Onion storage ability and an inventory of onion post-harvest fungi in Tunisia

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Onion storage ability and an inventory of onion post-harvest fungi in Tunisia

Sta Baba Rafika1, Daami-Remadi Mejda1, Ben Kheder Mohamed2 and Chaar Hatem3

1Institut National de la Recherche Agronomique de Tunisie, Pôle Régional de la Recherche et du Développement Agricole du Centre-est, 4042, Chott Mériem, Tunisia ([email protected]). 2Ecole Supérieure d’Horticulture et

d’Elevage de Chott Mériem, Centre Technique d’Agriculture Biologique, 4042, Chott Mériem, Tunisia. 3Institut National Agronomique de Tunisie, 43 Av. Charles Nicolle, 1082, Tunisia

Abstract Of 13 onion cultivars, red onions had longer storage life than white onions. Storage life was correlated with dry matter content. Botrytis spp., Aspergillus spp., Fusarium oxyspo-rum and Penicillium spp. were frequently isolated from rotted bulbs. Fusarium oxysporum pathogenicity tested on healthy bulbs led to a typical basal rot; this is the first report of Fusarium oxysporum f.sp. cepae in Tunisia. Copyright © 2006 John Wiley & Sons, Ltd

Key words: onion, cultivars, dry matter, storage, fungi, rots

Introduction

One of the main factors limiting long-term storage of onions is disease, which causes consider-able losses (Gruszecki and Tendaj 2000). This can be affected by cultivation conditions, the method and date of harvest, and storage conditions (Schwartz and Mohan 1996; Lorbeer 1997). Losses in store also depend on the genetic characteristics of the cultivars (Patil and Kale 1989; Siergrist 1992). This study evaluated the storage of some exotic and traditional onion cultivars in Tunisia and the major fungal agents which infect onion bulbs during storage.

Materials and methods

Observations on bulbs of 13 cultivars (Table 1) grown in summer 2003 were made at the National Institute of Agricultural Research of Tunisia (INRAT) Experimental Station of Sahline. Bulbs were harvested by hand and left to cure in the field for seven days. Fifty bulbs per variety and per plot were evaluated in storage in a traditional well-ventilated shed using a completely randomized block design with three replicates. Rotten and sprouting bulbs were counted and removed at 85, 115, 145, 175 and 205 days of storage.

For each date, analysis of variance was applied using variety as the fixed factor and block as the random factor. The effect of variety was tested by the interaction between variety × block. Means were compared by Duncan’s multiple range test at the 5% level. A Principal Component Analysis (PCA) was done to examine the relationship between the parameters

Tropical ScienceTrop. Sci. 2006, 46(2), 105–112Published online in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/ts.34

Accepted 9 June 2005

106 Sta Baba Rafika et al.

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Copyright © 2006 John Wiley & Sons, Ltd Trop. Sci. 2006, 46, 105–112 DOI: 10.1002/ts

at harvest, bulb weight, bulb and neck diameter, scale number and dry matter content, and also the parameters relevant to storage, and numbers of sound bulbs, rotten bulbs and sprout-ing bulbs. Hierarchical classification was done to identify related groups of varieties.

Bulbs showing symptoms of rotting, softening, fungal fructifications or visible mycelium development were examined in a series of microbiological isolations. Isolates from rotten bulbs were cultured and the fungi were identified by the methods of Abawi and Lorbeer (1965). Isolates of Fusarium oxysporum were tested to check if it was F. oxysporum f.sp. cepae, the causal agent of onion basal rot: spores from rotten bulbs were inoculated into healthy-looking bulbs which had been disinfected, and the inoculated bulbs were placed on damp, sterilized paper and incubated at 25°C for one month. If rotting occurred, the identity of F. oxysporum was confirmed by re-isolation of the fungal agent.

Results and discussion

The characteristics and yields of the cultivars were obtained in the 2003 yield trial (Table 2). After 85 days there was little difference between the varieties, all with between 86 and 100% sound bulbs (Table 3). The white and yellow varieties then deteriorated much more quickly than the red. By 175 days, several white varieties were less than 20% sound, whereas two of the red varieties, Rio Raji Red and Rouge d’Amposta, were still 75% sound after 205 days. In France, coloured onions are considered to have better storage life than white onions (CTIFL, 1981). In North Africa, coloured onions are generally used for late summer produc-tion and then kept for winter use because of their good storage ability.

