Juan Carlos Díaz-Pérez

Surface and internalized Escherichia coli O157:H7 on field-grown spinach and lettuce treated with spray-contaminated irrigation water.

Numerous field studies have revealed that irrigation water can contaminate the surface of plants; however, the occurrence of pathogen internalization is unclear. This study was conducted to determine the sites of Escherichia coli O157:H7 contamination and its survival when the bacteria were applied through spray irrigation water to either field-grown spinach or lettuce. To differentiate internalized and surface populations, leaves were treated with a surface disinfectant wash before the tissue was ground for analysis of E. coli O157:H7 by direct plate count or enrichment culture. Irrigation water containing E. coli O157:H7 at 10(2), 10(4), or 10(6) CFU/ml was applied to spinach 48 and 69 days after transplantation of seedlings into fields. E. coli O157:H7 was initially detected after application on the surface of plants dosed at 10(4) CFU/ml (4 of 20 samples) and both on the surface (17 of 20 samples) and internally (5 of 20 samples) of plants dosed at 10(6) CFU/ml. Seven days postspraying, all spinach leaves tested negative for surface or internal contamination. In a subsequent study, irrigation water containing E. coli O157:H7 at 10(8) CFU/ml was sprayed onto either the abaxial (lower) or adaxial (upper) side of leaves of field-grown lettuce under sunny or shaded conditions. E. coli O157:H7 was detectable on the leaf surface 27 days postspraying, but survival was higher on leaves sprayed on the abaxial side than on leaves sprayed on the adaxial side. Internalization of E. coli O157:H7 into lettuce leaves also occurred with greater persistence in leaves sprayed on the abaxial side (up to 14 days) than in leaves sprayed on the adaxial side (2 days).

Infrequent Internalization of Escherichia coli O157:H7 into Field-Grown Leafy Greens

Several sources of contamination of fresh produce by Escherichia coli O157:H7 (O157) have been identified and include contaminated irrigation water and improperly composted animal waste; however, field studies evaluating the potential for internalization of O157 into leafy greens from these sources have not been conducted. Irrigation water inoculated with green fluorescent plasmid-labeled Shiga toxin-negative strains (50 ml of 10(2), 10(4), or 10(6) CFU of O157 per ml) was applied to soil at the base of spinach plants of different maturities in one field trial. In a second trial, contaminated compost (1.8 kg of 10(3) or 10(5) CFU of O157 per g) was applied to field plots (0.25 by 3.0 m) prior to transplantation of spinach, lettuce, or parsley plants. E. coli O157:H7 persisted in the soil up to harvest (day 76 posttransplantation) following application of contaminated irrigation water; however, internalized O157 was not detected in any spinach leaves or in roots exposed to O157 during the early or late growing season. Internalized O157 was detected in root samples collected 7 days after plants were contaminated in mid-season, with 5 of 30 samples testing positive for O157 by enrichment; however, O157 was not detected by enrichment in surface-disinfected roots on days 14 or 22. Roots and leaves from transplanted spinach, lettuce, and parsley did not internalize O157 for up to 50 days in the second trial. These results indicate that internalization of O157 via plant roots in the field is rare and when it does occur, O157 does not persist 7 days later.

Growth and yield of muskmelon in response to plastic mulch and row covers

The objective of this study was to determine the effect of plastic mulch alone or in combination with row covers on plant biomass, growth analysis parameters, and yield of muskmelon plants (Cucumis melo L.). The design was a randomized complete block with four replications. The treatments were: (1) no mulch, no row cover (control); (2) black plastic mulch (BPM); (3) BPM plus row cover removed 10 days after seeding (das) (BPM+CE); (4) BPM plus row cover removed 20 das (BPM+CI); (5) BPM plus row cover removed 32 das (BPM+CL). Plants grown using soil mulch plus row cover presented higher values of plant biomass, specific leaf area (SLA), relative growth rate (RGR), and net assimilation rate (NAR) compared to control plants. Early and total yield were highest in plants grown under row covers. In non-covered plants, early and total yield were both higher in BPM plants relative to the control. Heat accumulation by the crop measured as soil degree-days showed a higher correlation with early and total yield compared to heat accumulation measured as air degree-days.

