Amnon Lichter

Ethanol controls postharvest decay of table grapes

Abstract Postharvest deterioration of table grapes generally results from berry decay and/or desiccation of stems and pedicels. Conventional methods to avoid these problems include SO2 fumigation or release from generator pads containing a metabisulfite salt, and packaging of the fruit in polyethylene liners. SO2 is usually effective in preventing decay as long as its level is sufficiently high. However, high levels can result in fruit damage, unpleasant aftertaste, and allergies. Our objective was to examine the effect of applying a postharvest ethanol dip on the decay of table grapes. Immersion of detached berries in 70% ethanol eliminated most of the fungal and bacterial populations on the berry surface, but had little effect on survival of yeasts. In vitro development of spores of the major postharvest pathogen of table grapes, Botrytis cinerea was arrested by 40% ethanol. Dipping of grape bunches in 50, 40 or 33%, but not in 20% ethanol, prior to packaging, resulted in inhibition of berry decay that was equivalent to, or better than that achieved with SO2, released from generator pads. Decay control was generally feasible for a cold storage period of 4–5 weeks and sometimes more. Ethanol did not impair bunch appearance, berry bloom or berry firmness and ethanol-treated berries obtained higher organoleptic scores than SO2-treated berries.

Production of acetaldehyde and ethanol during maturation and modified atmosphere storage of litchi fruit

The emission of the metabolites, acetaldehyde (AA) and ethanol, from litchi fruit was monitored during maturation and storage. Juice extracted from the arils at various stages (green, breaker, pink and red pericarp) during the season, contained increasing amounts of AA and ethanol. These increases, in mature fruit, were accompanied by a pronounced decrease in the soluble solids content (SSC) and an increase in titratable acidity of the juice. In parallel to juice analysis at harvest, red pericarp fruit were packed in laminated films, creating modified atmosphere packaging (MAP). Late-harvested whole fruit in MAP produced more AA and ethanol than early-harvested ones. Intensified production of AA and ethanol was found in later-harvested litchi, during cold storage and shelf life in MAP, and late-harvested fruit also showed increased decay after cold storage. These results suggest that mature litchi fruit deteriorated when kept longer on the tree during the harvesting season. This may be ascribed to a fermentation process that began on the tree and caused deterioration during MAP storage.

Post-Harvest Botrytis Infection: Etiology, Development and Management

Botrytis is regarded as the most important post-harvest fungal pathogen that causes significant losses in fresh fruits, vegetables and ornamentals. Its ability to attack a wide range of crops in a variety of modes of infection and its ability to develop under conditions prevailing during storage, shipment and marketing make its control a challenge. Harvested crops are particularly vulnerable to Botrytis infection because unlike vegetative tissue harvested commodities are senescing rather than developing. Control of Botrytis on harvested crops has relied mainly on pre-harvest chemical fungicides for reducing inoculum density and incipient infections before harvest. Control programmes were developed specifically for each crop and largely depend on epidemiological and etiological information. The future of many of these chemicals, however, is now doubtful and their use has come under scrutiny. This is due to severe restrictions and regulations imposed especially on post-harvest chemical treatments for the majority of freshly harvested fruits and vegetables. To develop better and more efficient methods for controlling post-harvest Botrytis rot it is essential to understand the relationship between infection of various plant parts in the field and incidence of grey mould in storage. This relationship has still not been fully elucidated in tomato, kiwifruit, strawberry, grapes and roses. These crops are discussed in this chapter as examples for different research strategies to tackle the problem. It is concluded that control methods based on holistic strategies which incorporate modelling and prediction systems, early detection techniques, biological and physical methods, and cultural practices, should be tailored to meet the demands of each crop.

Hot water brushing: an alternative method to SO2 fumigation for color retention of litchi fruits

