SURVEY OF POTATO DISEASE INCIDENCE

PROJECT FINAL REPORT

A survey to determine the incidence and severity of potato diseases both in the growing crop and on tubers

in England and Wales

N J Bradshaw ADAS, Henstaff Court Business Centre, Groesfaen, Cardiff CF72 8NG

J A Turner and S J Elcock

Central Science Laboratory, Sand Hutton, York YO41 1LZ

ABSTRACT Generation of statistically valid datasets on the status and importance of diseases and influences of current management practices can make a very significant contribution to DEFRA policies for movement towards more sustainable potato production. The changing status and importance of potato diseases has major impacts not only on the UK potato industry but also on consumers, particularly with concerns over pesticide residues and food quality, and on the environment. Data can assist in the development of strategies to improve seed health, pesticide minimisation and regulation, improvement of food quality and the overall competitiveness of the UK potato industry. The aim of this project was to gather data on the incidence and severity of potato diseases in England and Wales, to collect associated data on agronomic practice, pesticide use and storage regimes, and to gather information on site and environmental factors. The survey has generated data covering a five year period from 1996 to 2000 which have been used to identify the importance of individual potato diseases in England and Wales and to investigate the influence of a wide range of factors such as cultivar, planting date, seed class, seed grade, geographical location, pesticide use and the effects of storage regimes on disease. Full reports of these analyses can be found in the Annual Potato Disease Survey Reports (available from CSL). Diseases were assessed on seed stocks prior to planting, on the growing crop in the spring just before the haulm met along the rows, in the summer just prior to senescence, on the ware tubers after harvest and again after a period in store. Each year from 1996 to 2000, a total of 80 crops of the four main cultivars Cara, Estima, Maris Piper and Record (replaced by Pentland Dell since 1997) were surveyed. Crops were selected in proportion to the area of the above cultivars grown in each county based on planting statistics provided by the British Potato Council and between them, these cultivars accounted for 40% of the acreage planted with second early and maincrop potatoes each year. The data collected were entered onto an INFORMIX relational database at the Central Science Laboratory for subsequent analysis. Data have supplemented the existing DEFRA Potato Disease Survey Database at CSL to generate a dataset covering the period 1993-2000. Summary information and interactive searches which interrogate the database directly can be accessed via the web at http://www.csl.gov.uk/prodserv/cons/crop/survey/. Common scab, silver scurf, black dot and black scurf were the most common surface blemish diseases found on the seed tubers although the severity of each disease varied from year to year. Common scab, silver scurf and black dot were also the most severe diseases on the ware tubers with common scab and black dot again showing annual variation. In some, but not all years of the survey, there was a weak but statistically significant relationship between disease severity on seed and ware tubers for silver scurf, common scab and powdery scab (although the severity of the latter was low). Conversely, there was no correlation between seedborne infection with black dot and the disease on the ware tubers at harvest. Internal rust spot (IRS) was the most common internal disorder of the seed and ware tubers. Gangrene, dry rot and spraing were present in both seed and ware stock but at low levels. Tuber blight and black leg were rarely found in seed stocks but blight was more prevalent in ware tubers following the blight epidemics of 1997-2000. Quality of seed prior to planting was high with the majority of growers selecting Super Elite seed. There was no consistent reduction in surface and internal diseases/blemishes in certified seed compared to home saved seed but levels of virus were significantly lower in certified stocks. Disease levels recorded on ware crops during the survey have been used in conjunction with data from the BPC to calculate economic losses due to disease. Results show that in all years over a quarter of crops would have failed the maximum tolerance for the pre-pack washed market and that there is little evidence of the benefits of using high quality seed. Highest losses occurred in 1995 when 48% of samples would have failed.

The most common fungal disease in the spring affecting a mean of 72% of crops over all years of the survey (range 59-81%) was stem canker. Stem canker was the most severe disease within crops affecting 10-27% plants in the spring and 4-17% plants in the summer. The effect of stem canker on the yield and quality of potatoes is not known and further investigation is required. Foliar blight was recorded in the summers of 1997, 1998, 1999 and 2000 with the highest levels being recorded in the ‘blight years’ of 1998 & 2000 affecting a mean of 7 & 3% plants respectively. The incidence of the virus diseases Potato Leaf Roll Virus (PLRV), Potato Virus Y (PVY) and Mild Mosaic Virus (PVX) in seed stocks and during the growing crop inspections was low. There was a good relationship between the visual (crop) inspections for PLRV & PVX and infection in the ware tubers but for PVY the relationship was poor. Higher levels of PVY were found in the ware tubers than the crop inspections would have suggested. This may be due to late or latent infection at the time of the summer assessment and may have implications for seed certification and inspection. This illustrates the benefits of tuber indexing and routine virus testing of ‘home-saved’ seed. The effect of cultivar reaction to different diseases generally reflected their NIAB resistance ratings where these are available (Anon., 2000). In some but not all instances the NIAB resistance ratings for disease resistance/susceptibility were robust. Common scab was more severe on Maris Piper (rating 1) compared with Cara (rating 6) on seed tubers. However on harvested tubers, common scab was almost as severe on Pentland Dell (rating 5) as Maris Piper. Powdery scab was more severe on Pentland Dell (rating 7) compared with Cara (rating 3) and Record (rating 7). Resistance ratings are not available for black scurf, silver scurf or black dot. The results for this survey, however, provide indications of cultivar reactions. Black scurf was more severe on Pentland Dell than on Estima, silver scurf was more severe on Estima and Record than on Cara, Pentland Dell and Maris Piper, and black dot was more severe on Maris Piper and Record than on Cara. Foliar blight was found in crops in 1997, 1998, 1999 & 2000 which in view of the relatively small number of crops inspected indicated the prevalence of the disease. Fungicide use for the control of late blight was shown to be the main pesticide input to the growing crop and the numbers of treatments varied considerably from crop to crop each year. Nevertheless, inputs did not vary consistently according to annual changes in disease pressure, with some crops receiving up to 18 fungicide applications even in years when blight was not perceived to be a problem. Research into the effects of excessive use of blight sprays has indicated that there is no direct yield penalty incurred by applying up to eight fungicide sprays in blight free situations and that little improvement occurred after more than nine applications in diseased crops. These data show that there is scope for the development of a more robust and sophisticated Decision Support System for UK growers than is currently available to ensure that fungicide use is justified and more effectively targeted. Principal conclusions from the project are: 1. Most growers are planting high quality seed but the benefits in reducing disease levels on

resultant ware could not be demonstrated, other than for virus diseases. 2. Quality of harvested tubers destined for the pre-pack washed market was low with at least 25%

of stocks each year not meeting tolerance limits set by retailers. 3. Use of blight fungicides is excessive with up to 18 sprays used in a season. There is little

evidence of recognition of annual changes in disease risk in relation to annual fungicide use. In high blight years intervals between fungicide applications were reduced below legal limits. This is a major area for technology transfer and pesticide minimisation.

