ADDRESSING PEST AND DISEASE PROBLEMS FOR THE UK POTATO INDUSTRY

Transcription

1 POTATOES ADDRESSING PEST AND DISEASE PROBLEMS FOR THE UK POTATO INDUSTRY Martin Hims and Steve Hill, on behalf of Plant Health Group, Central Science Laboratory (CSL), York, UK, discuss a range of potato pest and disease problems The surveillance of pests and diseases by the UK Central Science Laboratory (CSL) through commercial testing, seed certification, international consultancy and multi-faceted R&D provides an blend of complementary experience and expertise which has enabled CSL researchers to resolve a range of potato problems in recent years in collaboration with industry representatives. Potato disease surveys Each year CSL has collected potato disease information on seed, the field crop and stored progeny tubers countywide from representative cultivars. Survey highlights include data on virus incidence, stem canker and pesticide use (Figures 1 and 2). The inability to visually identify Potato Virus Y (PVY) in the field just prior to burning-off and the high levels recorded in harvested progeny, illustrate the risks of using stocks for seed without first having them virus indexed. Stem canker, causing considerable industry concern, has been present in up to 70% of surveyed crops with up to 20% plants affected in a third of these. Tuber blight is wholly unacceptable in today s market and yet the average number of field applications of fungicide varies considerably. During non-blight years ( ) it was 8.4 and in the blight year of 1997, 10.7 with a range of 0 18 sprays over all surveyed crops, demonstrating that there are opportunities to successfully tailor fungicide programmes for blight control. Disease surveys, supplemented by pest monitoring, are a fundamental activity, providing a comprehensive awareness of pest and disease incidence and an unrivalled base from which to investigate problems and find solutions tailored to appropriate industry sectors. Survey data also provides the only means to assess, objectively, the impact of industry changes on pests and diseases so that research priorities are identified and scarce resources targeted appropriately. Increasing use, by the potato industry, of the CSL Commercial Diagnostic Service provides valuable additional intelligence and an early warning of industry problems. Non-indigenous potato diseases and pests The responsibility for maintaining international vigilance for non-indigenous pests and diseases completes the overall coverage provided by CSL. A good example of the significance of this role and of the way the organisation adapts to deal with industry problems is that of potato brown rot. It was first addressed as a non-indigenous disease, the objective being to develop robust but sensitive diagnostic techniques for detection and identification of the brown rot bacterial pathogen (caused by Ralstonia solanacearum) (Figure 3). Figure 1. Prevalence of potato diseases in the UK ( ). Figure 2. Fungicide use on potatoes in the UK ( ). 50 Figure 3. Brown rot (caused by Ralstonia solanacearum). Photo courtesy of CSL Pe s t i c i d e O u tl ook A pr il This journal is The Royal Society of Chemistry 2001 D O I : /b p

2 POTATOES EU and other diagnostic methods were considered either inappropriate or insufficiently reliable for the testing programme necessary after the disease was first confirmed in a UK crop. Nor were the methods considered wholly appropriate for proving that seed tuber imports from t h e Netherlands, where brown rot was widespread, were free of the pathogen. Only by devising improved techniques and using them in combination could consistently reliable methods be developed. The diagnostic protocols developed at CSL have now been validated as part of an EU scheme and several of them have been proposed for testing water, soil and processing waste as well as plant material. After the disease was confirmed in the UK, the emphasis shifted to clarify its epidemiology here. Groundbreaking research identified the hitherto unsuspected role of aquatic weeds in the survival of the pathogen as well confirming the role of infested water- Figure 5. Damage caused by root-knot nematodes of genus Meloidogyne. Photo courtesy of CSL. courses in its spread. The emphasis of CSL research has now shifted to aspects of its control. Current and future work will concentrate on because nematodes often have few distinguishing characters, disease avoidance, particularly in the context of potato and the variability of some of these makes positive identificaprocessing and waste disposal. tion difficult. Isozyme phenotypes of esterase and malate Potato ring rot, (Figure 4) caused by Clavibacter michiga - dehydrogenase (MDH) are species-specific and useful as renensis subsp. sepedonicus, is not yet found in the UK but is liable characters for Meloidogyne identification. Fractions of present in North America and latterly in many EU countries. nematode extracts containing these isozymes are currently The disease is very damaging and is therefore one reason for being evaluated for use in electrophoresis tests to enhance restrictions in seed potato trading. The