INTRODUCTION

Onion (Allium cepa L.) is an important horticultural crop grown worldwide

and is sold as dry bulb. Dry onion was one of the 20 most important agricultural

commodities, based on value, in the UK in the last 15 years (FAOSTAT, 2010). Over

350,000 tonnes of marketed onions are produced annually on about 8700 hectares in

the UK. The main production regions are Lincolnshire, Essex, Suffolk,

Cambridgeshire, Bedfordshire, Yorkshire and Kent (O`Connor, 2005).

The most common fungal pathogens which cause losses for UK onion

producers are Botrytis aclada, Botrytis squamosa, Peronospora destructor,

Sclerotium cepivorum, Urocystis cepulae and Fusarium oxysporum f. sp. cepae

(Maudeet al., 1984; Clarkson et al., 2000; Gilles et al., 2004; Coventryet al., 2002;

Tayloret al., 2012). F. oxysporumf. sp.cepaeW. C. Snyder & H. N. Hansen (FOC)

was described as the causal agent of onion basal rot, a soil-borne vascular disease.

Onion basal rot causes economic losses of around £10–11 million annually in the

UK, and is therefore a high priority in the research strategy of the British Onion

Producers Association (Alaister Findlay, BOPA, pers. comm.). Onion basal rot is

mainly a problem for individual growers on particular fields and losses of up to 60%

F. oxysporum f. sp. cepae (FOC) can cause symptoms at every stage of the

plant development and infects the roots and basal plate of sets and bulbs, causing a

brown basal rot that becomes covered with a white-pink mycelium (Figure 2A). Leaf

tips yellow, leaves wilt beginning with the older outer leaves and the plants die

(Figure 2B). FOC can also cause seedling damping off, or delayed emergence of

seedlings (Cramer, 2000).

FOC invades the roots or the stem plate of onion bulbs by direct penetration

or through wounds, and then grows through intercellular spaces until invading the

xylem vessels and xylem parenchyma. Xylem vessels blocked by tyloses are

observed in both roots and, more commonly, in the stem plate. The pathogen can

break through the stem area and grow in the fleshy leaf base tissues and between the

leaves. Chlamydospores are formed in the root cortex and in and around the xylem

vessels, while microconidia are produced by hyphae growing between the scale

tissues in the bulb (Abawi and Lorbeer, 1971).

A B

Figure 2.Onion basal rot symptomsADevelopment of mycelium on basal plate andBwilting and yellowing.

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The pathogen can spread in stores through the wounds of dormant bulbs

(Özer & Köycü, 2004) and within and between fields on equipment and infected

plant material (Maude, 2006; Cramer, 2000). The pathogen can survive as

chlamydospores (thick-walled survival structures) for up to 10 years in the soil or on

soil debris as saprotrophic mycelium where onions are grown continuously

(Entwistle, 1990). The disease is more serious when soil temperatures exceed 25°C

and soil moisture is high.

FOC isolates show different levels of aggressiveness in onion seedlings, but

separate races have not been identified (Cramer, 2000). FOC comprises seven known

vegetative compatibility groups (VCGs 0420 to 0426) and several single-member

VCGs (SMVs). However, it should be noted that the VCG analysis was carried out

only on isolates collected in Japan, South Africa, Colorado (United States), and

Turkey (Southwoodet al.,2012).

Recent studies from the Netherlands, Uruguay, Serbia, Turkey and

Washington State, USA reported a complex of F. oxysporum and F. proliferatum

(teleomorph:Gibberella intermedia) species causing onion basal rot. Additionally, in

Turkey F. solani (teleomorph: Nectria haematococca), F. acuminatum (teleomorph:

G. acuminata) and F. redolens were also found to be very aggressive in onion

(Galvan et al., 2008; du Toit et al., 2003; Klokocar-Smit et al., 2008; Bayraktar et

al., 2011). In the Netherlands F. avenaceum (teleomorph: G. avenacea) was

identified as weakly pathogenic (de Visseret al., 2005). In an early study,F. equiseti

(teleomorph: G. intricans), F. verticillioides (teleomorph: G. moniliformis) and F.

tricinctum (teleomorph: G. tricincta) were also all found to be pathogenic to onion

Basal rot symptoms caused by FOC can occur on onion (A. cepa), shallot (A.

cepavar.ascalonicum), Welsh onion (A. fistulosum), chive (A. schoenoprasum) leek

(A. ampeloprasum var. porrum) and garlic (A. sativum) (Bayraktar et al., 2010;

Coskuntuna & Özer, 2008; Özer & Köycü, 2004; Swift et al., 2002; Figure 3). From

diseased garlic F. proliferatum, F. verticillioides, F. culmorum and from diseased

Welsh onion F. redolens, F. verticillioides, F. solanihave also been isolated (Dugan

et al., 2007; Dissanayake et al., 2009ab; Seefelder et al., 2002; Stankovic et al.,

2007; Shinmura, 2002; Palmeroet al., 2012).

Leeks can suffer from foot and or basal rot depending on which Fusarium

species has infected the plant. Foot rot is caused byF. oxysporumandF. avenaceum

(Figure 3), while basal rot is caused byF. culmorum (Armengol et al., 2001; Koike

et al., 2003; Gilardiet al., 2009; Oxspringet al., 2004).

Members of the wildAlliumflora have not been reported as potential hosts of

F. oxysporumin the UK. The wild speciesA. roylei, A. galanthum, andA. pskemense

showed an intermediate level of resistance againstF. oxysporum andF. proliferatum

(Galvan et al., 2008). There is some information suggesting that FOC can colonize

without causing symptoms economically important weed species such as oxalis

(Oxalis corniculata) and fat-hen (Chenopodium album) (Abawi and Lorbeer, 1972).

As with many other soil-borne diseases, onion basal rot is difficult to control.

Chemical control is rather limited in the UK, partly because thiram

(dimethylcarbamothioylsulfanyl-N,N-dimethylcarbamodithioate) is the only

recommended fungicide in the UK, and this can only be used for seed-treatment

(Health and Safety Executive, HSE). The other reason is the increasing demand for

organic products and more sustainable approaches to disease control. Biocontrol

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were shown to be effective against onion basal rot (Coskuntuna and Özer, 2008;

Cramer, 2000), but only B. subtilis (sold as Serenade ASO) is registered for onion

treatment in the UK (Fungicide Resistance Action Group, FRAG). Work is in

progress to optimise field-scale control of Fusarium basal rot and white rot of onion

usingTrichodermaamended substrates and pellets, and onion residues (Ralph Noble,

East Malling Research, pers. comm.). Good management practices can help to

prevent the occurance and spread of onion basal rot such as use of at least a 4-year-

crop rotation, cleaning agricultural machinery between fields, temperature and

humidity controlled-storage and soil solarisation (Klein et al., 2011). It is not clear

why onion basal rot has become an increasing problem in the UK since 2006, but

theories include the role of recent erratic weather, withdrawal of fungicides and

emergence of new and/or more aggressive isolates or a combination of the above (A.

A D E E B C D

Figure 3.Fusarium rot symptoms of variousAlliumspecies.A-BFoot rot of leek (Photo credit: Hitch, C. J., SARDI)AFusarium avenaceumcauses a brown rot at the base of the shank.BF. oxysporumcauses a pink discoloration on the shank. CBasal rot of leek,F. culmorumcauses an orange discoloration on the shank and mycelial growth on leaves.DBasal rot of shallotEBasal rot of garlic (Photo credit: Putnam, M., Oregon State University).

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