Suggestions for further work:

1. Determine the importance of stem canker/black scurf on the yield and quality of potatoes. 2. Develop robust sampling and predictive models for the main soil–borne diseases of potatoes. 3. Develop a more robust and sophisticated Decision Support System for the control of late blight.

BACKGROUND Generation of statistically valid datasets on the status and importance of diseases and influences of current management practices can make a very significant contribution to DEFRA policies for movement towards more sustainable potato production. The changing status and importance of potato diseases has major impacts not only on the UK potato industry but also on consumers, particularly with concerns over pesticide residues and food quality, and on the environment. Data can assist in the development of strategies to improve seed health, pesticide minimisation and regulation, improvement of food quality and the overall competitiveness of the UK potato industry. The main scientific objectives were: (i) to measure the incidence and severity of potato diseases on seed tubers at planting, on the

growing crop, on tubers at lifting and following a period in store (ii) to collect agronomy/ husbandry data, site & environmental data (iii) to identify in-season, geographical, meteorological, husbandry and crop relationships (iv) to identify annual variation over a five-year period. Potatoes are susceptible to over 30 bacterial, fungal and virus diseases and 12 common disorders, all of which have economic significance in the UK. The ware potato crop in Great Britain in 1999 occupied 155,600 ha with an estimated value of £709m per annum (Anon., 2000). Mean annual crop losses due to established diseases may account for up to 15% in ware and seed crops, or £95m annually. Diseases vary in the type and magnitude of their effects on crops. Potato blight (Phytophthora infestans) is capable of causing total crop loss by destroying the haulm; it can also infect tubers prior to storage providing opportunities for secondary bacterial rots to develop in store. Verticillium wilt (Verticillium dahliae), black leg (Erwinia carotovora var. atroseptica) and the virus diseases, such as potato leaf roll, potato virus X and potato virus Y and other virus complexes in specific cultivars also reduce yield. Diseases, such as stem canker (Thanatephorus cucumeris) and skin spot (Polyscytalum pustulans) delay emergence and can lead to a loss of revenue due to missing the critical early market. Tuber quality is reduced by several diseases which produce economically significant surface or internal blemishes and affected tubers are rejected by customers, such as supermarkets (Hide, 1992). The incidence and severity of the 30 or more diseases and disorders are influenced by a large variety of interacting factors throughout the life of the crop, in seed and ware production and during storage (Elcock et al., 1998). Due to its clonal nature, the ‘life’ of a crop may extend for several years, increasing the potential for accumulation of disease incidence and severity. There has been limited work on the incidence and severity of potato diseases in ware growing areas (Ebbels, 1983; Hide, 1981), but this survey, which has been undertaken annually since 1993, is the first comprehensive survey of England and Wales. Some of the early data have been summarised by Hardwick & Turner (1996a & 1996b).

METHODOLOGY Each year from 1996 to 2000, 80 crops of potatoes comprising four cultivars, Cara, Estima, Maris Piper and Record (replaced by Pentland Dell from 1997 due to reduced planting area of Record), grown on farms in England and Wales were surveyed for the presence of diseases. These cultivars also possess a range of disease resistance characteristics (Anon., 1996-2000) and this would be expected to have an influence on disease incidence and severity. The number of farms sampled was in proportion to the area of the four cultivars grown in each county based on planting statistics provided by the British Potato Council. Between them, the above cultivars accounted for approximately 40% of the area planted with second early and maincrop cultivars. Prior to planting, seed tubers from each crop were sampled and assessed for surface and internal diseases and blemishes (Anon., 1976) and were also tested for virus infection using a growing-on test in the glasshouse (Hill, 1977). The growing crops were assessed for foliar and stem diseases in the field on two occasions, first as the haulm met along the rows in the spring and again in summer just prior to crop senescence. Tubers produced from these crops were assessed for surface and internal diseases and blemishes on two occasions; the first assessment within 14 days of harvest and the second, usually in January, following a period in store. Harvested tubers were assessed again for virus infection during the winter using the growing-on test. More details of the survey methodology can be found in the survey report in Annex1. Isolates of Helminthosporium solani from seed and ware tubers were assessed for resistance to thiabendazole between 1996-1999. Isolates were taken from slices of washed tubers, with silver scurf lesions where present, and incubated in humid conditions at 15°C. If no lesions were visible, slices were taken from the stolon end of the tubers. After 7-14 days the slices were examined for conidiophores of H. solani. Where found, conidia were picked off with a sterile needle, streaked onto 2% malt extract agar and the plates incubated at 20°C. One isolate was obtained from each tuber and up to 12 isolates from each sample. When colonies were at least 1 cm diameter (2-3 weeks), 2 mm diameter plugs of agar from the leading edge were placed onto two replicate plates each of 2% malt extract agar containing 0, 5 and 100 mg thiabendazole/litre (ppm) and incubated at 20ºC. Up to five different isolates of H. solani were inoculated on to each replicate plate. Growth was assessed after 2-3 weeks and plugs that produced no growth at 5mg/litre thiabendazole were regarded as sensitive to the fungicide. A comprehensive questionnaire was completed for each crop detailing husbandry and pesticide use. Disease assessment data and information on crop husbandry/pesticide use were entered into an INFORMIX relational database at the Central Science Laboratory for subsequent analysis. The latest results can be found on the CSL Web site at http://www.csl.gov.uk/prodserv/cons/crop/survey/. An interactive graphics system displaying data from the potato disease survey is also available on the CSL website.

In 1997, discussions were held with Logica Space Systems and the British Potato Council on the

potential for utilising the database in remote sensing studies. This resulted in a project funded by the British Potato Council to evaluate the feasibility of using satellite remote sensing for the detection of potato late blight (Bradshaw, 1997).