disease is of particular CSL s ability to distinguish quarantine nematodes from other concern because infected tubers may not exhibit symptoms, so nematode species. that visual inspection to prevent movement and trading of Colorado beetle (Figure 6), the old enemy, continues to be diseased tubers is inadequate. Significant progress has been a major and important threat to the UK despite its exclusion made in the development of reliable and robust molecular and/or eradication for more than 120 years. Trade, social techniques for detecting the bacterial pathogen in such latent and leisure activities increase the potential for importing infections and in detecting the pathogen in soil, in weeds adult beetles into the UK where it could happily establish and on surfaces associated with potato production. Future since CSL models have shown that development rates in work will use these techniques to evaluate on farm eradication the Cotentin peninsula of France, where it causes severe measures. damage if not controlled, are similar to those in southern CSL along with the Plant Health and Seeds Inspectors for UK. CSL has used another model, CLIMEX, in a geographengland and Wales maintain continued vigilance for the ical information system to predict the potential distribution arrival of non-indigenous, quarantine pests of potato. Examples of Colorado beetle in the UK under both current and future of such are the plant-parasitic, root-knot nematodes of the climate conditions. A prediction for 2060, allowing for genus Meloidogyne, M. chitwoodi and M. hapla (Figure 5). higher temperatures as a result of climate change, shows an Correct identification is vital to their exclusion but tradi- increase in potential range of 102%, including virtually all tional microscope techniques present particular difficulties traditional seed and ware potato growing areas. Figure 4. Potato ring rot (caused by Clavibacter michiganensis). Photo courtesy of CSL. Figure 6. Colorado beetle still a potential threat to potatoes in the UK. Photo courtesy of CSL. Pe s t i c i d e O u t l o o k A p r i l

3 addition, disease forecasting is becoming more important to producers as a means of justifying the use of pesticides to the customer. Forecasts will, therefore, play an increasing role in defining protocols for food production, security and its safe delivery to the consumer. Figure 7. Potato blight (caused by Phytophthora infestans) Photo courtesy of CSL. Potato blight Potato blight (Figure 7) continues to exert a major impact on the economics of potato production 150 years after its arrival in the UK. Survey data highlight the use of fungicides for blight c o n t rol and the continued tendency to apply these as insurance sprays. Blight epidemiology is well understood but attempts to rationalise fungicide use by predicting blight development have been unsuccessful. In an attempt to re v e r s e this unsatisfactory situation, re s e a rch at CSL has evaluated, s i m u l t a n e o u s l y, diff e rent blight forecasting schemes of vary i n g complexity from the USA and Continental Europe; none were wholly reliable under UK conditions. Currently national, regional and local blight predictions based on the simple UK Smith Period model use data obtained from the Meteorological Office s synoptic network of weather stations. These are not necessarily sited in areas of potato production, but nevertheless they have provided an indication of blight favourable weather. However, weather conditions can vary markedly across even a small geographical area rendering blight predictions inaccurate and undermining grower confidence. In a CSL re s e a rch project, the w e a t h e r data required for the Smith Period model were recorded from three sites at different altitudes within a 4 km radius and showed tremendous variation in predicted risk. Given this scenario CSL has developed an in-field blight prediction system for the UK that can utilise m e t e o ro l o g i c a l data re c o rded from portable weather stations and a weathermonitoring algorithm designed to give a warning when favourable conditions for infection by the blight fungus occur. This algorithm calculates a fungicide spray threshold and advises a repeat application if conditions remain conducive to blight development after the protection afforded by the earlier application is lost. The forecast system, called Blight Early Warning System (BLEWS) is still under development but shows considerable promise. An important advantage of the system is that it allows for adjustment to the time interval between treatments so that sprays are not applied when the blight risk is low. The provision of robust forecasting systems and their widespread adoption forms a key component of the strategy to minimise pesticide use and improve competitiveness. In Aphid population and virus dynamics Similar research stems from the long-standing involvement of CSL staff in providing technical support for the development and administration of the seed potato cert i f i c a t i o n scheme. The deregulation of potato seed production in England and Wales has allowed growers to utilise lowland