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One of the main outcomes of this study is an understanding of the widespread spatial and temporal variation in disease incidence in strawberries in Great Britain. Whilst temporal variation is already well understood with high and low disease severity years, often depending on the disease epidemiology and effects of weather, spatial variation can often be hard to capture on a national scale. This kind of variation has been observed previously for diseases of wheat throughout the UK (Hardwicket al., 2001). With strawberries, spatial variations in the four studied diseases were found to be independent of the size of the strawberry sector in that region or county, and following spatial statistical analysis carried out on the data, were also found not to be due to chance. Thus other reasons are responsible. The most important of these is probably the weather. Whilst it was not possible to correlate the disease events with prevailing weather events in the counties where disease outbreaks occurred, a lot of epidemiological studies have been carried out on the diseases to establish the effect of weather variables on disease incidence and severity (Xiao et al., 2001, Maas, 1998, Hickman and English, 1951a, Blanco et al., 2004). Prevalent weather patterns in the different counties could affect both crop development and diseases differently. Counties with weather patterns that are more conducive to the development of a certain disease would be expected to have a higher incidence of that disease. For instance, red core is known to be affected by high rainfall and flooding, and could even develop in freely draining soils if the rainfall was high enough (Hickman and English, 1951a), as was found to be the case with the large number of disease infestations of red core in the strawberry sector in Cornwall.

Another factor affecting spatial variation in disease incidence was the soil substrate in which strawberries are grown. This is particularly so for soil borne diseases such

as Phytophthora fragariae and Verticillium wilt. In the former case, higher disease

incidence was found even in areas which have a lower rainfall than that described as the ideal amount for the disease to occur (Reid, 1949). In these cases, such as in Kent, the majority of the outbreaks occurred in soils that were not freely draining and, subsequently, were prone to water logging. This created the ideal conditions for red core to develop and in doing so explained why a lower than ideal mean October to March rainfall still ended up with a much higher than expected number of recorded infestations of red core in that county. In fact, the most severe infestations

were on soils that were naturally wet, with a much larger area per farm being scheduled, not just in counties with a high number of infestations, but even in counties with a lower than expected number of infestations, such as Norfolk. This clustering of red core infestations had already been observed in the 1940s (Hickman and English, 1951a), although this current study shows it to be statistically significantly on a national level.

The incidence of Verticillium wilt, on the other hand, depends on the cultivation history of the field in which the strawberries are being grown. If infestations of

Verticillium dahliae have previously been recorded in the field, the inocula can lie

dormant for up to 25 years (Raffle and O’Neill, 2006) and the quantity of inoculum in the soil could increase as the field is repeatedly cropped with strawberries or another plant susceptible to the disease (Harris and Yang, 1996). This could explain the high number of infestation records in Kent because not only have strawberries been an important crop in the county since the 1920s (Figure 2-3), but so have Hops (Harvey, 1963) which, being susceptible to the same disease (Raffle and O’Neill, 2006), could have led to an increase in inoculum levels of the disease in Kentish soils.

The use of susceptible cultivars in the past (Ellis, 1970) has also influenced the incidence of disease in the strawberry sector. In all four diseases, the two cultivars with the highest record of disease incidences were pre-Second World War cultivars, in a time when developing disease resistance was not yet well understood (Harris et al., 1934). In fact, the cultivars were so susceptible toP. fragariaein particular that it is reported that there was “devastation en masse” of strawberry beds in the affected areas (Wardlaw, 1927). In an experiment carried out by Hickman and English (1951b), the cultivars Madame Lefevre and Huxley’s Giant were found to be the two varieties most susceptible to P. fragariae. In the disease datasets collected through this study, these two cultivars also had the highest records of red core infestations over the 90 year period, even though they went out of use by the early 1960s. Conversely, Cambridge Favourite, which was the most popular cultivar in the 1960s and 1970s, obtained relatively few records of disease infestations in this study, being ranked eighth and responsible for only 3.7% of the infestations where the cultivar was known. This could be attributed to its partial resistance toP fragariae(Gooding, 1972, van de Weget al., 1989) which was thought to be derived from its tolerance to