RESULTS Details of disease levels in each of the survey years are available in the annual survey reports (Bradshaw, Turner & Elcock, 1997; 1998; 1999; 2000 & 2001). The figures referred to in the text can be found in the final annual survey report (2000/01) which is at Annex 1 of this report. Statistical analysis Actual means are quoted in the tables although Analysis of Variance was performed on transformed data (using an angular transformation (ASIN(SQRTx/100)*57.29578). Correlations between disease severity on seed tubers and harvested tubers were carried out using Pearson’s Correlation Coefficient. Disease incidence on the seed before planting The most frequently encountered surface diseases on seed tubers were silver scurf, common scab and black scurf. These diseases affected the majority of the seed stocks examined in all years of the survey, being recorded on a mean of 98%, 83% and 60% of seed stocks respectively over the period of the survey. Within stocks, silver scurf was also the most prevalent surface disease affecting a mean of 56% of tubers compared with 23% and 7% of tubers for common scab and black scurf (Fig 3, Annex 1). There were significant seasonal differences in the severity of all surface blemish diseases as shown in Table 1.

Table 1. Disease severity on seed tubers, 1996 to 2000 - mean % surface area affected

Disease 1996 1997 1998 1999 2000 p value

Black scurf 0.18 bc 0.20 bc 0.31 ab 0.09 c 0.42 a <0.01 Silver scurf 16.93 a 8.85 c 11.25 bc 8.73 c 14.40 ab <0.001 Black dot 0.08 a 0.20 ab 0.91 c 0.74 c 0.37 bc <0.001 Skin spot 0.02 a 0.09 abc 0.03 ab 0.08 bc 0.14 c <0.01 Common scab 1.95 b 1.50 ab 0.94 a 0.94 a 1.10 a <0.01 Powdery scab 0.03 ab 0.01 a 0.09 b 0.26 c 0.11 b <0.001 NS = not significant (p>0.05) values sharing the same letter in each row are not significantly different according to least significant difference test.

Silver scurf was the most severe surface disease in all years, particularly in 1996 and 2000 when severity was significantly higher than in 1997 and 1999 (P<0.001). This may be a reflection of store hygiene or storage conditions during the previous winter period when silver scurf lesions are known to develop on infected tubers. Despite the prevalence of common scab and black scurf in stock and within stocks, neither of these diseases severely affected seed tubers. The incidence and severity of powdery scab was extremely low but was most severe in 1999, a year with above average rainfall. Of the internal diseases/disorders, internal rust spot (IRS) was most frequently found in stocks (15%), followed by dry rot (8%), gangrene (6%) and spraing (4%). In all cases the incidence within stocks was low and except for IRS in 1998, less than 1% of tubers were affected. Both tuber blight and black leg were rarely found in seed stocks (Fig 4, Annex1). Disease incidence and severity in the spring and summer.

Stem canker was the most common fungal disease in the spring affecting a mean of 72% of crops over all years of the survey (range 59 - 81%). With the exception of stem canker, the incidence of fungal diseases on the haulm in the spring was low in all years (Fig 6, Annex 1). There were significant differences in the annual incidence of stem canker which increased during the period of the survey from 15% plants infected in 1996 to 27% in 2000 (Table 2). However, similarly high levels were recorded in 1993 (25% plants) in the previous MAFF funded potato disease survey (ROAME HP0113). Black leg was less common, affecting on average 9% of crops (range 3 -16%) and, although there were significant differences between years, the incidence of black leg within crops was low (<1.15% plants affected) (P<0.01). There were no significant annual differences in the incidence of skin spot on the haulm (P>0.05). At the summer assessment just prior to senescence, and over the period of this project, black leg, black dot and stem canker were found in an average of 27, 36 and 42% of crops surveyed. The incidence of these and other diseases in each of the survey years is shown in Fig 7, Annex 1 and in Table 3, together with data from the previous three years. There were significant differences in the percentage of plants affected by all of the diseases recorded with the exception of Botrytis and foliar blight. Botrytis is a ubiquitous fungus and is not considered to be a pathogen of potatoes but is commonly found colonising damaged plant tissue. Symptoms of Botrytis are sometimes confused with foliar blight infection by growers.

Table 2. Disease incidence on the haulm in the spring, 1993 to 2000 - mean % plants affected

Disease 1993 1994 1995 1996 1997 1998 1999 2000 p value Stem canker 24.55 cd 17.43 b 18.15 bc 15.15 ab 9.70 a 14.94 b 20.0 bcd 26.53 d <0.001 Black leg 0.90 bc 0.99 c 0.90 abc 0.10 a 0.15 a 0.96 bc 0.95 bc 0.30 ab <0.01 Skin spot - 3.80 1.70 0.75 1.00 2.18 3.30 3.34 NS - = not assessed NS = not significant (P>0.05) values sharing the same letter in each row are not significantly different according to least significant difference test.

Table 3. Disease incidence on the haulm in the summer, 1993 to 2000 - mean % plants affected

Disease 1993 1994 1995 1996 1997 1998 1999 2000 p value Black leg 3.60 d 1.45 ab 1.30 a 1.17 a 3.12 cd 1.76 bc 2.85 c 2.35 cd <0.01 Verticillium wilt 1.42 e 0.21 bcd 0.42 d 0.30 cd 0.68 d 0.05 ab 0.20 abc 0 a <0.01 Black dot 4.30 ab 2.18 a 6.81 bc 6.99 cd 10.81 de 7.15 c 13.10 ef 13.15 f <0.001 Stem canker - - - 11.80 c 3.07 a 5.39 a 16.85 d 17.10 d <0.001 Sclerotinia - - - 1.11 b 3.84 c 1.22 ab 0.20 a 0.70 ab <0.05 Botrytis - - - 5.42 4.31 8.37 3.35 2.25 NS Blight - - - - 1.20 6.88 1.95 3.40 NS - = not assessed NS = not significant (P>0.05) values sharing the same letter in each row are not significantly different according to least significant difference test.

Stem canker was the most severe disease in the summer and in 1996, 1999 & 2000 the percentage of plants affected was significantly higher than in 1997 & 1998 (P<0.001). The incidence of black leg

in the summer was higher than in the spring and was significantly higher in 1993, 1997, 1999 & 2000 compared with other years (P<0.01). Sclerotinia stem rot was found in 8% of crops examined between 1996 and 2000 (range 3 - 14%) with the highest incidence in 1997 which was significantly higher than all other years (P<0.05). Late blight (Phytophthora infestans) was recorded in 1997-2000 and in the ‘blight years’ of 1998 to 2000 in 24 & 13% of crops respectively. The incidence of foliar infection was particularly severe in 1998. Disease severity on ware tubers at harvest At harvest, black scurf, silver scurf, black dot and common scab were found in an average of 59% (range 54 -63%), 61% (range 51 - 63%), 56% (range 43 - 67%) and 85% (range 80 - 90%) of ware stocks respectively. The mean number of tubers affected within stocks in each year of the survey is shown in Fig 8, Annex 1. Whilst there were annual differences in the mean number of tubers affected by each disease, the largest differences occurred with common scab. This is probably a reflection of the rainfall, which, when occurring at or soon after tuber initiation, is known to reduce the levels of infection. This is clearly demonstrated by comparing the annual incidence of powdery scab (favoured by higher soil moisture) with common scab.