a reas for growing seed, which in some years may be under considerable threat from PVY. By monitoring aphid vectors at the field/crop level growers can rationalise management strategies to minimise the risk of PVY infected seed. PVY is transmitted non-persistently by a number of aphid species with v a rying degrees of eff i c i e n c y. In collaboration with Cambridge University Farms and a number of English lowland seed g rowers, aphid vectors have been monitored at 16 sites in each year since Using water traps set at the crop canopy level, weekly indices of vector pre s s u re are calculated, based on t h e numbers of individual vector species trapped and their efficiency at transmitting PVY This index is accumulated t h roughout the growing season to give an indication of the potential risk of PVY infection occurring at each site. Considerable variation is observed between sites and years over the years of the study period but nevertheless growers have found this scheme to be a useful tool in planning haulm destru c t i o n dates to minimise PVY infection. The epidemiology of PVY is complex and involves factors other than vector pressure such as seed-borne virus input levels, mature plant resistance, varietal resistance and insecticide usage. CSL research continues to determine the significance of these individual factors and to produce a more refined tool that growers can use to rationalise the decision making process and minimise the risk of PVY infection within lowland seed production. Field-based virus diagnostic kits Whilst the spread of virus diseases into potential seed potato crops is important, an equally relevant aspect of this production relates to the health of the parent seed. Whether in certification or where growers are concerned to produce healthy viru s - f ree seed for their own use, the ability to identify v i rus-infected parent plants is vital. Accurate diagnosis of v i rus disease is essential to ensure valid inspection for cert i f i- c a t i on and to enable reliable judgements on home-saving of seed. Many modern potato cultivars express only mild symptoms when PVY is tuber-borne. Distinguishing such symptoms from those of milder viruses or non-pathogenic conditions can be difficult and is often subjective. Using the internationally recognised skills of CSL staff in the development of diagnostic techniques a field kit for the confirmation of PVY has been developed along with PVA, PVS, PVV and PVX. Other so-called field test kits are typically multistep methods which need 10 to 30 minutes to perform, need refrigerating and it is doubtful whether they can be per- 52 Pesticide Outlook April 2001

4 Figure 8. Pocket Diagnostics test kits for potato viruses. LEFT closeup of test kit (second line in upper kit indicating the presence of virus. Photo courtesy of CSL. RIGHT test kit in use. fo rmed practically in a field or on-site location. CSL staff have now developed single-step, antibody-based field kits that diagnose five potato virus diseases direct from leaf sap. The test provides a result in two minutes or less and is in the style of the well-known home pregnancy testing kits (Figure 8). More information on these Pocket Diagnostic test kits for potato and other plant viruses is available at http: // Third party validation confirms that Pocket Diagnostics are a valuable and consistently reliable tool in the diagnosis and management of virus diseases and the production of healthy potato seed. 1 New PVY strains Potato virus research has also been a CSL speciality in relation to the need to understand and thereby prevent the introduction of non-indigenous viruses. Recent work has helped resolve the nature of a new and virulent PVY strain, P V Y N T N. First re p o rted in Hungary in 1984, this virus disease has subsequently been found further and further west in Europe. Infection by the virus causes a superficial ringshaped necrosis on the surface of the tubers (potato tuber necrotic ring disease or PTNRD) which renders them unsaleable. In some cultivars the incidence and severity of symptoms has led to 100% crop loss. A risk assessment of this relatively new virus isolate to the UK potato crop has been based on CSL research into its detection and epidemiology. Relationships of the PVY NTN isolates have been examined within the taxonomy of the PVY group as a whole using the sequence of the coat protein gene for a large number of isolates. Sub-groups were identified according to the biotype of the different isolate. Two main groups, necrotic (N) and ordinary (O) were observed as well as a sub-group within the N group, in which all members had a 1 Please note that Chris Danks, a researcher at CSL, was awarded the 2001 Westminster Medal at a House of Commons Reception for Younger Scientists, Engineers and Technologists on 19 March 2001, for his work on the development of Pocket Diagnostics. recombinant coat protein and belong to the NTN biotype. A subgroup within the ordinary strains possibly composed of PVY C isolates was also observed. A PCR assay has been developed which identified isolates falling into each group, and which can be used to confirm the