drought, thus compensating for the loss of roots through disease infestation. In comparison, twice the number of disease incidence records were obtained for Elsanta, which was the most common variety grown since the early 1980s. This lies in its susceptibility to the pathogen (Perry and Raffle, 2004). The same could be said for Verticillium wilt, the other soil borne disease covered by this study. In Keyworth and Bennett (1951), Huxley’s Giant and Madame Lefevre were considered as severely susceptible to the disease, whilst Cambridge Vigour was described as very highly susceptible (Talboys and Bennett, 1969) and through this study these three cultivars obtained three of the four highest records of infestations from Verticillium wilt. Notwithstanding the improvement in disease resistance in strawberry cultivars today (Shawet al., 2010), the disease still remains one of the most devastating of all soil-borne strawberry diseases in the UK (Raffle and O’Neill, 2006, Lole et al., 2009). In view of this, more growers have been raising their crops out of the ground into soil-less substrates which are free of soil borne diseases (Lietenet al., 2004). Powdery mildew and grey mould have been amongst the two most common diseases in England and Wales since the 1920s (MAFF, 1922, British Mycological Society, 1929, Dennis and Foister, 1942, Baker, 1972, Garthwaiteet al., 2006) and have been continuously recorded in the datasets studied. Since the mid 1960s, over 80% of fungicides used on strawberries were targeted at these two diseases (Sly, 1975, Steed

et al., 1978, Sly, 1982, Wilder, 1987, Davis et al., 1992, Garthwaite and Thomas,

1996, Woods, 1999, Garthwaite and Thomas, 2000, Garthwaite and Thomas, 2003, Garthwaiteet al., 2006). Until the late 1990s, a greater proportion of fungicides were used to combat B. cinerea. This changed after 2001 with the increased use of protected cultivation (2006, Garthwaite et al., 2009), when P. aphanis became the pathogen against which most spray applications were made. This impact of protection on powdery mildew in strawberries could be seen in the datasets whereby the majority of disease records where protected methods were used were attributable to this pathogen. The impact of this cultivation practice on powdery mildew in strawberries has been known for a few decades (Jordan and Hunter, 1972), as protected cultivation is known to favour the development of P. aphanis and reduce the incidence of B. cinerea (Xiao et al., 2001). This increased use of protection by the UK strawberry industry in the last decade (refer to Figure 2-22) has led to the

replacement of B. cinerea by P. aphanis as the most common and sprayed against disease in strawberries in the UK.

The use of long term, national disease datasets like these can be useful in providing the opportunity to study the development and influence of plant diseases on a sector during a relatively long time scale. Nevertheless, there are limitations in the data in terms of content and their reliability. For instance, data might not be available on all of the variables for each record, such as the production methods and cultivars used. Moreover, one cannot assume that this is a comprehensive list and that every single outbreak in each farm in England or Wales since 1920 is actually recorded in these datasets. There inevitably will be disease outbreaks that were not reported to the authorities, and reports might also depend on the consistency in the reporting of disease outbreaks by regional inspectors. These reasons, together with the lack of an exact date for the disease outbreaks, have made it impossible to link the disease outbreaks to obvious, real-time historic weather events. In fact statistical analysis used to correlate disease presence or absence during a year with over 2500 potential weather predictor combinations was done to determine which weather events were responsible for the disease events. These proved futile as no solid correlations were obtained, and eventually had to be left out of this study.

Despite these limitations, the records do provide much useful data and long term trends, showing spatial clustering in plant disease. The data also provides evidence for disease being a driver of change in the strawberry sector, such as the development of disease resistance in strawberries since the 1940s. More detailed records were available for P. fragariae, which were collected from 1946 to 1995. These years correspond to when legislation was introduced to prevent the spread of this disease (1946), and the disease became notifiable by law (1952) requiring registration. By 1993 registration was no longer required except in nurseries selling runners with plant passports. Some of the best data for this disease were from this period since it was more likely that an infestation was recorded as there was a legal obligation to do so. Nonetheless, there is evidence that the rest of the datasets for the other three diseases and for the earlier records of red core are reliable. Firstly, the earliest records of the diseases encountered in these datasets were similar to that found in the literature, such as the first records of red core andVerticilliumwilt in 1920 and 1933

respectively in these datasets being around the same time as those in published literature (MAFF, 1922, Keyworth and Bennett, 1951).

Secondly, evidence for events that are mentioned in the literature such as the unhealthy plant stock and cultivars that led to a decline in the strawberry sector between 1920 and the second World War (Ellis, 1970), is also evident in these datasets; the three most popular varieties in that phase were responsible for just under half of the total records for which cultivars were known and taken over the 90 years covered by the study. Moreover, evidence of the movement of infected plant material through trade, in particular through a major outbreak of Verticillium wilt in several nurseries after the second World War (Hickman and English, 1951a, Keyworth and Bennett, 1951), is seen in the disease datasets through a drastic increase in nationwide Verticillium records post World War Two (see Figure 3-1). Thirdly, whilst it was known in the industry that certain diseases were more severe in certain areas (Baker, 1972), this study has shown evidence of spatial variation on a national scale in England and Wales.