Table 4. Disease severity on harvested tubers, 1993 to 2000 - mean % surface area affected

1993 1994 1995 1996 1997 1998 1999 2000 p value

Black scurf 0.28 0.34 0.25 0.21 0.21 0.23 0.27 0.25 NS Silver scurf 1.48 0.79 2.26 0.92 2.08 1.24 1.30 1.96 NS Black dot 3.82 bc 2.92 abc 2.13 ab 1.69 a 4.65 c 2.08 ab 2.22 ab 2.21 ab <0.05 Skin spot 0.01 0.04 tr 0 0.01 0.02 0.01 0.02 NS Common scab 2.34 ab 3.50 b 3.45 b 2.39 ab 1.10 a 1.28 a 2.91 b 3.17 b <0.001Powdery scab 0.08 ab 0.03 a 0.04 a 0.01 a 0.17 ab 0.59 c 0.30 b 0.07 ab <0.001 tr = < 0.005 NS = not significant (P>0.05) values sharing the same letter in each row are not significantly different according to least significant difference test.

The severity of surface diseases and the annual variation is given in Table 4, and includes data for the period 1993-95. There were no significant differences in the severity of black scurf, silver scurf and skin spot (P>0.05). The incidence of common scab and powdery scab also fluctuated each season, as did the severity and there were significant differences for both diseases across the years (P<0.001). In wetter seasons when powdery scab was more severe (reflecting higher soil moisture), common scab was less severe. In all years of the survey internal rust spot was the most frequently encountered internal blemish on harvested tubers affecting a mean of 32% of harvested crops (range 17 - 41%). The proportion of crops affected by gangrene, dry rot and spraing was low, affecting on average 2.6%, 3.9% and 3.9% of ware stocks respectively. Tuber blight was recorded in all years except 1996. Within ware stocks at harvest, internal rust spot was the most prevalent disease (Fig 9, Annex 1) but levels were low and did not exceed a mean of 3.5% tubers affected (1997). The incidence of all other internal diseases was low and below 0.5% tubers affected.

Disease severity on ware tubers after a period in store During the period of the survey the mean proportion of crops affected by black scurf increased slightly from 58.8% at harvest to 67% crops affected following storage. The biggest increase occurred with silver scurf which was present in an average of 61% of crops at harvest but affected 87% crops after storage. This is probably due to store management and cross contamination from other stocks. Storage did not affect the incidence of other surface diseases. After storage there were increases in the incidence of gangrene (from 3 to 10% crops) and dry rot (from 4 to 9%) but not for spraing or internal rust spot. The proportion of ware stocks affected by tuber blight decreased probably due to secondary bacterial soft rotting, masking symptoms or causing complete disintegration. Within stocks, the proportion of tubers affected by silver scurf, gangrene and dry rot increased after storage (Figs 11 & 12, Annex 1). The incidence of other surface and internal diseases/blemishes remained unaffected by storage. Incidence of virus diseases The incidence of the severe virus diseases Potato Leaf Roll Virus (PLRV) and Potato Virus Y (PVY), and Mild Mosaic Virus (PVX) were assessed on the seed tubers prior to planting, on plants in spring and summer and on ware tubers after harvest. The results for the period of this project (1996-2000) and the previous survey (1993-1995) are given in Table 5.

Table 5. Incidence of virus diseases between 1993 and 2000 – Mean of all sites assessed each year.

1993 1994 1995 1996 1997 1998 1999 2000 p value % seed tubers PVX - - - 0.26 0 1.25 0.18 0.01 NS PVY - - - 0.83 0.17 0.88 1.18 0.77 NS PLRV - - - 0.15 0.21 0.10 0.03 0.06 NS % plants (spring)

PVY 0.30 0.10 0.25 0 0.10 0.35 0.15 0.76 NS PLRV 0.05 0 0 0 0.05 0 0 0.15 NS % plants (summer)

PVY 0.41 1.14 0.52 0 0.21 0.59 0.95 0.35 NS PLRV 1.06 1.82 0.83 0.46 0.99 0.59 0.60 0.15 NS % harvested tubers

PVX 0.30 2.28 1.26 0.10 0 1.78 0.39 1.93 NS PVY 14.30 a 13.76 a 24.62 b 9.77 a 11.45 a 8.49 a 10.30 a 8.30 a <0.001 PLRV 3.51 c 1.13 ab 2.00 b 1.08 ab 0.81 ab 0.28 a 0.50 a 0.71 ab <0.01 - = not assessed NS = not significant (P>0.05) values sharing the same letter in each row are not significantly different according to least significant difference test.

There were no significant differences between the annual incidence of PVX, PVY and PLRV in seed stocks or from assessments of the growing crops in the spring or summer (P>0.05). In all cases, the

incidence of virus infection was low. There were no significant differences in the annual levels of PVX in the ware tubers (P>0.05). The highest incidence of PVY in harvested tubers was recorded in 1995 which was significantly higher than all other years (P<0.001). There were no significant differences in PVY incidence in the other survey years. The highest incidence of PLRV occurred in 1993 when the level of harvested tuber infection was significantly higher than in all other years (P<0.01). There was a relatively low incidence of PLRV in the harvested ware tubers and the in-field visual inspection was a good predictor of tuber infection levels. In contrast, PVY incidence in the harvested ware tubers was higher and the in-field visual inspection was a poor predictor of tuber infection levels. Effect of planting date on disease levels There was little effect of planting date on disease levels. The only consistent trend was a higher incidence of stem canker in the crops planted latest (Fig.13, Annex 1). Effect of irrigation in disease levels Data did not clearly demonstrate a significant benefit of irrigation in the control of common scab but this was largely due to wet weather conditions in the years 1997,1998, 1999 and 2000. In all years except 1997, black dot was shown to be higher in irrigated crops (Fig. 14, Annex 1). Effect of cultivar on disease levels The severity of disease on the seed tubers (1996-2000), on plants in the spring and summer and on harvested ware tubers (1993-2000) of the different cultivars is shown in Tables 6-9. The cultivar Record was surveyed in 1993, 1994, 1995 and 1996 but was replaced by Pentland Dell in 1997, 1998, 1999 and 2000. Annual variation in cultivar susceptibility is shown in Figs. 15 - 19 in Annex 1. Seed tubers There were no significant differences between the different cultivars in the severity of black scurf, black dot, skin spot and powdery scab (P>0.05). The severity of silver scurf on Record was significantly higher (P<0.001) than on the other cultivars surveyed. The severity of common scab was significantly higher on Maris Piper compared with Cara which reflects their NIAB resistance ratings of 1 (susceptible) and 6 (moderately resistant) respectively (P<0.001). Neither of these two cultivars was significantly different from the other three (P>0.05).