presence of NTN in infected tubers. Two further isolates were identified however, which belong to the NTN bio-type but which do not have a recombinant coat protein suggesting that the recombination event is not important for the induction of symptoms, but is often associated with it. This suggests there are at least two genotypes of PVY which cause PTNRD. During 1998 PVY NTN was confirmed for the first time in English potato crops. The primary and secondary (tuber-borne) symptoms of PVY NTN on more than 20 UK grown potato cultivars were recorded and photographs and descriptions made. Secondary foliar symptoms were generally different and less distinctive than the primary ones and differed markedly between cultivars. Only a limited number of cultivars exhibited tuber symptoms. New methods for detecting tuber-borne potato viruses A common disease in Europe and North America leading to potato tuber rejection is spraing, appearing as brown, necrotic arcs in tuber flesh. Spraing is induced by two soilborne viruses, tobacco rattle tobravirus (TRV) and potato mop-top pomovirus (PMTV), transmitted by nematodes of the genera Trichodorus/Paratrichodorus and Spongospora s u b t e rr a n e a re s p e c t i v e l y. Accurate determination of the causal agent is important, as control measures differ for the two viruses. To improve the diagnosis CSL has developed a multiplex TaqMan assay which allows detection of both viruses in one tube. The new assay is 1000x more sensitive than the traditional PMTV ELISA and 100x more sensitive than the TRV RT-PCR. The results show the new assay allows the rapid, specific and sensitive detection of TRV and PMTV in a single assay, saving both time and labour compared to existing techniques. Pesticide Outlook April

5 Current methods for detecting virus in dormant tubers concentrate on the growing on of tuber eye cuttings and ELISA testing of the sprouts, a time consuming method, taking from 4 to 6 weeks to complete. The ability to detect plant viruses in tissues where their titre is very low (e.g. PVY in potato tubers) using PCR is well documented. However, the wide-scale use of PCR is limited, due to problems in handling large numbers of samples. Work at CSL has concentrated on the development of PCR for routine use, in part i c u l a r by investigating improved product detection. Initial work concentrated on the development of a multiplex assay which includes bro a d - s p e c t rum PVY detection with an intern a l positive control which could be used for large scale testing. The results indicate that TaqMan has considerable potential in helping towards the development of PCR for the large-scale, routine detection of potato and other viruses. Large-scale comparisons of the TaqMan assay and the growing on test are on-going. Expertise on potato pests and diseases arises for a variety of different reasons within CSL. The examples given demonstrate the extent to which all aspects of the CSL work inter-relate to strengthen each other. Increasing contacts with the industry through the British Potato Council, NFU, seed producers, processors and merchants complement our long-standing commitment to dealing with problems of both indigenous and non-indigenous pests and pathogens. Our combination of biology and diagnostic research in the context of statutory control and commercial production provides an ideal basis for a well-integrated and wellfocussed approach to potato problems. For further information on the work of CSL in potatoes and other areas see Dr Martin Hims is Business Development Manager for the Plant Health Group at the Central Science Laboratory (CSL). Martin s career began in ADAS Plant Pathology at Newcastle, where he specialised in diseases of vegetables and arable crops. Moving to Wye in 1984, Martin continued to work on diseases of arable crops, leading to his appointment to the post of Head of Disease Assessment Branch at Plant Pathology Laboratory Harpenden in On assimilation of Plant Pathology Laboratory into CSL and its move to York, Martin took responsibility for the wider development of the Plant Health Group business. Professor Stephen Hill is Head of the Plant Health Group at CSL. Trained as a plant pathologist, Stephen became ADAS National Plant Virologist in 1976, eventually moving to take responsibility for virology at the then MAFF Plant Pathology Laboratory at Harpenden in As the pest and disease science at Harpenden became part of the emerging CSL Agency in 1992, Professor Hill took overall responsibility for the re-named Plant Health Group, embracing both plant pathology and entomology. CSL relocated to Sand Hutton near York in PREVIOUS ARTICLES IN PESTICIDE OUTLOOK ON POTATOES Potato blight control in Ireland (Cooke) Pesticide Outlook, 1992, 3(4), 28. Potato blight global challenges and new problems (Cooke and Deahl) Pesticide Outlook, 1998, 9(6), 22. Promoting international collaboration for potato late blight disease management (the CEEM Project) (Raman et al.) Pesticide Outlook, 2000, 11(5), 181. Late blight of potato and its control in Russia (Dyakov and Derevjagina) Pesticide Outlook, 2000, 1 1(6), Pesticide Outlook April 2001