Postharvest Practices for Managing the Quality of Longans and Rambutans

Research was conducted to integrate preharvest disease control methods and postharvest practices to manage diseases, improve fruit quality, and extend the shelf-life of longans and rambutans exported from Hawaii. The main pathogens of longan and rambutans were isolated and identified as Lasmenia, Colletotrichum, Pestalotiopsis and Phomopsis. In vitro sensitivity of these pathogens to registered fungicides was established, and Serenade (a patented strain of Bacillus subtilis) was 100% effective at all test concentrations. However, when Serenade was applied as a preharvest field treatment, it did not control postharvest diseases or improve quality of rambutans or longans. In postharvest studies, optimum storage temperatures and packaging systems were established. Package treatments included microperforated bags, clamshell containers, Peakfresh film, and Lifespan film. For longans, the sensory quality was maintained, overall disease incidence minimized, and shelf-life extended when microperforated packages or clamshell containers were stored at 10°C. The modified atmospheres (15% CO2 and 7% O2) inside Peakfresh packages adversely affected longan flavor. Rambutans stored in the clamshell, microperforated bag, or Peakfresh packages had higher visual quality ratings and lower disease incidences when stored at constant 10°C compared to simulated shipping temperatures. Rambutans held under simulated shipping temperatures in microperforated bags or clamshells had disease incidences that were 2-3 times higher than when stored at constant 10°C. Rambutans stored in Peakfresh packages had the best overall quality ratings and lowest disease incidence, but lowest flavor ratings. INTRODUCTION Longan (Dimocarpus longan) and rambutan (Nephelium lappaceum) are two of the most important specialty fruit crops grown in Hawaii. Increased export of these fruits to U.S. mainland markets is necessary to accommodate a rapid expansion in production. Consumer acceptance of these high value crops requires that fruit arrive at their final destination in excellent condition with minimal external and internal defects. Currently, postharvest diseases limit the successful export and marketing of specialty tropical fruit. In addition to postharvest disease symptoms, fresh rambutans dehydrate and skin color darkens during postharvest storage, limiting shelf-life (Landrigan et al., 1996). Longan pericarps also dry and darken in color. These factors may be minimized through modified atmosphere packaging (MAP) in which high humidity, high CO2 and low O2 levels are created inside semipermeable films through fruit respiration and transpiration (Kader et al., 1989). Enhanced CO2 (9-12%) can retard color loss and increase shelf-life by 3-4 days for rambutans (O’Hare et al., 1994). Although packaging reduces water loss (the main cause of spintern blackening for rambutans), the humid atmosphere can cause greater postharvest diseases, especially at higher temperatures that may occur during retail marketing. Knowledge of O2 and CO2 concentrations within a package is important because concentrations beyond those a [email protected] Proc. IS on Postharvest Pacifica 2009 Eds.: E.W. Hewett et al. Acta Hort. 880, ISHS 2010 474 tolerated by a commodity can induce physiological disorders or anaerobic respiration and off-flavor development (Kader et al., 1989). Optimum temperatures must be maintained during postharvest handling to maximize shelf-life and fruit quality, especially when MAP is used. Recommended postharvest conditions are 8 to 15°C at 90-95% RH for rambutans (Mendoza et al., 1972; O’Hare, 1995), and 4 to 7°C at 90-95% RH for longans (Paull and Chen, 1987). However, cultivars can differ in sensitivity to chilling injury, modified atmospheres, and disease susceptibility, and the spectrum of fungi associated with diseased fruit can vary with storage temperature. Also, temperature fluctuations are common during shipping and retail handling and can impact the effectiveness of packaging. The objectives of this work were 1) to identify fungal pathogens associated with rambutan and longan fruits and establish the baseline sensitivity (in vitro) of the major fungal pathogens to registered fungicides, 2) to determine the effectiveness of preharvest fungicide applications on postharvest disease development and fruit quality, and 3) to develop packaging strategies to extend the shelf-life of rambutans and longans. MATERIALS AND METHODS A survey was conducted on Hawaii Island to determine the causal agents of diseases affecting rambutan and longan production. Leaves and fruit were collected from representative samples of symptomatic plant material. Fungi were isolated on artificial media using a tissue dissection method in which three small pieces of tissue from the advancing margin of a lesion were cut, surface sterilized with 0.5% sodium hypochlorite, and plated onto potato dextrose agar (PDA) or water agar (WA). Plates were incubated at 26°C and examined after 7 d of incubation. Fungi were identified using morphology, physiology and molecular techniques. Pure cultures were obtained by hyphal tip transfer from plate cultures. Fungal isolates were evaluated for sensitivity to fungicides registered for use in Hawaii (Abound, Trilogy and Serenade) by growing them on PDA plates amended with label-approved concentrations of fungicides. Abound is a broad spectrum, systemic azoxystrobin fungicide. Trilogy is an extract from neem oil, and Serenade is a patented strain of Bacillus subtilis. Mycelial plugs (5-mm) were cut from the margin of fungal colonies, transferred to three replicate plates, and maintained at ambient temperatures (23 to 25°C) under fluorescent lighting (12 h/day). Colony diameter after 7 d was measured across two axes, averaged, and the diameter of the mycelial plug subtracted from the average. Fungal growth was compared to PDA plates containing no amendments to determine the percent inhibition. Each experiment was conducted at least twice. In-field applications were made at commercial orchards of rambutan (‘Jit Lee’) and longan (‘Biew Kiew’) with Serenade, the registered fungicide shown to be most effective in laboratory tests on fungi isolated from fruits. Applications were made according to label specifications and to simulate grower practices. Each experiment was a randomized complete block design with single trees as replications. Four replications were used for each treatment per crop (rambutan or longan). Approximately 20 kg of fruit were randomly harvested from each tree for disease incidence, postharvest quality and shelf-life evaluations. Untreated trees served as controls. Rambutans were harvested in Dec. 2007 from fungicide-treated and untreated trees, removed from the panicles, and sorted. Fruit (500 g) were placed into fiberboard boxes, microperforated bags (MP), clamshells (CL), Lifespan bags (LS), or Peakfresh bags (PF). Each package type was placed into fiberboard boxes and stored at 10°C. A further packaging study was conducted in Feb. 2009 using fruit harvested from a commercial orchard (untreated trees). Rambutans (‘R167’; 1800 g) were packaged in MP bags, CL, PF bags, or LS bags and stored at 1) constant 10°C (control) or 2) held at 20°C for 2 days (simulated harvest, packing, quarantine treatment, and air transport), transferred to 10°C for 5 days (simulated wholesale storage and distribution), and removed to 20°C for 2 days (simulated retail conditions). For longans, a packaging experiment was conducted in Oct. 2008 in which fruit