Table 6. Disease severity on seed tubers (1996-2000) - mean % surface area affected

Cara

(1996-2000)

Pentland Dell

(1997-2000)

Estima

(1996-2000)

Maris Piper

(1996-2000)

Record

(1996)

P value

Black scurf 0.40 0.26 0.15 0.26 0.37 NS Silver scurf 5.36 a 5.91 ab 12.54 bc 13.62 c 30.49 d <0.001 Black dot 0.08 0.67 0.62 0.41 0.05 NS Skin spot tr 0.13 0.02 0.11 0.09 NS Common scab 0.44 a 0.54 ab 0.84 ab 2.03 b 1.14 ab <0.001 Powdery scab tr 0.09 0.22 0.04 tr NS tr = < 0.005 NS = not significant (P>0.05) values sharing the same letter in each row are not significantly different according to least significant difference test.

On the haulm in the spring. There were no significant differences between cultivars in the incidence of black leg, PVY and PLRV (P>0.05). This is surprising in view of the high NIAB resistance rating of Cara for PVY (7) but may be a reflection of the low levels of infection recorded at the spring assessment. Stem canker incidence was significantly lower on Estima compared with Pentland Dell, Maris Piper and Record (P<0.001).

Table 7. Disease incidence on the haulm in the spring (1993-2000) - mean % plants affected

Cara

(1996-2000)

Pentland Dell

(1997-2000)

Estima

(1996-2000)

Maris Piper

(1996-2000)

Record

(1996)

P value

Stem canker 17.96 ab 20.39 b 12.36 a 21.74 b 21.43 b <0.001 Black leg 0.58 0.10 1.07 0.47 0.63 NS Skin spot 0.18 a 0 a 0.16 a 1.04 b 0.24 a <0.001 PVY 0.09 0.10 0.70 0.07 0 NS PLRV 0.04 0 0.02 0.05 0 NS NS = not significant (P>0.05) values sharing the same letter in each row are not significantly different according to least significant difference test.

On the haulm in the summer. Although not significant (P>0.05), the higher incidence of PVY on the cultivar Record reflects the susceptible NIAB rating (2) for this disease. PLRV was significantly higher on Cara and Pentland Dell compared with the other cultivars despite their moderately susceptible NIAB resistance rating of 5 (P<0.01). Black leg was more common on Estima and Record more closely reflecting their susceptibility ratings (NIAB 2).

Table 8. Disease incidence on the haulm in the summer (1993 - 2000) - mean % plants affected

Cara

(1996-2000)

Pentland Dell

(1997-2000)

Estima

(1996-2000)

Maris Piper

(1996-2000)

Record

(1996)

P value

PVY 0.53 0.48 0.34 0.47 1.18 NS PLRV 1.96 b 2.00 b 0.41 a 0.65 a 0.18 a <0.01 Black leg 0.84 a 1.14 a 3.82 b 1.74 a 2.18 ab <0.001 Verticillium wilt 0.13 0.38 0.55 0.19 1.24 NS Black dot 8.27 b 0.95 a 6.59 ab 12.00 b 2.00 a <0.001 Stem canker (96,97,98)

16.48 15.24 8.75 9.63 17.67 NS

Sclerotinia (96,97,98)

0 0.19 1.92 1.74 0 NS

Botrytis (96,97,98) 1.90 1.52 6.96 4.56 4.00 NS Blight (97, 98) 1.68 3.43 3.02 3.89 - NS - = not assessed NS = not significant (P>0.05) values sharing the same letter in each row are not significantly different according to least significant difference test.

Harvested tubers. On the harvested tubers there were significant differences between cultivars in the severity of black scurf, silver scurf, black dot, common scab and powdery scab. Black scurf was more severe on Pentland Dell than on Estima or Maris Piper (P<0.01), silver scurf was more severe on Estima and Record than on Cara, Pentland Dell and Maris Piper (P<0.05), and black dot was more severe on Maris Piper and Record than on Cara (P<0.01). There are no NIAB resistance scores for these diseases. Also, common scab was more severe on Maris Piper (NIAB 1) compared with Cara (6), Estima (6) and Record (5)(P<0.001). Powdery scab was more severe on Pentland Dell (7) compared with Cara (3) and Record (7) (P<0.05). On the basis of the data collected, the official resistance ratings for disease resistance/susceptibility appear robust whereas in other instances they may not.

Table 9. Disease severity on harvested tubers (1993-2000)- mean % surface area affected

Cara

(1996-2000)

Pentland Dell

(1997-2000)

Estima

(1996-2000)

Maris Piper

(1996-2000)

Record

(1996)

P value

Black scurf 0.29 ab 0.45 b 0.28 a 0.18 a 0.31 ab <0.01 Silver scurf 0.47 a 1.58 a 2.09 b 1.24 a 2.35 b <0.05 Black dot 1.08 a 1.65 ab 2.30 abc 3.73 c 3.04 bc <0.01 Skin spot Tr Tr 0.01 Tr 0.06 NS Common scab 0.89 a 3.22 bc 1.33 a 4.00 c 2.16 ab <0.001 Powdery scab 0.04 a 0.39 b 0.30 ab 0.10 ab 0.02 a <0.05 tr = < 0.005 NS = not significant (P>0.05) values sharing the same letter in each row are not significantly different according to least significant difference test.

Correlation between severity of silver scurf, common scab, powdery scab and black dot on the seed and harvested (ware) tubers of different cultivars Significant correlations between disease severity on seed and ware tubers were found for some diseases in some years, and these relationships were investigated further by analysis of correlations between seed and ware tuber disease severity for different cultivars (Table 10). The correlation between severity of black dot on seed and ware tubers was statistically significant (P<0.01) for Pentland Dell and the correlations between severity of silver scurf and powdery scab on seed and ware tubers were statistically significant for Maris Piper (P<0.05). No other correlations were significant.