Onion varieties differ in their ability to store, and the cultivar is the most important factor affecting storage life (Justin 1988; Patil and Kale 1989; Siergrist 1992). The duration of onion storage is closely linked with bulb dormancy (Miedema, 1994), which is affected by farming conditions and method and date of harvest (Brewster 1997).

Table 1. Origin and colour of the cultivars

Cultivars Origin Colour

Jaune des Cevennes Gautier YellowContessa Asgrow WhiteImaï Early Yellow Asgrow YellowRedwing Bejo RedCastillo Bejo YellowRobin Bejo RedMerveille de Pompeï Tezier WhiteDe Vaugirard Petoseed WhiteRed Creole Sunseed RedMarquesa Asgrow WhiteVGE 1072024 Asgrow WhiteRio Raji Red Rio Colorado RedRouge d’Amposta Tezier Red

Onion storage in Tunisia 107

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Table 2. Characteristics and yield of onion cultivars

Cultivar Bulb Bulb Neck Number Dry matter Date of Yield weight diameter diameter of scales (%) harvest (kg/m2) (g) (cm) (cm) (2003)

Jaune des 144f 7.1c 0.3d 8c 12.9a 10/06 2.16fCévennesRio Raji Red 141f 7.4c 0.1e 9c 13.3a 10/06 2.11fContessa 127g 6.7d 0.1e 7d 12.7a 29/05 1.90gImaï Early 161f 6.9d 0.2g 7d 11.2b 29/05 2.41fYellowRedwing 337b 8.9a 2.0a 12a 12.3a 06/08 5.05bCastillo 409a 9.3a 2.3a 12a 10.0c 06/08 6.13aRobin 252c 8.3b 2.0b 12a 12.7a 06/08 3.78cMerveille de 223d 8.6b 0.6c 8c 10.3b 10/06 3.34dPompeïDe Vaugirard 215e 8.8a 0.5c 9c 10.0c 10/06 3.22eRed Creole 324b 8.9a 1.5b 9c 10.8b 06/08 4.86bMarquesa 253c 8.2b 0.4d 8c 9.2c 28/05 3.79cVGE 1072024 276c 8.1b 0.3d 9c 9.0c 28/05 4.14cRouge 216e 8.4b 1.3b 11b 13.5a 06/08 3.24ed’AmpostaMean 237 8.15 0.92 9.3 11.4

Values in a column with the same letter are not significantly different at p < 0.001.

Table 3. Numbers of sound bulbs during storage (of 50)

Cultivar Days of storage

85 115 145 175 205

Jaune des 43.3c 37.0d 28.0e 20.0c 13.7cCevennesRio Raji Red 50.0a 48.7b 45.7a 42.3b 37.0aContessa 50.0a 44.7b 42.3b 28.0b 17.0cImai Early 50.0a 46.0b 37.3c 27.0b 13.7cYellowRedwing 45.3b 35.7d 32.0c 22.7c 18.3cCastillo 46.7b 41.0c 29.7d 20.0c 6.0dRobin 50.0a 46.3b 42.3b 31.0b 20.3bMerveille de 47.0b 41.7c 22.0e 8.0e 0.7ePompeïDe Vaugirard 48.0b 38.3c 23.0e 5.7h 2.0eRed Creole 50.0a 45.3b 40.0b 33.0b 26.3bMarquesa 47.7b 45.0b 20.0g 8.0e 1.3eVGE1072 024 45.7b 43.0c 35.0c 14.0d 5.7dRouge 49.7a 49.7a 48.7a 47.3a 4.27ad’Amposta



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The proportion of sound bulbs decreased steadily following a logistic asymmetric model (Figure 1). When this was adjusted for each colour group, it illustrated the poor storage capability of white cultivars and the relatively good storage of red (Figure 2).