Abstract Distribution of high-quality litchi ( Lychee chinensis Sonn.) fruits to global markets depends exclusively on postharvest treatments to suppress peel browning. The current standard treatment of litchi fruits in Israel includes fumigation with sulfur dioxide (SO 2 ) followed by dipping the fruits in hydrochloric acid containing the fungicide prochloraz. As part of the effort to reduce the use of potentially hazardous chemicals in agriculture, we developed a new procedure that may enable SO 2 to be avoided. Instead of fumigation, litchi fruits are sprayed with hot water while being brushed in a revolving drum, after which the fruits are subjected to hydrochloric acid treatment. Fruits that are processed in this way maintain a uniform red color for at least 35 days, without apparent deterioration in external or internal quality, or taste. Physiological studies demonstrate that polyphenol oxidase (PPO) activity is reduced by the hot water brushing (HWB) procedure as compared with controls but not to the same extent as inhibition by SO 2 treatments. In addition to its effect on PPO activity, HWB may lead to reduced pH of the pericarp, or more uniform distribution of the acid in it. This result suggests that HWB may act by bruising the external layer of the pericarp allowing the peel to be uniformly exposed to the acid which may inhibit PPO activity and maintain the anthocyanins in their red-pigmented form.

Cracking of cherry tomatoes in solution

Tomato fruit cracking occurs both during ripening and after harvest. Cracked fruits cannot be marketed and the cracks form sites for fungal penetration and infection. An assay based on immersion of the fruit in water was developed to study factors involved in fruit cracking. Adding calcium to the water reduced cracking whereas chelating agents increased cracking. Mineral analysis of the fruit following calcium treatment demonstrated an increase in bound calcium, while CDTA reduced the amount of soluble calcium. Decrease in fruit weight associated with water loss during storage was correlated with a decrease in the cracking potential of the fruit. Conversely, ripening during storage resulted in an increase in the cracking potential. Immersion of the fruit in acidic phosphate or citrate buffers promoted cracking whereas neutral or basic buffers prevented cracking. The cracking potential of cherry tomatoes was high after morning harvest, and it declined at noon and was low after evening harvest. It is anticipated that this study will assist to evaluate positive or negative practices which may influence cracking of cherry tomatoes after harvest.

Stress on avocado fruits regulates Δ9-stearoyl ACP desaturase expression, fatty acid composition, antifungal diene level and resistance to Colletotrichum gloeosporioides attack

Abstract The expression of the avocado homologue avfad9 encoding Δ 9 -stearoyl-ACP desaturase was enhanced by multiple stimuli: inoculation with Colletotrichum gloeosporioides, exposure to ethylene or CO 2 , low temperature (4 °C) and fruit wounding. This enhanced expression was correlated with an increase in the preformed antifungal (Z, Z)-1-acetoxy-2-hydroxy-4-oxo-heneicosa-12,15-diene. Treatments of fruits with ethylene that enhanced the up-regulation of avfad9 also increased the concentration of 18:2 fatty acid and the incorporation of 14 C-linoleate into the antifungal diene. Fruits with enhanced Δ 9 stearoyl-ACP desaturase expression were more resistant to C. gloeosporioides . It is suggested that the enhanced Δ 9 stearoyl-ACP desaturase expression is involved in elevation of unsaturated 18:2. It is also concluded that similar treatments enhance the incorporation of labeled 18:2 into the antifungal diene and elicit the concurrent enhanced resistance to fungal attack.

Mechanisms modulating fungal attack in post-harvest pathogen interactions and their control

As biotrophs, insidious fungal infections by post-harvest pathogens remain quiescent during fruit growth, but at a particular phase, during ripening and senescence, the pathogens transform to necrotrophs and elicit the typical decay symptoms. Exposure of unripe hosts to pathogens initiates defensive signal-transduction cascades that limit fungal growth and development. Exposure to the same pathogens during ripening and storage activates a substantially different signalling cascade that facilitates fungal colonization. This review will focus on modulation of post-harvest host-pathogen interactions by pH and reactive oxygen species (ROS). Modulation of host pH in response to a host signal is bidirectional and includes either alkalinisation by ammonification of the host tissue, or acidification by secretion of organic acids. These changes sensitise the host and activate the transcription and secretion of fungal hydrolases that promote maceration of the host tissue. This sensitisation is further enhanced at various stages by the accumulation of host or fungal ROS that can further weaken host tissue and amplify fungal development. Several specific examples of coordinated responses that conform with this scheme are described, followed by discussion of the means to exploit these mechanisms to establish new approaches to post-harvest disease control.