Table 10. Correlation between disease severity (% surface area affected) on the seed and harvested (ware) tubers of different cultivars, 1996-2000

Cultivar Pearson’s Correlation Coefficient Silver scurf Common scab Powdery scab Black dot Cara 0.262 NS 0.274 NS -0.076 NS -0.076 NS Pentland Dell (97,98) 0.056 NS -0.076 NS -0.103 NS 0.468 ** Estima 0.115 NS 0.00 NS 0.094 NS -0.009 NS Maris Piper 0.226 * 0.067 NS 0.912 * 0.146 NS * = P<0.05, ** = P<0.01, *** = P < 0.001, NS = not significant (P>0.05)

Effect of seed quality on disease levels The effect of seed quality on disease levels was difficult to analyse from the survey data due to the confounding influences of the large number of diseases, seasonal effects, seed source and cultivar. Detailed analyses of the effects of seed class and seed source are given in Figs 20 – 29 in Annex 1. Analyses of disease levels on seed stocks between 1996 and 2000 showed that there was no clear trend towards reduction in severity of surface and internal diseases/blemishes in certified seed stocks compared to home saved seed (Table 11). However, Super Elite and Elite stocks were shown to carry significantly less virus infection than home-saved seed.

Table 11. Disease severity on seed tubers (1996-2000)

Super Elite Elite Home-saved P value % surface area affected

Black scurf 0.24 0.41 0.15 NS Silver scurf 13.12 14.1 10.09 NS Black dot 0.47 0.28 0.72 NS Skin spot 0.08 b 0.11 b tr a <0.01 Common scab 1.25 1.05 1.08 NS Powdery scab 0.09 0.06 0.19 NS % tubers affected

Black leg 0.05 0.00 0.00 NS Gangrene 0.18 0.25 0.22 NS Dry rot 0.23 a 0.71 b 0.13 a <0.01 Internal rust spot 0.65 0.93 0.36 NS Spraing 0.27 0.29 0.04 NS PVX 0.14 a 0.00 a 1.95 b <0.03 PVY 0.38 a 0.25 a 3.05 b <0.01 PLRV 0.09 0.09 0.33 NS tr = < 0.005 NS = not significant (P>0.05)

values sharing the same letter in each row are not significantly different according to least significant difference test.

The relationships between seed quality and disease levels in harvested ware potatoes in 2000 is shown in Fig. 27, Annex 1. In order to investigate these relationships further, the severity of silver scurf, common scab, powdery scab and black dot on the seed tubers was correlated with disease severity on the ware tubers from 1996-2000 and the results are shown in Table 12. In 1996, 1997, & 1998 there was a weak but significant correlation between seed borne silver scurf and ware infection. Correlations were found between disease severity on seed and ware tubers in some years for both common scab and powdery scab. Conversely no significant correlation was found for black dot in any year.

Table 12. Correlation between disease severity (% surface area affected) on the seed and harvested (ware) tubers, 1996-2000

(Pearson’s Correlation Coefficient)

1996 1997 1998 1999 2000 Silver scurf 0.367** 0.482*** 0.242* 0.104 NS -0.059 NS Common scab -0.005 NS 0.539*** -0.017 NS -0.058 NS 0.280* Powdery scab 0.349** -0.036 NS 0.696*** 0.163 NS 0.023 NS Black dot -0.014 NS 0.136 NS 0.155 NS 0.176 NS 0.172 NS * = P<0.05, ** = P<0.01, *** = P <0.001, NS = not significant (P>0.05)

In order to estimate the potential economic impact of potato diseases and the likely benefits of choice of seed grade or cultivar, the survey data have been subjected to analyses based on determining the percentage of crops each year which would pass the quality standards set by the pre-pack washed market. Maximum tolerance levels of skin defects for retail pre-pack washed market have been determined by the BPC (Table 13). Table 13. Maximum tolerance levels of skin defects

Disease Maximum tolerance Silver scurf 10% cover on 50% of sample Black dot 10% cover on 50% of sample Black scurf 2% cover on 20% of sample Common scab 2% cover on 50% of sample Powdery scab 2% cover on 50% of sample Total skin defects 10% cover on 50% of sample

Source: Joanne Knowles (BPC) 2000, Cost Benefit Analysis of using High Quality Seed Potatoes - Draft Report) These tolerances have been applied to results from potato disease surveys carried out between 1993 and 2000. Results show that in all years over a quarter of crops would have failed the maximum tolerance for the pre-pack washed market (Table 14) and that there is little relationship between seed quality and ware quality and little evidence of the benefits of using high quality seed. Table 14. Percentage of harvested samples grown from home-saved, Super Elite or Elite seed

failing at least one maximum tolerance (ANOVA performed on untransformed data)

Home-saved Super Elite Elite All samples p value 1993 100.0 54.6 17.2 38.5 1994 33.3 59.3 43.5 44.6 1995 33.3 63.3 22.2 47.5 1996 0.0 36.6 25.0 27.6 1997 0.0 40.0 36.8 29.4 1998 0.0 39.1 30.0 27.7 1999 21.4 39.1 37.5 34.3 2000 25.0 48.8 25.0 40.5 <0.001

Between 1993 and 2000, there were significant differences between home-saved, Super Elite and Elite ware tubers (P<0.001), with significantly more Super Elite ware samples failing at least one maximum tolerance. Similar analyses on the quality of two pre-pack cultivars show that considerable differences in quality have occurred in some years, especially in 1995 when 77% of Maris Piper samples failed the tolerance limits compared to 20% of Estima samples (Table 15).

Table 15. Percentage of Estima and Maris Piper samples failing at least one maximum tolerance (1993-2000)

Estima Maris Piper All samples p value

1993 31.6 40.0 38.5 1994 31.6 58.3 44.6 1995 20.0 76.9 47.5 1996 24.0 37.9 27.6 1997 28.6 40.7 29.4 1998 20.0 36.4 27.7 1999 29.2 43.2 34.3 2000 44.0 42.9 40.5 <0.05

Between 1993 and 2000, there were significant differences between Estima and Maris Piper ware tubers (P<0.05) significantly more Maris Piper samples failing at least one maximum tolerance. Effect of storage conditons on disease levels The main factors affecting disease levels in store were ventilation (convection or forced) and the use of refrigeration. Most influential was use of refrigeration, which over the period of the survey resulted in marked reductions in the severity of silver scurf, black dot, common scab, dry rot and internal rust spot (Table 16 and Figs. 32 & 33, Annex 1). Highest disease levels were generally found in tubers from stores relying on natural convection for ventilation whereas levels in tubers from stores using forced ventilation were intermediate. Data on the effects of storage conditions on disease levels have been summarised by Elcock et al. (1998).