Transpiration rates in eggplant fruit as affected by fruit and calyx size

Abstract Fruit transpiration was measured on eggplants of a wide range of sizes. The separate contributions of the skin and calyx to whole-fruit (skin plus calyx) transpiration were determined. The calyx was the main route for fruit water loss, accounting for at least 60% of fruit transpiration. Fruit transpiration rate declined as eggplants increased in size. This decline in fruit transpiration was due to both a reduction in the portion of the fruit covered by the calyx and a reduction in the surface area/mass ratio of the fruit as they increased in size. The calyx covered 95 and 10% of fruit surface area in a 5 and 550 g fruit, respectively. The high rate of water loss through the calyx indicates that postharvest treatments intended to reduce water loss from the calyx may be beneficial in extending shelf life in eggplant fruit.

Quality changes in sapote mamey fruit during ripening and storage

Abstract Physical and chemical changes in sapote mamey ( Pouteria sapota (Jacq.) H.E. Moore and Stearn) fruit during ripening and storage at various temperatures were evaluated. Ripening was associated with flesh softening, an increase in soluble solids content (SSC), and a change in flesh color from yellow or pale pink to a dark pink or red. No changes in fruit skin color or in flesh acidity were observed as ripening progressed. Ripe fruit had 30% or higher SSC, orange or red flesh (hue angle=52; chroma=45; L =60), acidity of 6–8 mM H + , and flesh firmness (compression force) ≤50 N. Flesh turned brown ( L * value declined) in overripe fruit. Fruit held at 27, 25, or 20°C ripened in 3.5, 5 or 7 days after harvest, respectively. Fruit kept at 10°C showed minor changes in color and firmness and a slow rate of SSC increase. Fruit stored at 10 or 15°C and then ripened at 20°C had portions of the flesh with a much higher firmness and poorer development of red color compared to other parts of the fruit. This uneven ripening was probably a result of chilling injury. The number of fruit with injury was higher at 10°C than at 15°C, and increased with storage time. The rates of fruit weight loss relative to the initial fruit weight were 0.58, 0.98 and 1.83% d −1 at 10, 20 and 27°C, respectively.

Ripening in papaya fruit is altered by ACC oxidase cosuppression

Papaya (Carica papaya) is a very important crop in many tropical countries but it is highly susceptible to parasitic diseases, physiological disorders, mechanical damage and fruit overripening. Here we report a study on ACC oxidase cosuppression and its effects on papaya fruit ripening. Papaya ACC oxidase was isolated using PCR and embriogenic cells transformed by biolistic using the CaMV 35S promoter to drive the expression of the PCR fragment in sense orientation. Fifty transgenic lines were recovered and 20 of those were grown under field conditions. Southern analysis showed incorporation of the transgene in different copy numbers in the papaya genome. Fruits were evaluated in terms of texture (firmness), colour development, respiration and ethylene production. A sharp reduction in ethylene and CO2 production was detected, whereas softening and colour development of the peel were also altered. Overall, transgenic fruits showed a delay in ripening rate. A reduction in mRNA level for ACC oxidase in transgenic fruit was clearly detectable by northern blot. More studies are necessary before this technology can be used to extend the shelf life of papaya fruit.

Evaluation of the fungicidal properties of plant extracts to reduce Rhizopus stolonifer of ‘ciruela’ fruit (Spondias purpurea L.) during storage

Rhizopus stolonifer is one of the main postharvest pathogens of ‘ciruela’ fruit (red mombin) (Spondias purpurea L.) during handling and storage. To evaluate the fungicidal potential of plants indigenous to the state of Morelos, Mexico, aqueous extracts of leaves or stems of 19 different plant species were tested against Rhizopus development in vitro and in vivo. Extracts were applied to fruit of three botanical varieties of ciruela: fruit skin turning green to red, green to yellow or green to orange, grown throughout the year over dry and wet seasons. In vitro evaluations were carried out to observe mycelial growth, sporulation and conidial germination. Evaluations on fruit were percentage and disease severity, soluble solids content (SSC) and weight loss after 4 days storage at ambient temperature. In general, leaf extracts had better fungicidal effects than stem extracts. For in vitro studies, leaf extracts inhibited sporulation and spore germination more than mycelial development. Leaf extracts of Annona cherimola, Bromelia hemisphaerica and Carica papaya inhibited Rhizopus sporulation and rot development on the yellow variety whereas extracts of Casimiroa edulis reduced R. stolonifer rot on red ciruela. Infection spread from 25 to 100% of the fruit surface depending on extract or ciruela variety. After storage, SSC values were not significantly different. Less weight loss was observed for the orange variety than the other varieties. Further studies need to be undertaken to isolate the active compounds from those extracts with fungicidal potential.

