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Fungal pathogens have been identified as being the primary cause of parsnip

(Pastinaca sativa) canker, which is a major contribution to the downgrading and

rejection of parsnip crops, leading in some years to considerable economic losses (Gladders, 1997). The occurrence and type of symptoms can be highly variable depending on a number of factors, including cultivar, cropping history, weather conditions and previous disease incidence (McPherson, 2013). In the UK, fungal parsnip canker is responsible for average crop losses of 20%, often more severe in wet seasons, resulting from a greater number of spores being produced (Channon 1963b), whilst hot, dry conditions were found to retard the development of canker symptoms (Cerkauskas, 1985). Other countries have reported much greater losses, with Australia reporting up to 80% reduction in marketable yield in spring-harvested crops for the past 40 years (Minchinton, 2008).

There has been a general lack of research into the cause of parsnip canker, with

Wilkinson (1952) first isolating Itersonilia spp. from parsnip canker. Further work

by Channon (1956) described a range of pathogens as the causes of parsnip canker. More recently, the research of Gladders (1997) and McPherson (2013) gave an updated report on the causes of root blemishes in the UK which included

C. destructans, M. acerina, Fusarium spp. and Phoma spp. However, these

studies focussed on control strategies for the prevention and treatment of root canker pathogens. Channon (1963a) described two main types of canker typically present on the crown and shoulder of parsnip roots in the UK. These symptoms

were: a black canker, caused by Itersonilia pastinacae (I. pastinacae), Phoma

spp., or both; and an orange-brown canker of unknown cause. Results indicated

581 roots sampled (63%), whilst pathogenic Phoma spp. was responsible for approximately 15% of symptoms. Further work by Channon (1963a) suggested a ‘consortium’ of fungi causing black cankers, while fungi isolated from brown cankers failed to induce similar symptoms. However, the development of brown cankers appeared to be associated with growth splitting, where root tissue was split open, generally as a result of irregular moisture conditions. Channon

(1963a) used Koch’s postulates to show that I. pastinacae isolated from brown

cankers produced typical black canker symptoms following re-inoculation on parsnip.

Outside of the UK, I. perplexans has been implicated as the primary causal agent

of parsnip canker in Australia (Minchinton, 2008), being responsible for 21.6% of positively identified isolations. In addition, a range of other fungal pathogens have also been identified as causal agents of parsnip canker in Australia. These

include: Fusarium spp., Cylindrocarpon spp., Mycocentrospora acerina and

Phoma spp. Phoma spp. were identified as being the primary cause of parsnip

canker in Canada, with Phoma complanata identified as causing up to 80% of

crop losses (Cerkauskas, 1985). This pathogen was reported to be a seed-borne pathogen and a strong correlation between severity of foliar symptoms and incidence of root canker was found (Cerkauskas, 1987). Despite having a

narrower host range than I. pastinacae, P. complanata was found to be capable

of over-wintering and surviving in soils for up to five months (Cerkauskas, 1987).

Mycocentrospora acerina has also been described as a causal agent of canker

symptoms similar to those of I. pastinacae (Channon, 1965). Symptoms were

generally described as black lesions, often surrounded by a brown/red ring. Channon (1965) found there was no difference in size or severity of symptoms

on wounded or unwounded roots when inoculated with M. acerina, while the

pathogen’s wide host range (Hermansen, 1992) and capacity to grow and infect at temperatures below 0°C suggests an ability to develop symptoms under

acerina being isolated from Australian parsnip crops (Minchinton, 2008); however, despite its wide host range it has not been documented as a pathogen of parsnip in other countries.

Cylindrocarpon destructans has been identified as another fungus causing

black/dark brown parsnip canker symptoms (Channon & Thomas, 1981). Isolates from lesions were confirmed as pathogenic and able to re-infect damaged/wounded parsnip roots, leading to the development of symptoms. This

fungus has a narrower host range than Itersonilia spp. and M. acerina, and is

generally considered to be a weak pathogen, particularly in relation to woody pine species, although it is considered to be a major pathogen of ginseng, grape

vines and parsnip (Channon & Thomson, 1981; Lombard et al., 2015; Cabral et

al., 2012). Whilst C. destructans has been identified as a parsnip pathogen in the

UK and Australia, its global impact is more closely associated with infection of grape vines and ginseng (Probst, 2011).

