280 min. The rate of mesocarp color (CIE a*) development was reduced in treated fruit, particularly in inner tissue. Rate of softening of mesocarp tissue was reduced after heat treatment; inner more than outer. Fruit treated at 50C remained more firm than control fruit 9 days after treatment, whereas fruit treated at 46C were more firm than controls 3 days after treatment, but were similar by 9 days. Electrolyte leakage from inner mesocarp tissue disks increased with increasing time at 50C, but was unchanged in fruit treated at 46C. However, after 3 days, electrolyte leakage returned to control levels. Ethylene-forming enzyme (EFE) activity of inner meso-carp tissue was greatly reduced in fruit treated at 50C (all times), and at 46C (220 and 280 min). After 3 days, EFE activity of fruit from most treatments had recovered to levels higher than controls. These data indicate that fruit may be able to recover from heat stress. Mild heat stress may increase postharvest shelf life by reducing the rate of softening.

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Elizabeth J. Mitcham and Roy E. McDonald

`Keitt' and `Tommy Atkins' mango (Mangifera indica L.) fruit were evaluated for selected ripening criteria at six ripening stages, from mature green to overripe. `Tommy Atkins' mangos developed more red and yellow pigmentation (CIE a* and b*) in peel and mesocarp tissues than `Keitt'. The outer mesocarp of `Keitt' remained firm longer than `Tommy Atkins', and the inner mesocarp was softer than the outer at each stage in both cultivars. Cell wall neutral sugars, particularly arabinosyl, rhamnosyl, and galactosyl residues, decreased with ripening in both cultivars. `Keitt' had more loosely associated, chelator-soluble pectin, accumulated more soluble polyuronides, and retained more total pectin at the ripe stage than `Tommy Atkins'. Both cultivars had similar polygalacturonase (EC 3.2.1.15) activity which increased with ripening. The amount and molecular weight of cell wall hemicellulose decreased with ripening in both cultivars. These data indicate that enzymatic and/or nonenzymatic processes, in addition to polygalacturonase activity, are involved in the extensive softening of mango fruit.

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

The performance of three relatively new nondestructive cherry firmness devices and a penetrometer were evaluated and compared with the firmness testing performance of an Instron Universal Testing Machine. The inherent variability of the nondestructive devices was estimated by repeated measurement of a uniform, symmetrical, and resilient rubber ball. Analysis of residuals from correlations between each device and the Instron from firmness testing on common samples of sweet cherries (Prunus avium L.) confirmed the relative variability of the nondestructive devices, and estimated measurement reliability of the penetrometer. Subjective firmness sensing by compression of cherries between the fingers of human evaluators proved to be less reliable than the devices tested. Sweet cherry firmness correlated reasonably well with skin color, with the strength of the correlations from each device agreeing with device ranking in terms of firmness measurement reliability. Firmness correlated poorly with soluble solids, titratable acidity, and specific gravity; soluble solids correlated well with specific gravity; and skin color correlated reasonably well with both soluble solids and specific gravity. Fruit surface pit volume, induced by a specific impact force from a ball bearing, correlated relatively well with fruit firmness measured by the penetrometer, but poorly or inconsistently with measurements from the remaining firmness devices.

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H. Ahmadi, W.V. Biasi, and E.J. Mitcham

Effects of short-term exposure to a 15% CO2 atmosphere on nectarines [Prunus persica (L.) Batsch (Nectarine Group) `Summer Red'] inoculated with Monilinia fructicola (Wint.) Honey (causal agent of brown rot) were investigated. Nectarines were inoculated with spores of M. fructicola and incubated at 20 °C for 24, 48 or 72 hours and then transferred to storage in either air or air enriched with 15% CO2 at 5 °C. Fruit were removed from storage after 5 and 16 days and were examined for brown rot decay immediately and after ripening in air for 3 days at 20 °C. Noninoculated nectarines were stored and treated likewise for evaluation of postharvest fruit attributes to determine their tolerance to 15% CO2. Incubation period after inoculation, storage duration, and storage atmosphere had highly significant effects on fruit decay. `Summer Red' nectarines tolerated a 15% CO2 atmosphere for 16 days at 5 °C. Development of brown rot decay in fruit inoculated 24 hours before 5 or 16 days storage in 15% CO2 at 5 °C was arrested. After 3 days ripening in air at 20 °C, the progression of brown rot disease was rapid in all inoculated nectarines, demonstrating the fungistatic effect of 15% CO2. The quantity of fungal cell wall materials (estimated by glucosamine concentration) was compared to visual estimation of decayed area and visual rating of fungal sporulation. The glucosamine assay defined the onset and progress of brown rot infection more precisely than either of the two visual tests.

