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  • Author or Editor: T. G. McCollum x
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Decay caused by fungal pathogens accounts for significant postharvest losses. Although the application of synthetic fungicides can reduce postharvest decay, increasing public concern over using fungicides as well as the resistance that develops to them indicate that alternative means of decay control are needed. Freedom from disease before harvest is the norm rather than the exception. Numerous defense mechanisms, both preformed and inducible, are involved in plant resistance to fungal pathogens. Understanding how natural defense mechanisms are regulated and how to maintain them in harvested products may provide the basis for new strategies to reduce postharvest losses caused by pathogens. Host–pathogen interactions have been well studied in growing plants but much less extensively in harvested organs. The interaction between host and pathogen is dynamic; changes in both organisms are required for disease development. Following harvest, the incidence of decay increases indicating that changes in the host render it more susceptible to pathogen development. Recent studies by plant physiologists and pathologists have contributed to our understanding of changes in harvested tissues that render them less resistant to decay as well as changes in the host that are induced in response to fungal infection.

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Summer squash (Cucurbita pepo L.) fruit were harvested 8 ± 1 day after anthesis and held at 2 or 15C. Samples were transferred from 2 to 15C after 3, 6, 9, or 12 days of storage. Respiration and ethylene production were measured 24 and 48 hr after fruit had been transferred to 15C. Electrolyte leakage from pericarp disks was determined 48 hr after transfer to 15C. Weight loss during storage was greater at 15 than at 2C; however, chilled fruit had a greater rate of weight loss after transfer to 15C than did nonchilled fruit. Surface pitting became obvious after 6 days at 2C and was progressively more severe with increased duration of chilling. Respiration rates of nonchilled fruit decreased with increased storage duration, whereas respiration rates of chilled fruit increased with increased storage duration. Ethylene production was <0.1 nl g-1hr-1 in nonchilled fruit. Ethylene production was enhanced in chilled fruit and increased with increased duration of chilling to a peak of 1.5 nl-g-1-hr-1 after 9 days at 2C, then decreased. Electrolyte leakage was not influenced by chilling.

Open Access

Storage of `Marsh' white seedless grapefruit (Citrus paradisi Macf.) for 2 weeks at 5C resulted in the development of chilling injury (CI). Electrolyte leakage from chilled fruit did not increase significantly until CI had become severe, and was therefore considered to be a poor index of CI. In contrast to electrolyte leakage, respiration and ethylene evolution were consistently higher in chilled than in nonchilled fruit, even prior to the onset of visual symptoms of CI. Respiratory rates ranged from 8.0 to 10.7 and 4.6 to 6.7 ml/kg/hr in chilled and nonchilled fruit, respectively. Ethylene evolution was not detected from nonchilled fruit, whereas chilled fruit produced from 45.6 to 249.3 ml/kg/hr ethylene. Ethylene production was maximum following 2 weeks at 5C. Results of this study indicate that increases in electrolyte leakage do not occur until considerable tissue damage has occurred, whereas stimulation of respiration and ethylene evolution occur early in the development of CI.

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Storage of `Marsh' white seedless grapefruit (Citrus paradisi Macf.) for 2 weeks at 5C resulted in the development of chilling injury (CI). Electrolyte leakage from chilled fruit did not increase significantly until CI had become severe, and was therefore considered to be of limited value as an early indicator of CI. In contrast to electrolyte leakage, respiration and ethylene evolution were significantly higher in chilled than in nonchilled fruit, even before the onset of visual symptoms of CI. Respiration rates ranged from ≈8 to 11 and 5 to 7 ml CO2/kg per hour in chilled and nonchilled fruit, respectively. Ethylene evolution was not detected from nonchilled fruit, whereas chilled fruit produced from 45 to 250 nl ethylene/kg per hour. Results of this study indicate that electrolyte leakage does not increase until visible pitting of the flavedo has occurred, whereas stimulation of respiration and ethylene evolution occur early in the development of CI.

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Grapefruit (Citrus paradisi) flavedo is a rich source of peroxidase (POD) (EC 1.11.1.7). Changes in POD have been related to senesence and environmental stress in a variety of plant tissues. However, due to the large number of POD isoenzymes as well as the broad substrate specificity, measurement of POD activity in crude extracts is of limited value for gaining an understanding of the role of POD in vivo. We have begun to purify and characterize POD isoenzymes from grapefruit flavedo. HPLC gel permeation chromatography reveals 2 peaks of POD activity with apparent MW of 66 kD and 30 kD. Native PAGE (8% bis-acrylamide, pH 8.8) followed by activity staining indicates that the PODs differ in Pi; the 30 kD POD migrates anodally, whereas the 66 kD POD does not migrate. Isoelectric focusing has been used to separate flavedo PODs into acid (Pi ca 4.0) and basic (Pi > 8.5) forms. Treatment of grapefruit with ethylene (2 ppm 72 hours) induces a basic POD not present in freshly-harvested fruit or in nonethylene-treated controls.