Table 16. Effect of storage regime on mean disease severity on stored tubers (1996-2000)

Convection Forced Refrigerated p value % tubers affected Black scurf 0.4 0.3 0.2 NS Silver scurf 9.0 b 6.5 b 5.3 a <0.01 Black dot 5.3 b 2.8 ab 2.0 b <0.01 Common scab 2.6 1.6 1.5 NS Powdery scab 0.16 0.18 0.22 NS % tubers affected

Black leg 0.3 0.3 0.4 Ns Dry rot 1.1 0.5 0.7 NS Internal rust spot 4.0 b 1.5 a 1.9 a <0.001

Fungicide use Foliar applied fungicides In all years of the survey, fungicides applied to the growing crop were for the control of late blight. The number of applications varied from crop to crop and from season to season. (Fig 34, Annex1). The use of fungicides for blight control between 1993 and 1999 has been discussed by Bradshaw et al. (2000). Between 1993 and 1996, the number of applications ranged from one to 17 sprays with the majority of crops receiving between six and 12 applications (mean 8 treatments). In 1997, 1998 and 1999, seasons when blight was more severe, the number of applications ranged from four to 18 sprays with the majority of crops receiving between six and 15 applications (mean 11 treatments). The mean interval between planting and the first spray application reduced in 1998 in response to the blight epidemic experienced the previous year. The average interval between applications also decreased in response to the blight epidemic, from a mean of 9.3 days between 1993 and 1996 to 8.1 days in 1998. Between 1993 and 1996, 22% of crops received more than 10 fungicide sprays and this increased to a mean of 53% in 1997/98 and 43% in 1999. This strongly suggests that a considerable number of fungicide applications are unnecessary which may be due to a lack of understanding by growers of the epidemiology of blight. Fungicide use was more intensive in higher blight-risk years and there were indications that some fungicides were applied at intervals closer than the legal restrictions permit under the Control of Pesticides Regulations (Anon., 1986). In accordance with the FRAC (Fungicide Resistance Action Committee) guidelines (Urech & Staub, 1985), phenylamide-containing fungicides were applied early in the growing season throughout the period of the survey. Fungicide seed treatments In all years of the current survey, the majority (>60%) of seed stocks were treated with a fungicide prior to delivery to the farm (Fig 38, Annex 1). Products containing the fungicide imazalil dominated product choice and were used principally for the control of H. solani, the silver scurf pathogen. The other major fungicide use was with products for the control of stem canker. Fungicide resistance testing Hide & Hall (1993) and Carnegie et al. (1994) previously reported resistance in H. solani populations to thiabendazole. During the period of this survey, isolates of H. solani were routinely tested from both seed and ware tubers. A summary of the results from 1996-2000 is shown in Table 17 together with results from the previous survey (HP0113). Resistance to thiabendazole in H. solani populations tested from 1996-1999 remained at levels similar to those in the earlier survey years. The data generated in 2000 should be treated with caution because of the small number of samples tested.

Table 17. Sensitivity of H. solani isolates to thiabendazole

Survey year No crops tested Total No isolates tested

% isolates resistant to thiabendazole

1993 64 357 73.1 1994 64 364 65.9 1995 29 122 51.6 1996 31 178 62.9 1997 31 221 67.9 1998 29 196 65.3 1999 16 50 72.0 2000* 6 18 38.9

* Data from the 2000 growing season should be treated with caution as samples from only 6 crops could be tested. This was due to bacterial contamination of the agar plates, which was confounded by mite infestation.

CONCLUSIONS This survey together with its’ predecessor (Project Code HP0113) have provided data on the occurrence and relative importance of fungal, bacterial and virus diseases in the UK potato crop. Information has been collected on tuber diseases on both seed and ware, on ware tubers at harvest and after storage and on the growing crop after emergence and just prior to crop senescence. This information is unique and has not been collected in such detail before. The results have profound implications for future research priorities, for Plant Health and in pesticide regulatory matters. Common scab, silver scurf, black dot and black scurf were the most commonly encountered diseases in seed stocks although there were annual differences in the severity of each disease. There was a weak but statistically significant relationship in most years between the severity of silver scurf on the seed and on the ware tubers. In some but not all years of the survey, a similar relationship was found for common and powdery scab but not for black dot. However, it should be noted that the levels of powdery scab found in the survey were extremely low. Silver scurf, black dot and common scab were the most important diseases of the ware tubers. Stem canker was by far the most prevalent disease of the growing crop in the spring and summer and there were significant seasonal differences in the incidence of this disease. Analysis of fungicide seed treatment usage in relation to stem canker clearly demonstrated their effectiveness. Severe virus infection (PVY & PLRV) was low in the seed stocks tested and remained low during the growing season when crops were inspected. In all years, the in-field visual inspection for PLRV was a good indicator of likely seed infection but there was a poor correlation with PVY. This may be due to late (or latent) infection at the time of the summer assessment but may have implications for seed certification and inspection and illustrates the potential benefit of tuber indexing as part of the certification scheme. The poor correlation for PVY certainly has implications for home-saved seed and the need for tuber virus testing. Quality of seed prior to planting was high with the majority of growers selecting Super Elite seed. There was no consistent reduction in surface and internal diseases/blemishes in certified seed compared to home-saved seed but levels of virus were significantly lower in certified stocks. Further analyses of relationships between disease levels in seed prior to planting and disease levels in harvested ware crops showed significant correlations for silver scurf, common scab, powdery scab in some but not all years, but no relationship between black dot levels. Analyses of the quality of ware crops monitored between 1996 and 2000 indicated that in each year at least a quarter of all crops would have failed the maximum tolerance limits for surface diseases set by retailers for the pre-packed washed market. There was no evidence that use of high quality seed resulted in an improvement in the proportion of crops passing the criteria. Comparison of two cultivars, Estima and Maris Piper, revealed large differences in quality in some years. These data have significant implications for the industry particularly with the increasing demand for quality skin finish directed at the pre-pack market and the increasing dominanace of multiple retailers reducing market outlets. Foliar blight was found in crops in 1997, 1998, 1999 & 2000 which, in view of the relatively small number of crops inspected, indicated the prevalence of the disease. Fungicide use for the control of late blight was shown to be the main pesticide input to the growing crop and the numbers of treatments varied considerably from crop to crop each year and to a lesser extent from year to year. Use of blight fungicides was often excessive with some crops receiving up to 18 fungicide applications even in years when blight was not perceived to be a problem. In high blight years intervals between fungicide applications were reduced below legal limits. This is a major area for technology transfer and pesticide minimisation.