Expression of Δ12 fatty acid desaturase during the induced accumulation of the antifungal diene in avocado fruits

SUMMARY The preformed (Z,Z)-1-acetoxy-2-hydroxy-4-oxo-heneicosa-12,15-diene (AFD) is the most active antifungal compound in avocado; it affects the quiescence of Colletotrichum gloeosporioides in unripe fruit. One of the genes encoding Delta(12) fatty acid desaturase (avfad12) was hypothesized to take part in the biosynthesis of AFD, and its expression pattern and enzymatic activity were determined in relation to the content of AFD. Using avfad12-3 as a probe, high levels of expression were detected in young fruits and leaves, where the level of AFD was highest. In contrast, Northern analysis of RNA from mature leaves and fruits showed no transcripts from the avfad12 gene family and lower AFD content. The transcripts from the avfad12 gene family, the enzymatic activity of Delta(12) fatty acid desaturase, and the level of AFD in unripe-resistant fruits increased transiently when the fruits were inoculated with C. gloeosporioides or exposed to ethylene (40 microL/L), low temperature (4 degrees C) or 1 mm H(2)O(2), but ripe fruits were not affected. The effect of H(2)O(2) on the transcripts from the avfad12 gene family is of specific importance, because reactive oxygen species were produced by unripe-resistant host fruit soon after inoculation of C. gloeosporioides. In addition, the fungus itself produced H(2)O(2) in culture medium at pH 5.0, which is similar to the pH of unripe-resistant fruit, but not at pH 7.0. Treatments that enhanced Delta(12) fatty acid desaturase activity increased the concentration of the AFD precursor, linoleic acid, and its incorporation into AFD; these treatments also caused a delay in decay development. The present results demonstrate temporal relationships among the transcripts from the avfad12 gene family, the synthesis of the precursor of AFD (linoleic acid), the AFD content and quiescence of C. gloeosporioides in unripe fruits.

Abscisic acid improves colour development in ‘Crimson Seedless’ grapes in the vineyard and on detached berries

Summary ‘Crimson Seedless’ is a high quality, red, table grape (Vitis vinifera) cultivar, which may fail to develop adequate red colour in warm climates. In addition, most bunches contain some green berries when the rest of the bunch has become red. Abscisic acid (ABA) is a plant hormone which increases in grape berry skin at the onset of maturation and is involved in the regulation of anthocyanin accumulation. A commercial formulation, S-ABA (ProTone®), was sprayed at 400 mg l−1 or 800 mg l−1 in a vineyard at the beginning of veraison, or 200 mg l−1, 400 mg l−1, or 600 mg l−1 S-ABA was applied to detached berries or to small bunches (five-to-six berries) in the laboratory by spraying or by allowing uptake through the pedicel. In the vineyard, the application of S-ABA affected berry colour, changing the red berry colour of the control [hue angle (h°) = 3.6] to black for S-ABA-treated clusters (h° = –45°). Ripeness parameters (soluble solids content, titratable acidity, and berry size) were not affected by S-ABA treatment, but treated berries were less firm (250 g mm−1) than untreated control fruit (335 g mm−1) using a compression durometer. Anthocyanin accumulation in berries treated with 400 mg l−1 S-ABA was almost double that of control berries, although the anthocyanin compositions were similar. Storage of grapes at 0°C for 3 weeks, with no protection from decay, indicated that S-ABA did not increase their sensitivity to fungal infection. Green or breaker-stage detached berries, or bunches, developed a red colour by spraying with S-ABA or by uptake through the pedicel. Detached berries may therefore serve as an efficient system with which to test the effects of S-ABA and its interactions with other factors which influence colour development.

Mechanisms Modulating Postharvest Pathogen Colonization of Decaying Fruits

As biotrophs, insidious fungal infections of postharvest pathogens remain quiescent during fruit growth while at a particular phase during fruit ripening and senescence the pathogens transform to necrotrophs causing typical decay symptoms. Exposure of unripe hosts to pathogens (hemi-biotroph or necrotrophs), initiates defensive signal-transduction cascades that limit fungal growth and development. Exposure to the same pathogens during ripening and storage activates a substantially different signalling cascade which facilitates fungal colonization. This chapter will focus on modulation of postharvest host-pathogen interactions by pH and the consequences of these changes. Host pH can be raised or lowered in response to host signals, including alkalization by ammonification of the host tissue as observed in Colletotrichum and Alternaria, or acidification by secretion of organic acids as observed in Penicillium, Botrytis and Sclerotinia. These changes sensitize the host and activate transcription and secretion of fungal hydrolases that promote maceration of the host tissue. Several particular examples of coordinated responses which follow this scheme are described.