More recently, Pythium spp. have also been implicated as part of a disease

complex causing parsnip root rot. Parsnip and parsley (Petroselinum crispum, a

related umbelliferous species) crops in south-east Australia suffer from 80-100% crop losses due to seedling damping-off, minor root lesions and mature root rot

(Petkowski et al., 2013). A total of nine Pythium spp. isolated from diseased roots

were all found to be pathogenic upon inoculation of fresh parsnip tissue. Of the species isolated, several have been associated with a range of root symptoms in different plant species, including parsnip, in Canada, the Netherlands, South

Africa, Germany and the UK (Petkowski et al., 2013). In glasshouse pathogenicity

tests, Pythium sulcatum and Pythium intermedium were found to be most

pathogenic in causing damping-off symptoms in parsnip seedlings (Petkwski et

al., 2013).

Pathogen persistence in soils has been suggested as an important factor in the

demonstrated by Smith (1966), where the pathogen was found to survive in the soil in infected roots for at least 12 months. It was additionally found to survive as chlamydospores in the soil for at least 12 months in Australia (Minchinton,

2008). These results, alongside studies on M. acerina and C. destructans

(Evenhuis, 1995; Minchinton, 2008), suggested that chlamydospores produced

by I. pastinacae, M. acerina and C. destructans were able to survive in the soil for

long periods, indicating that, without regular crop rotation with non-host plants, these pathogens can persist in causing infection (Minchinton, 2008).

In addition to fungal pathogens, Chamaepsila rosa (carrot root fly) is considered

to be a major contributor to the incidence of parsnip canker within the UK. Larvae cause damage by tunnelling through the tissue, predisposing roots to fungal infections (Stone, 1954). Controlling carrot root fly incidence was also found to correlate with a significant reduction in parsnip canker incidence (Collingwood & Croxall, 1954). Carrot fly control usually involves pre-drilling soil prior to seeding, and treating crops with insecticides such as phorate and

diazinon (Minchinton, 2008; Sivasubramanaim et al., 1999). Carrot root fly has a

global distribution, being found in North America, UK, Europe and Eurasia. Whilst not an issue in Australia, carrot root fly has also been documented in New Zealand, and it is therefore considered a major biosecurity issue for carrot and parsnip growers in Australia (Minchinton, 2008).

The most recent investigation into the range of root disease symptoms observed on parsnips in the UK was carried out by McPherson (2013) using samples from fields and packhouses in Norfolk and Yorkshire during the years 2010/2011. They identified 14 different types of symptoms from which seven different species of

fungi were isolated, with C. destructans being frequently found (Table 2.1). The

range of symptoms were significantly greater than previously identified on parsnips by Channon (1963a; 1963b; 1965) and, due to multiple organisms being recovered from lesions of each symptom type, it can be concluded that a

complex of fungal pathogens are responsible for parsnip canker and other root rot symptoms (Minchinton, 2008).

Table 2.1 – Root symptoms and associated isolated pathogens for field and packhouse parsnip roots in 2010/2011 (after McPherson, 2013).

Other than the McPherson (2013) study, little is understood about the extent and cause of parsnip canker within the UK. The aims of the work described in this chapter are:

• To collect samples of diseased parsnip roots;

• To isolate and identify the causal agents of parsnip canker using

morphological and molecular techniques;

Symptom Organisms recovered

Crown canker Botrytis cinerea, Fusarium sp.

Black crown lesion Cylindrocarpon destructans, Fusarium sp.,

Itersonilia sp., Verticillium sp.

Black shoulder lesion Cylindrocarpon destructans

Young red crown lesion

(watery) Cylindrocarpon destructans, Fusarium sp.

Young red shoulder

lesion (watery) Botrytis cinerea, Fusarium sp.

Red/brown mid-root

canker Cylindrocarpon destructans

Deep soft rot Cylindrocarpon destructans, Fusarium sp.

Dry scars Botrytis cinerea, Cylindrocarpon destructans, _{Fusarium sp., Phoma sp.}

Dark narrow scars Cylindrocarpon destructans, Fusarium sp.

Ginger blotch Botrytis cinerea, Cylindrocarpon destructans, _{Fusarium sp., Phoma sp.}

Red spots/speckles and

stripes Cylindrocarpon destructans, Fusarium sp. sp., Phoma

Cavity spot - like Botrytis cinerea, Fusarium sp., Cylindrocarpon sp.

Red-root scars Cylindrocarpon destructans, Phoma sp.