Análisis general

Allium

species in the UK

Onion bulbs showing basal rot symptoms were collected from stores or

directly from fields. Field samples were taken by walking diagonally across each

field and taking bulb samples with symptoms of basal rot from at least ten plants per

a field (Table 2). Approximately 1 cm3sections of diseased and healthy basal tissue

of Allium samples (Table 2) were surface sterilised (twice in 70% ethanol for 5

minutes each followed by two rinses in sterile distilled water) and transferred onto

water agar (20 g agar in 1L of distilled water) amended with antibiotics (1 g

streptomycin sulphate and 0.12 g neomycin per L media to inhibit bacterial growth)

and incubated at 25°C for 3 days. Isolates were sub-cultured onto potato dextrose

agar (PDA, Merck) via hyphal tip transfer to obtain pure cultures. After 7-10 days

incubation, isolates with different colony morphologies characteristic of Fusarium

were selected for further studies.

Fusarium isolates (168) were recovered from onion bulbs showing basal rot

symptoms from 17 samples collected in the UK (Table 2). Imported onion bulbs and

sets were also used for isolations (35 isolates) to investigate the possibility of new

Fusarium isolates being introduced into the UK from other countries (Table 2).

These were imported originating from the Netherlands, Belgium and France, while

imported onion bulbs were from Spain, the Netherlands, Chile and New Zealand.

Garlic, shallot and leek plants were collected from production and processing sites in

the UK in order to define which Fusarium species were present on other Allium

crops (Table 2). Nineteen isolates were obtained from these.

Additionally, Fusarium isolates (77) were also obtained following isolations

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were air dried and sieved before plating on peptone PCNB agar (PPA) and Komada`s

media (Nash & Snyder, 1962; Komada, 1975). To obtain individual Fusarium

colonies, 1 x10-2 and 1 x10-3 dilutions were used (starting with 1 g of soil in 10 ml

sterile distilled water). After 10 days isolates were sub-cultured onto PDA.

A field in Warwickshire (Table 2, sample reference 17) was chosen for

sampling as the onions recovered showed basal rot symptoms (Figure 4A). Five soil

samples (1 – 5) were collected from this field. A second field (Table 2, sample

reference 6) in Suffolk showed severe basal problems at the half of the field (Figure

4B). Five soil samples were collected from both symptomless (1 - 5) and diseased (6

- 10) bulbs. A third field in Essex (Table 2, sample reference 8) was sampled three

years after onions were grown and also a year before the next onion crop was due to

be grown (Figure 4C). The cropping history of the field was well documented; onion

was grown in 4-year-rotation with winter wheat and sugar beet. Five samples (11 –

15) were collected. The same onion field in Essex (Table 2, sample reference 8) was

sampled intensively shortly before harvesting in September 2010. To study the

spatial variation of Fusarium species in these fields, a total of 70 bulbs were taken

from positions approximately 10 m apart by walking diagonally and transversally

across the field. The area sampled had a long history of onion basal rot and the first

incidence of the disease in the 1980’s was reported in the same region and by the

same grower. A four-year crop rotation was used to grow onions on this field in

alternation with winter wheat and sugar beet.

All Fusarium isolates were stored on Spezieller Nährstoffarmer Agar (SNA,

Nirenberg, 1976) at 4°C or as spore suspension in 15% glycerol-water solution at -

Table 2. Origin, date of collection, source, host, sample material and number of isolates obtained of samples collected for isolation ofFusariumspecies.

Reference number

Location of onion field

Date of

collection Source Host

Sample material

No. of isolates

1 Bedfordshire Jul-10 stores onion bulb 4

2 Bedfordshire Oct-09 stores onion bulb 6

3 Bedfordshire Oct-09 stores onion bulb 4

4 Bedfordshire Oct-09 stores onion bulb 12

5 Essex Mar-09 stores onion bulb 8

6 Essex Oct-09 stores onion bulb 10

7 Essex Aug-10 field onion bulb 49

8 Essex Sep-10 field onion bulb 31

9 Suffolk Oct-09 stores/field onion bulb 5

10 Suffolk Jun-09 stores onion bulbc 7

11 Suffolk Dec-09 stores onion bulb 5

12 Suffolk Jul-09 field onion bulb 7

13 Suffolk Nov-10 stores onion bulb 7

14 Lincolnshire Mar-09 field onion bulb 4

15 Lincolnshire Aug-10 stores onion bulb 1

16 Nottinghamshire Oct-09 stores onion bulb 2

17 Warwickshire Dec-08 stores onion bulb 6

17 Warwickshirea Mar-09 field onion soil 36

6 Suffolk Oct-09 field onion soil 24

8 Essexb Oct-09 field onion soil 17

18 The Netherlands 2009 stores onion set 5

19 France 2009 stores onion setc 1

20 Belgium 2009 stores onion setc 1

21 New Zealand 2009 stores onion bulb 1

22 The Netherlands 2010 stores onion bulb 4

23 Spain 2010 stores onion bulb 16

24 Chile 2011 stores onion bulb 7

25 Cambridgeshire Jun-10 glasshouse leek stalk 8

26 Bedfordshire Nov-10 store garlic cloves 8

27 Bedfordshire Nov-10 store shallot bulb 3

Total 299

a

:Year after onion crop grown, b Year before onion was grown, wheat field at the time of samplingc: plant material did not show symptoms of onion basal rot

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Figure 4.Soil sampling points (marked with circles) of three onion fields with onion basal rot incidence. A Five soil samples (1 - 5) were collected from field in Warwickshire with severe basal rot incident (reference number 17); B Field in Suffolk (reference number 6), onions showing onion basal rot symptoms were found only from half of the field (shaded in green). Five samples were collected from symptomless half (1 – 5) on the field and five samples (6 – 10) were collected from diseased half of the field; and C Five samples (11 - 15) were a field in Essex (reference number 8) with onion basal rot history.

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