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M.L. Arpaia

Although postharvest handling schemes have improved during recent years, it is still possible to observe considerable variability in fruit quality between individual lots. Preharvest factors such as irrigation, nutrition and pest management practices, as well as rootstock and environmental variables, may greatly influence quality after harvest and may well account for some of the differences between individual lots. The influence of preharvest factors on postharvest quality of tropical and subtropical fruit will be discussed using pertinent examples from the literature. Emphasis will be given to those factors which can be manipulated to improve quality.

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M. L. Gottlieb and M. L. Arpaia

A major outlet for California Valencia orange fruit is the export market. Transit time to Pacific Rim markets varies from ca. 20 to 40 days. This coupled with delays in packing and cooling ranging from 1 to 3 days may negatively affect fruit quality at the destination market. A study was conducted which examined cooling/packing delays (6, 24, 48, 72 hrs), storage temperature (5, 11C) and duration (3, 6 wks) following packing to evaluate the effect of these factors on the postharvest quality of Valencia orange. The following parameters were monitored: peel penetration force, peel color (L*, Chroma, Hueo), weight loss, external appearance (0-5), decay, and changes in carton pack height. Weight loss prior to packing was directly related to the duration of the prepack delay. This relationship, however, did not continue through storage and simulated marketing. Fruit subjected to delayed packing, however, had lower pack heights and higher external ratings following storage. Storage at 11C resulted in decreased penetration force, higher levels of decay and greater color development and weight loss. Storage for 6 weeks resulted in decreased penetration force, higher weight loss and greater color development.

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S. L. Ontai, M. L. Arpaia, and J. S. Reints Jr.

In southern California, avocados are often left in the field for up to 12 hours after harvest. Fruit in the bin may reach up to 40C during the summer months and may take up to 24 hours to cool to the recommended storage temperature. A study was conducted using `Hass' avocados over two growing season during the months of July and August to determine the effect of delayed cooling on fruit quality. Fruit were held at 20, 30 or 40 C for 0, 6, 12 or 24 hours before storage at 5C for 0, 2, 4, or 6 weeks. Fruit quality was determined by flesh firmness, time to ripe, vascular and flesh discoloration and the presence or absence of decay. The level of damage seen in storage varied with the harvest. Overall, after 4 or 6 weeks in storage, there was a considerable increase in either vascular or flesh discoloration and decay especially when fruit had been held at 30 or 40C prior to storage. The results indicate that harvested avocados should be kept as cool as possible in the field and that fruit should be processed within 12 hours for storage periods greater than 2 weeks.

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P. W. Robinson, M. V. Mickelbart, and M. L. Arpaia

Measurements of flowering, yield, vegetative and root growth were begun in Spring 1992 to establish a phenological model for `Hass' avocado. Although rootstock (Thomas', Topa Topa', Duke 7' and `D9') did not affect the timing or intensity of bloom, differences in yield were observed. Flowering occurred in March - April in both years, although the intensity of bloom in 1993 was drastically reduced due to an extremely heavy 1992-93 crop. Vegetative flushes occurred in April (following bloom) and July in both years. In 1993, however, cumulative growth was ca. 10-fold greater. Rootstock did not affect the timing or intensity of vegetative growth in either year. In both years, vegetative growth preceded root growth. In 1992, there were no differences detected in the timing or intensity of root growth related to rootstock. In 1993, however, the `Topa Topa' rootstock produced more roots throughout the growing season. The timing and intensity of root growth during the spring flush were similar in both years. During Fall 1993, root growth rates, however, were consistently higher than those observed in 1992. Additionally, while root growth ceased in November 1992, roots have continued to grow through January 1994.

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M.L. Arpaia, G.S. Bender, and G.W. Witney

A project evaluating the performance of cv. Hass on eight clonal avocado rootstocks—G755A, G755B, G755C, Duke 7 (D7), Borchard (BR), D9, Toro Canyon, and Topa Topa was established in southern California in 1986. Two additional rootstocks, Thomas and G1033, were added in 1987. Of the trees planted in 1986, the BR and D7 rootstocks have consistently had the highest total yields for all rootstocks, whereas the three G755 selections have had the lowest productivity. No differences in productivity between the two rootstocks planted in 1987 have been detected. The influence of rootstock on the magnitude of alternate bearing will be discussed, although the oscillation in yield is greater for the higher-yielding rootstocks. Tree size has been measured throughout the study. The BR selection has consistently produced a larger tree, even though it has continued to have high productivity. There are no consistent differences between the other rootstocks. Yield efficiency, measured as the kg fruit/m3 of canopy volume has been calculated. In selections that are prone to severe alternate bearing, the swing in yield efficiency is also the greatest. The data thus far suggests that a yield efficiency of ≈2.5 kg fruit/m3 canopy volume is the maximum yield possible for California `Hass' avocado.

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X. Liu, P. Robinson, M.L. Arpaia, and G.W. Witney

Monthly samples were taken from 9-year-old `Hass' avocado trees on Duke 7 rootstock grown at the UC Southcoast Research and Extension Center in Irvine, Calif. Changes in starch and total soluble sugars were monitored from fine and coarse roots, trunk (above the bud union), small diameter stems, leaves, and fruit. When possible, seasonal carbohydrate changes were compared to root and shoot flushing patterns. In all of the vegetative plant organs monitored, total soluble sugars accounted for most of the carbohydrate. Starch accounted for ≈10% of the sample dry weight, whereas the total soluble sugars accounted for ≈18%. D-mannoheptulose and perseitol, both C7 sugars, were the predominant soluble sugars throughout the year. Fructose, glucose, and sucrose accounted for <5% of the total soluble sugars. During fruit development, soluble sugar content of the exo- and mesocarp tissues >25% of the dry weight. The significance of these findings will be discussed in relationship to tree phenology and carbohydrate partitioning.