Up to 145 days there was increasing rotting (Table 4), and by 175 days the number of newly rotten bulbs decreased so that by 205 days there were very few new rotten bulbs; this period was November and December, a cold period when high-temperature pathogens would

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Figure 1. Adjustment curve of sound bulbs (%) with storage.The model equation is: %hbi = 106.8 - 88.56/[1 + e2.8339-0.0335×temps]0.8512 R2 = 0.6557.

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Figure 2. Adjustment curve of sound bulbs (%) with storage and bulb colour.The models are:Red: %hbi = 145.8 - 2218.6/[1 + e17.2504-0.0222×temps]0.2252 R2 = 0.5284.Yellow: %hbi = 131.8 - 144.39/[1 + e3.9220-0.0293×temps]0.3752 R2 = 0.9102.White: %hbi = 103.0 - 97.56/[1 + e4.0793-0.0509×temps]0.7681 R2 = 0.8508.


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be inhibited. Rio Raji Red and Rouge d’Amposta had fewest rotten bulbs, perhaps due to the presence of fungistatic compounds in the tunic or outer dry skins. These cultivars had rela-tively narrow necks at harvest, another factor favouring a long storage life. Robin, Redwing and Red Creole had an intermediate number of rotten bulbs, which might be due to their thick necks, making the penetration of fungi easier, or perhaps their necks were not dry enough after curing.

Up to 115 days of storage there was practically no sprouting (Table 5). All the bulbs were then in a phase of true dormancy. After 145 days of storage, Castillo, Merveille de Pompeï, De Vaugirard and Marquesa had most sprouting bulbs, and after 175 days these still showed the highest levels of sprouting, together with VGE. The red cultivars had the lowest level of sprouting throughout. These differences are probably due to the variation in dormancy between cultivars. Coloured bulbs in our trials had a more pronounced dormancy.

A varied range of fungi were isolated, the most frequent being Aspergillus spp. On potato dextrose agar (PDA) the colonies were initially yellow and then black after fructification. Bertolini et al. (1983) frequently found Aspergillus niger on onion bulbs during storage, while El-Nagerabi and Ahmed (2003) reported that losses during storage in Sudan were due to A. niger. Penicillium spp. and Botrytis spp. were also found on stored bulbs on most cultures on PDA. Botrytis spp. on PDA developed a colony, which was initially white, and then pro-gressively grey after fructification. Conidiophores of Botrytis spp. were large, bifurcated at their ends and carried numerous blastoconidia. These were formed by budding, and were unicellular and rounded in shape. Sclerotia appeared as macroscopic black dots on the colony

Table 4. Numbers of rotten bulbs during storage (of 50)


85 115 145 175 205 Total

Jaune des 6.7a 5.3b 8.3a 2.7b 0.0a 23.0CevennesRio Raji Red 0.0e 0.7d 1.0c 1.0b 1.7a 4.4Contessa 0.0e 2.0d 3.3c 4.3b 0.7a 10.0Imai Early 0.0e 3.3c 7.0b 0.0b 1.0a 11.3YellowRedwing 4.7b 9.0a 2.0c 1.5b 0.3a 12.5Castillo 3.3c 5.7b 4.7c 0.3b 1.3a 15.3Robin 0.0e 3.7c 3.0c 4.0b 1.7a 12.4Merveille de 3.0c 3.7c 8.0a 3.3b 0.7a 18.7PompeïDe Vaugirard 2.0d 7.3a 4.3c 7.3b 0.0a 17.7Red Creole 0.0e 4.3c 5.3b 2.0b 1.7a 13.3Marquesa 2.3d 4.0c 6.7b 2.0b 0.7a 15.7VGE1072 024 4.3b 3.7c 4.7c 9.0a 0.0a 21.7Rouge 0.3e 0.0d 0.6d 0.7b 1.7a 3.3d’Amposta



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surface and on damaged bulb tissues. Justin (1988) reported that various species of Botrytis attack onion; particularly B. squamosa and B. allii.