There is scope for the development of a more robust and sophisticated Decision Support System for UK growers than is currently available to ensure that fungicide use is better targeted and justified. Analysis of fungicide use shows that a proportion of sprays are applied at intervals closer than the label recommendation. This has implications for the future registration of blight fungicides.

PUBLICATIONS Bradshaw NJ, Turner JA, Elcock SJ, 1997. Potatoes: Survey of diseases 1996/97. CSL/ADAS

Internal report. Bradshaw NJ, Turner JA, Elcock SJ, 1998. Potatoes: Survey of diseases 1997/98. CSL/ADAS

Internal report. Bradshaw NJ, Turner JA, Elcock SJ, 1999. Potatoes: Survey of diseases 1998/99. CSL/ADAS

Internal report. Bradshaw NJ, Turner JA, Elcock SJ, 2000. Potatoes: Survey of diseases 1999/00. CSL/ADAS

Internal report. Bradshaw NJ, Turner JA, Elcock SJ, 2001. Potatoes: Survey of diseases 2000/01. CSL/ADAS

Internal report. Bradshaw NJ, 1997. Feasibility study on the detection of blight (Phytophthora infestans) using

satellite data. ADAS Contract Report Ref: XAAXG Undertaken for the British Potato Council. Bradshaw NJ, Taylor MC, Hardwick NV, 1998. Potato blight forecasting in England & Wales.

Proceedings 1st Transnational Conference on Biological, Integrated and Supervised Control, Lille, France. 325-331.

Bradshaw NJ, Elcock SJ, Turner JA, Hardwick NV, 2000. Are potato blight fungicides being used

rationally? British Crop Protection Conference, Pests & Diseases 3, 847-852. Elcock SJ, Turner JA, Bradshaw NJ, 1998. Factors influencing internal tuber diseases and

blemishes in ware potatoes in England & Wales. In: Proceedings 7th International Congress of Plant Pathology. Edinburgh, Scotland 9-16 August 1998. Volume 2.

Hardwick NV, Taylor MC, Bradshaw NJ, Hall AM, 1998. Comparison of blight forecasting models in

England and Wales. Potato Blight Conference, Harper Adams Agricultural College, 13 January 1998.

Wale SJ, Clayton R, Elcock SJ, 2001. Relationship between powdery scab inoculum on seed and

disease development in progeny tubers. In: The Dundee Conference, Crop Protection in Northern Britain, 2002. In Press.

PRESENTATIONS Talks have been given which included data generated by this project at: - Cambridge University Potato Growers Research Association, 8th Annual Cambridge Potato

Conference: Control of potato blight - now and in the future. Robinson College, Cambridge 15/16 December 1997.

1st Transnational Conference on Biological, Integrated and Supervised Control, Lille, France.

January 1998 Potato Blight Conference: Harper Adams Agricultural College, 13 January 1998. British Potato Council: Grower meeting on blight control in early potato production, Haverfordwest

(22 January 1998). British Potato Council: Grower meeting on blight control, Ormskirk (10 February 1998). British Potato Council: Poster on blight forecasting. Potato Technology ‘98 (13/14 May 1998). British Potato Council; Seminar - Blight control strategies. Potato Technology ‘98 (13 & 14 May 1998) British Potato Council: Poster on Potato Disease Survey. Potato Technology ‘97. British Potato Council: Poster on Potato Disease Survey. Potato Technology ‘98. Presentation to National Association of Seed Potato Merchants (NASPM) - January 1998. Presentation to British Potato Council - Potato Disease Survey and Blight Forecasting - September ‘98 Presentation a delegation of farmers from Finland entitled ‘Potato Disease Surveys’ - April ‘98 PRESS ARTICLES Survey underlines disease risks. EYEwitness, October 1998, pp 19. REFERENCES Anonymous, 1976. Manual of plant growth stages and disease assessment keys. Pinner, UK: MAFF. Anonymous, 1986. The Control of Pesticides Regulations (as amended) SI 1986 No. 1510. Anonymous, 2000. The Potato Variety Handbook 1996. National Institute of Agricultural Botany,

Cambridge, UK. Anonymous, 2000. Potato statistics in Great Britain 1995-1999. Oxford UK: British Potato Council.

(source: www.potato.org.uk). Bradshaw NJ, Elcock SJ, Turner JA, Hardwick NV, 2000. Are potato blight fungicides being used

rationally? British Crop Protection Conference, Pests & Diseases 3, 847-852.

Carnegie SF, Cameron AM, Hide GA, Hall SM, 1994. The occurrence of thiabendazole-resistant isolates of Polyscytalum pustulans and Helminthosporium solani on seed potatoes in relation to fungicide treatment in store. Plant Pathology 43, 961-971.

Ebbels DL, 1983. Incidence of tuber diseases in classified seed potatoes harvested in England and

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Hardwick NV, Turner JA, 1996 (a) Potato diseases – is there a north/south divide in England and

Wales? Proceedings Crop Protection Conference in Northern England 1996 Dundee: ACPNB, 313-318.

Hardwick NV, Turner JA, 1996 (b). A survey of potato disease in England and Wales, 1993-1996.

Abstracts 13th Triennial Conference, European Association of Potato Research 1996 Veldhoven: EAPR, 631-632.

Hide GA, 1981. Fungus disease on potato seed tubers planted in England and Wales, 1963-76.

Annals of Applied Biology 98, 377-393. Hide GA, 1992. Towards integrated control of potato storage diseases. Aspect of Applied Biology

33, Production and protection of potatoes 1992. Cambridge: AAB, 197-204. Hide GA, Hall SM, 1993. Development of resistance to thiabendazole in Helminthosporium solani

(silver scurf) as a result of potato seed tuber treatment. Plant Pathology 42, 707-714. Hill S A, 1977. Virus testing of home-saved seed potatoes. Agricultural Development and Advisory

Service, Regional Agricultural Science Service Annual Report, 1976. London, UK: HMSO, 195-200.

Taylor MC, Hardwick NV, Bradshaw NJ, and Hall AM, 2000. Are excessive blight sprays detrimental

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