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James D. Oster, D.E. Stottlmyer, and M.L. Arpaia

A field experiment was conducted between 1992 and 1997 in a commercial orchard of mature ‘Hass’ avocados on Mexican seedling rootstock (Persea americana Mill.) to determine how yield was influenced by the amount of irrigation water applied and the frequency of application. Three amounts of water (targeted at 90%, 110%, and 130% of estimated crop evapotranspiration) were applied at three frequencies (one, twice, and seven times per week) with microsprinklers located beneath the tree canopy. The site was set up as a randomized complete block design with six blocks, each including one replicate of all irrigation treatments. One or two trees located at the center of the replicates were used to measure yields and tree size, and as the locations where samples of soil and soil water were obtained for analysis from beneath the tree canopy. The average electrical conductivity and chloride concentration of the irrigation water, corrected for rain, were 0.7 dS·m−1 and 1.8 mmol·L−1, respectively. From May 1994 to Nov. 1996, salinity of the saturated-paste extracts of soil samples obtained in the 0- to 120-cm depth interval averaged ≈2 dS·m−1 for all irrigation treatments. Irrigation treatments also had little influence on the maximum soil-water salinity, ≈4 dS·m−1, in and below the lower portion of the root zone. Consequently, irrigation treatments had little influence on the fraction of applied water that was not used by the crop, the leaching fraction. Chloride concentrations in leaves were affected by applied water but did not attain levels that are associated with leaf injury. Trees irrigated seven times per week had lower yields than trees that received less frequent irrigation. During the last 2 years of the experiment, when yields no longer increased with time, the yields for treatments irrigated once and twice per week increased with increasing amounts of applied water. We were able to explain the influence of both amount of applied water and soil salinity on avocado yields and leaching fraction using production function concepts. Yields increased with increasing amounts of applied water because of increased water availability for crop use before a soil-water salinity of ≈4 dS·m−1 restricted water uptake. The threshold salinity above which yield decline occurred was determined to be 0.57 dS·m−1 and yield declined by 65% per unit of salinity above the threshold. Our results suggest that maximum yields of ‘Hass’ avocado on Mexican seedling rootstock are not achievable when the average annual salinity of irrigation water, including rainfall, is greater than ≈0.6 dS·m−1.

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M. L Arpaia, L. G. Houck, P. Hartsell, S. L. Ontai, and J. S. Reints Jr.

Postharvest quarantine treatments of methyl bromide fumigation (MB) or a combination of MB and cold storage are allowed for the Mediterranean Fruit Fly (Ceratitis capitata) and other fruit flies. A study was undertaken to address the effect of MB on the fruit quality of `Hass' avocados. Fruit were obtained from two growers at a commercial packinghouse three times during the 1991 season. Fruit were fumigated at 21C or 30 C within 24 hours after harvest or after 1 week of storage. Fruit were evaluated after 0, 1, 2 or 4 weeks of storage at 5 C. Fruit quality was determined by flesh firmness, days to ripe, ease of peeling, weight loss, external discoloration, flesh or vascular discoloration and the presence/absence of decay. There was considerable variability between grower lots, however fruit that were fumigated had higher levels of weight loss, vascular or flesh discoloration and decay after 4 weeks of storage. The timing of fumigation had little effect on fruit quality. Generally, fruit which were fumigated at 30 C had less damage. These results suggest that `Hass' avocado could withstand MB as long as the fruit is marketed within 2 weeks of harvest.

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D. E. Stottlemyer, M. L. Arpaia, J. L. Meyer, G. W. Witney, and G. S. Bender

The influence of three irrigation treatments on flowering, yield, tree growth, root distribution, and leaf analysis of mature `Hass' avocado (Persea americana Mill.) was investigated over a six year period (1987-1992). Three irrigation treatments; 60, 80, and 100% of evapotranspiration (ETc) were applied using low-volume spray emitters. The differential irrigation treatments were maintained year round. Irrigation treatments did not affect the timing or intensity of bloom. Yield data from years 2-6 show a significant irrigation effect on cumulative weight and total number of fruit per tree. Trees receiving 100% ETc had higher yield/tree. This increased yield was due both to increased fruit numbers and individual fruit weight per tree. Tree growth was also significantly impacted by the irrigation treatments. Trees receiving 100% ETc exhibited the greatest amount of vegetative growth over the study. Yield efficiency (Kg fruit/m3 canopy) was not influenced by irrigation treatment. Irrigation treatment did not significantly influence nutrient analysis taken in the fall of each year.

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N.G. Beck, M.L. Arpaia, J.S. Reints Jr., and E.M. Lord

Deformations consisting of longitudinal ridges in the rind of Citrus fruits have recently been found in Southern California Citrus groves. Here, we report the correlation between ridge formation and applications of chlorpyrifos (Lorsban, Dow Chemical Company, Midland, MI) during the feather-growth stage of bud break. All chlorpyrifos formulations resulted in significant ridging. Addition of agricultural oil and 2,4-D (2,4-dichlorophenoxyacetic acid (2,4-D) to chlorpyrifos resulted in the greatest ridging damage and widened the window of susceptibility by 2 weeks in 1988. In 1989, no significant difference was seen between treatments of chlorpyrifos, although all were significantly greater than the control. The susceptible stages of bud growth are described, as are the non-susceptible stages which precede and follow it. Floral buds in which carpels are initiating are susceptible to fruit ridging upon application with chlorpyrifos. These ridges are the result of an increase in cell size of the flavedo tissue which may be the result of a polyploid chimera.