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W.R. Miller, E.J. Mitcham, R.E. McDonald, and J.R. King

Postharvest quality of `Climax' rabbiteye blueberries (Vaccinium ashei Read) was evaluated after exposure to dosages of 0, 0.75, 1.5, 2.25, or 3.0 kGy gamma irradiation (0.118 kGy·min-1) and after subsequent storage. Irradiation did not affect weight loss, but irradiated berries were softer than nontreated berries. There was also a trend toward increased decay as dose increased. Irradiation had no effect on powdery bloom or surface color; total soluble solids concentration, acidity, and pH were affected slightly. Flavor preference was highest for nonirradiated berries and generally declined as dosage increased. Irradiation at 2.25 and 3.0 kGy resulted in increased levels of xylosyl residues in cell walls, and xylosyl residues were the most abundant cell-wall neutral sugar detected in blueberries. There was no evidence of cell wall pectin loss in irradiated berries. Irradiation at 21.5 kGy lowered the quality of fresh-market `Climax' blueberries.

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Maria E. Monzon, Bill Biasi, Elizabeth J. Mitcham, Shaojin Wang, Juming Tang, and Guy J. Hallman

The external and internal quality of ‘Fuyu’ persimmon fruit (Diospyros kaki L.) was evaluated after heating with radiofrequency (RF) energy to 48, 50, or 52 °C, holding at the target temperatures for durations ranging from 0.5 to 18 minutes, hydrocooling, and ripening at 20 °C for 12 days. These treatment conditions were identified for control of third instar Mexican fruit fly larvae (Anastrepha ludens). The treatments had no commercially significant effect on firmness, soluble solids content, titratable acidity, or weight loss of the fruit. RF-treated persimmon fruit attained a deeper orange–red skin color than control fruit. There was a greater incidence of slight to moderate flesh browning in fruit heated to 50 and 52 °C as compared with 48 °C. Calyx browning increased slightly in all RF-treated fruit and was the highest in the longer treatments at each temperature. Heating persimmon fruit with RF to 48 °C and then holding for 6 or 12 minutes showed the least damage, and the latter treatment was longer than should be required for a quarantine treatment against the third instar Mexican fruit fly. Holding persimmons for 6.6 minutes at 48 °C should provide control of the Mexican fruit fly and maintain fruit quality. Confirmation tests with infested fruit should be conducted.

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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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Kenneth A. Shackel, H. Ahmadi, W. Biasi, R. Buchner, D. Goldhamer, S. Gurusinghe, J. Hasey, D. Kester, B. Krueger, B. Lampinen, G. McGourty, W. Micke, E. Mitcham, B. Olson, K. Pelletrau, H. Philips, D. Ramos, L. Schwankl, S. Sibbett, R. Snyder, S. Southwick, M. Stevenson, M. Thorpe, S. Weinbaum, and J. Yeager

To be useful for indicating plant water needs, any measure of plant stress should be closely related to some of the known short- and medium-term plant stress responses, such as stomatal closure and reduced rates of expansive growth. Midday stem water potential has proven to be a useful index of stress in a number of fruit tree species. Day-to-day fluctuations in stem water potential under well-irrigated conditions are well correlated with midday vapor-pressure deficit, and, hence, a nonstressed baseline can be predicted. Measuring stem water potential helped explain the results of a 3-year deficit irrigation study in mature prunes, which showed that deficit irrigation could have either positive or negative impacts on tree productivity, depending on soil conditions. Mild to moderate water stress was economically beneficial. In almond, stem water potential was closely related to overall tree growth as measured by increases in trunk cross-sectional area. In cherry, stem water potential was correlated with leaf stomatal conductance and rates of shoot growth, with shoot growth essentially stopping once stem water potential dropped to between −1.5 to −1.7 MPa. In pear, fruit size and other fruit quality attributes (soluble solids, color) were all closely associated with stem water potential. In many of these field studies, systematic tree-to-tree differences in water status were large enough to obscure irrigation treatment effects. Hence, in the absence of a plant-based measure of water stress, it may be difficult to determine whether the lack of an irrigation treatment effect indicates the lack of a physiological response to plant water status, or rather is due to treatment ineffectiveness in influencing plant water status. These data indicate that stem water potential can be used to quantify stress reliably and guide irrigation decisions on a site-specific basis.