Physical and chemical properties of pomegranate fruit accessions from Croatia

The objective was to evaluate physical and chemical properties of eight pomegranate accessions (seven cultivars and one wild genotype) collected from the Mediterranean region of Croatia. Accessions showed high variability in fruit weight and size, calyx and peel properties, number of arils per fruit, total aril weight, and aril and juice yield. Variables that define sweet taste, such as low total acidity (TA; 0.37-0.59%), high total soluble solids content (TSS; 12.5-15.0%) and their ratio (TSS/TA) were evaluated, and results generally aligned with sweetness classifications of the fruit. Pomegranate fruit had a high variability in total phenolic content (1985.6-2948.7 mg/L). HPLC-MALDI-TOF/MS analysis showed that accessions with dark red arils had the highest total anthocyanin content, with cyanidin 3-glucoside as the most abundant compound. Principal component analysis revealed great differences in fruit physical characteristics and chemical composition among pomegranate accessions.

Response of sapote mamey [Pouteria sapota (Jacq.) H.E. Moore&Stearn] fruit to hot water treatments

Abstract The aim of this study was to determine the effect of potential insect disinfestation hot water treatments (HWT) on sapote mamey fruit ripening and quality. The center of a 0.50 kg fruit took ∼45 min to reach the water temperature (40–60°C). Once the center of the fruit reached the water temperature, fruit were held for an additional 40–60 min in the water. After immersion, fruit were allowed to dry and were held in storage at 25°C for 4–6 days. Fruit treated at 40 or 45°C showed no significant changes in quality, while fruit treated at 50°C for 60 min ripened irregularly and had flesh with white and dark colorations. Fruit treated at 60°C for 60 min ripened more slowly and had no skin or flesh symptoms of heat injury, but had lower soluble solids and pH, higher firmness and slightly higher water loss than untreated fruit. The 60°C heated fruit also showed a lesser degree of flesh browning than untreated fruit. A panel of untrained judges found no significant difference in taste or aroma. Judges did give a higher score for the flesh color of fruit treated at 60°C compared with that of untreated fruit as there was less flesh browning and a firmer flesh relative to untreated fruit. Sapote mamey fruit subjected to 60°C HWT for 60 min had relatively minor effects on fruit quality and a delayed ripening. Hot water treatments may have potential as an insect disinfestation treatment for sapote mamey.

Absence of internalization of Escherichia coli O157:H7 into germinating tissue of field-grown leafy greens.

Both growth chamber and field studies were conducted to investigate the potential for Escherichia coli O157:H7 to be internalized into leafy green tissue when seeds were germinated in contaminated soil. Internalized E. coli O157:H7 was detected by enrichment in both spinach (Spinacia oleracea L.) and lettuce (Lactuca sativa L.) seedlings when seeds were germinated within the growth chamber in autoclaved and nonautoclaved soil, respectively, contaminated with E. coli O157:H7 at 2.0 and 3.8 log CFU/g, respectively. Internalized E. coli O157:H7 populations could be detected by enumeration within leafy green tissues either by increasing the pathogen levels in the soil or by autoclaving the soil. Attempts to maximize the exposure of seed to E. coli O157:H7 by increasing the mobility of the microbe either through soil with a higher moisture content or through directly soaking the seeds in an E. coli O157:H7 inoculum did not increase the degree of internalization. Based on responses obtained in growth chamber studies, internalization of E. coli O157:H7 surrogates (natural isolates of Shiga toxin-negative E. coli O157:H7 or recombinant [stx- and eae-negative] outbreak strains of E. coli O157:H7) occurred to a slightly lesser degree than did internalization of the virulent outbreak strains of E. coli O157:H7. The apparent lack of internalized E. coli O157:H7 when spinach and lettuce were germinated from seed in contaminated soil (ca. 3 to 5 log CFU/g) in the field and the limited occurrence of surface contamination on the seedlings suggest that competition from indigenous soil bacteria and environmental stresses were greater in the field than in the growth chamber. On the rare occasion that soil contamination with E. coli O157:H7 exceeded 5 log CFU/g in a commercial field, this pathogen probably would not be internalized into germinating leafy greens and/or would not still be present at the time of harvest.