We also found that onion bulbs could be invaded by Penicillium spp., which on PDA formed white colonies that turned green on fructification. Conidiophores were forked and phialides arranged in brush form, carrying on their ends the unicellular, round conidia arranged in chains. P. glabrum is frequently the cause of onion spoilage in Australia (Pitt and Hocking 1997). Onion storage fungi may be seed borne (Köycü and Özer 1997) or come from the field and the store environment (Messiaen et al. 1991).

A fungal culture from a bulb rotten at the base with no visible fructifications revealed on PDA a colony typical of Fusarium oxysporum, with an aerial, loose whitish-grey mycelium, in the form of a flattened cone. These colonies showed the presence of short phialides carry-ing false head microconidia, some macroconidia of medium size and intercalary chlamy-dospores on the mycelium. The pathogenicity test of this isolate showed that it was Fusarium oxysporum f.sp. cepae., a strongly phytopathogenic agent which infects onion roots and bulbs and can attack other alliaceous crops like shallot (Cramer 2000). The bulb symptoms are yellowing and apical drying of the leaves on the growing plant. This is the first record of this pathogen being present in Tunisia. This pathogen can survive for a long time in the field in the form of chlamydospores. Cultivation measures need to be taken, particularly the rotation of crops and better store hygiene, to reduce the incidence of onion storage problems.

In PCA, the only correlation was between the dry matter content at harvest and the pro-portion of sound bulbs. Lin et al. (1995) and Silué et al. (2003) found a positive correlation

Table 5. Numbers of sprouting bulbs during storage (of 50)


85 115 145 175 205 Total

Jaune des 0.0a 1.0a 1.0c 5.3b 6.3b 13.6CevennesRio Raji Red 0.0a 0.0a 2.0c 2.3c 3.7c 8.0Contessa 0.0a 0.3a 0.3d 10.0a 10.3a 20.9Imai Early 0.0a 0.0a 1.7c 7.3b 12.3a 21.3YellowRedwing 0.0a 0.3a 1.7c 6.3b 4.0c 12.3Castillo 0.0a 0.0a 6.7b 12.0a 10.7a 29.4Robin 0.0a 0.0a 1.0c 7.3b 5.7b 14.0Merveille de 0.0a 1.3a 11.7a 10.7a 6.7b 30.4PompeïDe Vaugirard 0.0a 1.0a 11.0a 13.7a 3.7c 29.4Red Creole 0.0a 0.3a 0.0d 5.7b 5.0b 11.0Marquesa 0.0a 0.0a 15.0a 10.0a 5.7b 30.7VGE1072 024 0.0a 0.7a 0.0d 12.0a 8.3b 21.0Rouge 0.0a 0.0a 0.3d 0.7c 3.3c 4.3d’Amposta



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between the level of dry matter and the storage life. High dry matter onions have a good ability to store (Justin 1988). An examination of changes in the values of each axis established the two first axes of PCA. The first factorial axis was positively correlated with the level of dry matter at harvest (DM) and the sound bulbs at the five dates (Figure 3). DM was nega-tively correlated with the level of rotten bulbs. This axis characterized the ability to store of cultivars. The second axis tended to be correlated with parameters that characterized yield, mean bulb weight, bulb and neck diameters, and scale number (SN). When the cultivars were represented graphically on the two axes of PCA (Figure 4), Rio Raji Red and Rouge

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Figure 3. Graphic representation of parameters on the two axes of PCA: BWt, bulb weight; BD and ND, bulb and neck diameter; SN, scale number; DM, dry matter level; hb, number of sound bulbs; rb, number of rotten bulbs and spb, number of sprouting bulbs.

RedwingCastillo

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d’Amposta had high levels of dry matter and sound bulbs, so they combined a good ability to store and good or medium yield. Conversely, Jaune des Cevennes, Merveille de Pompeï, De Vaugirard, Marquesa and VGE combined a poor ability to store with medium yields. Redwing and Castillo produced high yields but had only medium to poor ability to store. However, Contessa and Imai Early Yellow had a better ability to store but still had low yields. The red cultivars Robin and Red Creole had good ability to store and good yields.

Acknowledgement

We thank Dr Lesley Currah for helpful advice.

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Onion storage ability and an inventory of onion post-harvest fungi in Tunisia