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The diseases huanglongbing [HLB, associated with Candidatus Liberibacter asiaticus (CLas)] and Asian citrus canker [ACC, caused by Xanthomonas citri (Xcc)] are widespread in Florida and many other citrus-growing areas, presenting unprecedented challenges for citrus breeding. Because HLB and ACC weaken trees and compromise cropping, breeding is much less efficient using seed parents that have been exposed to these diseases. Therefore, it would be highly desirable to use unique disease-exposed selections only as pollen parents with pollen applied to disease-free trees. However, there may be a risk of introducing these diseases using such pollen sources. To assess this potential, abundance of the pathogens associated with these diseases was assessed in anthers and flowers using quantitative polymerase chain reaction. Because CLas is systemic, levels on mature leaves from the flower source trees were assessed to see if the presence of CLas in flowers was associated with leaf levels. Disease-exposed trees were tested in 10 genotypes from each of three broad genotypic categories, which reflect different levels of susceptibility to the diseases associated with the pathogens studied: Poncirus trifoliata hybrids (most resistant to HLB), Citrus maxima and hybrids (susceptible to both diseases), and C. reticulata and hybrids (considerable resistance to ACC). Of the 30 samples of each tissue type analyzed for CLas, 88% of mature leaves, 69% of flowers, and 88% of anthers had one or more CLas bacterium per sample. The trifoliate genotypic group had significantly lower levels of CLas than the pummelo and mandarin groups in mature leaf samples, but CLas levels were more similar between groups in anther and flower samples, and the pathogen was present in most of the trifoliate hybrids tested. Mean numbers of CLas detected per nanogram nucleic acid were 100 to 800 times higher in mature leaf samples, most characteristic of HLB symptoms, compared with anther samples. Xcc DNA was detected in 30% of flower samples and 23% of anther samples. No significant differences in Xcc levels were found between tissue type or genotypic group. However, regressions between Xcc levels in flowers and percent of plant pedigree derived from mandarin had a negative correlation and an r 2 of 0.159 (P = 0.029). The biology of CLas is consistent with the pathogen being present in anthers from unopened flowers, whereas the ACC pathogen detected inside flowers was likely the result of contamination despite great care in sample collection and handling. Where exceptional diligence to exclude HLB and ACC is appropriate, results suggest that there may be a risk of spreading these pathogens through use of pollen from trees on infected farms.

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The fungicides thiabendazole (TBZ) or imazalil were applied at 1 g·liter-1 at 24 or 53C to `Marsh' and `Redblush' grapefruit (Citrus paradis i Macf.) to reduce fruit susceptibility to chilling injury (CI) and decay. There was more CI and decay on `Marsh' grapefruit than on `Redblush'. CI was found to be lower in grapefruit that had been dipped at 53C than at 24C. CI was higher after water dips without fungicide. Imazalil was found to be more effective in reducing CI than TBZ. Fungicides reduced decay at both temperatures, and imazalil was better than TBZ. Results of this study confirm the benefits of high-temperature fungicide treatments for maintaining grapefruit quality and indicate some benefits of high-temperature fungicide treatments for reducing CI.

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Mature green `Sunbeam' tomato fruit (Lycopersicon esculentum Mill.) were treated in water for 1 hr at 27 (ambient), 39, 42, 45, or 48°C, and then either ripened at 20°C (nonchilled) or stored at 2°C (chilled) for 14 days before ripening at 20°C. The most-effective heat treatment was 42°C, which reduced decay 67% in chilled fruit and 53% in nonchilled fruit. Heat treatment had no effect on time required to ripen the fruit. Red-ripe tomatoes had higher respiration rates and evolved more ethylene following nonchilling storage, but heat treatment had no effect on respiration or ethylene evolution. Red color development was enhanced by heat treatment, and inhibited by chilling. At red ripe, fruit were firmer as a result of storage at the chilling temperature, while heat treatment had no effect on firmness. Heat-treated fruit were preferred in terms of taste and texture over nontreated fruit in informal taste tests, with the exception of the 45°C treatment. With increasing temperature of heat treatment, there was increased electrolyte leakage following chilling storage. Of the 15 flavor volatiles analyzed, the levels of five were decreased with increasing temperature of heat treatment. Storage at the chilling temperature reduced the levels of six flavor volatiles. Prestorage heat treatments can reduce decay with only minimal adverse effects on tomato fruit quality.

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Mature, green tomatoes were either gassed or not gassed with C2H4 for 24 h, immersed in 42C water for 60 min, or held in 38C air for 48 h or not treated, and then stored at either 2C or 13C for 14 days before ripening at 20C. During ripening, the fruit were evaluated for color development, internal quality, and decay and for volatiles when full ripe. Both high-temperature treatments reduced chilling injury and inhibited decay. Days to ripen after removal from storage at 2C or 13C was not influenced by heat treatment method. Color development, lycopene content, and internal quality characteristics of fruit were similar at the ripe stage, irrespective of heat treatment. Of 15 volatiles analyzed, seven showed decreased levels of concentrations as a result of C2H4 gassing, nine showed decreased levels when stored at 2C prior to ripening, and most were unaffected by the heat treatments. Heat treatments appear to be beneficial for maintaining tomato fruit quality.

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