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1. Late blight occurrence and LB severity by major production area in the country. What crops were affected (tomato, potato, others)? Disease sources (seed, cull piles, volunteer plants, soil)?

As most tomatoes are grown under glass, potato is the only crop severely affected by late blight in the UK.  The 2002 season was a difficult one for growers with above average spring rainfall delaying planting of many crops.  The extremely wet weather continued throughout most of the summer, favouring infection and making it difficult for growers to maintain spray schedules.  However, although the majority of potato crops had some blight (<1%) by the end of the season, few were severely infected.  Drier weather during September and early October allowed the majority of crops to be lifted in good conditions, although some still remain to be lifted as it has again been very wet from mid-October on.  In most cases, infection sources were unknown: two infected cull piles were identified and seed-borne infection is probably important in our conditions. In Northern Ireland neither volunteers nor soil-borne inoculum is reported to be important. Across the rest of the country both mating types are present and the presence of soil borne inoculum cannot be ruled out – there is insufficient data to comment on either its prevalence or significance.

The majority of second early and maincrop potatoes were planted according to schedule. Following a mild winter with very few sustained frosts, volunteer potatoes were reported in the following spring mainly in sugar beet crops.  Prolific haulm growth was also found on dumpsites. Although very few Smith Periods were recorded up to the end of May (thought to be due to low night temperatures), blight was reported on dump sites in the south-west of England (24 April) and in East Anglia (Norfolk, 16 May: Cambridgeshire, 30 May).

There were periods of very humid and misty weather over much of England & Wales in early June and by the third week of June there were reports of blight in crops in the north west of England, the west Midlands and the south west of England. These outbreaks were associated with infection on dumps and volunteers combined with extended spray intervals. By the end of June, blight was also present in crops in the south east of England and on dumps in the east Midlands. The epidemic continued to develop in July with outbreaks reported in crops in many areas.  However, with the exception of the ‘hot spot’ in NW of England there was no general epidemic. There were few outbreaks reported in East Anglia due to a period of dry weather in July coupled with intensive spray programmes.

At the end of July and into early August blight favourable conditions (Smith Periods) occurred in virtually all potato growing areas of England & Wales. These conditions again stimulated blight development and the disease became widespread in crops in the southern counties of England. By the middle of August blight was present in crops in most areas with infection ranging from light infections on the ‘heads’ of isolated plants to primary and secondary foci. 

In 2002, maincrops generally tended to produce fewer tubers and consequently, met their size specification earlier than normal.  A significant proportion of crops were therefore desiccated earlier than usual.  Drier weather conditions from the end of August and into September would also have reduced the risk of tuber infection during the desiccation period. The incidence of tuber blight at the time of writing is not known.

In Scotland wet weather during June, July and August led to infections in many crops with dumps and seed reported as the main sources of infection.  The appearance of a stem lesion in an otherwise uninfected area of crop would suggest a latent infection likely resulting from an infected seed.

2. LB impact (foliar damage, yield losses, tuber rot occurrences)?  

Intensive spray regimes resulted in few crops with yield reductions due to late blight but the impact on gross margins will have been significant. The wet weather and saturated soil conditions may have led to poorer growth and slight yield reductions in some crops. Increased levels of tuber blight were expected but, at least in Northern Ireland, there appears to be less than might have been anticipated. Growers are being warned to look out for rotting and subsequent problems in store.

3. Fungicide use (amounts, types, etc.)?

Data for fungicide usage for 2002 is not available but a wide range of products is used for blight control.  In general, spray programmes will have started early on second early/maincrops (rosette stage to haulm just meeting along the rows) resulting in between 8-14 sprays being applied. Spray intervals of 5 days or less were being recommended in Scotland. There is no standard approach to fungicide use in the UK; it is down to each grower’s approach to risk and willingness to pay for pesticides.  Some will use cheap protectant materials such as mancozeb routinely at close intervals throughout the season. Others will apply systemic fungicide during the rapid haulm growth stage and follow on with translaminar product which have curative activity or use the newer protectant materials which have come onto the market over the last few years. Early season systemics included either phenylamides (metalaxyl-M + mancozeb or oxadixyl +cymoxanil+mancozeb) or propamocarb + chlorothalonil or mancozeb.  The principal mid-season translaminar was a dimethomorph & mancozeb mixture with mancozeb and fluazinam being the main protectants.  The majority of growers used fentin-based products at the end of the season. This year, because of the very high risk conditions, many growers used translaminar fungicides tank-mixed with fentin-based fungicides and these combinations appear to have been successful in dealing with low levels of blight or where spray intervals had been stretched due to adverse weather.  One useful development in the UK in 2002 is the availability of a cymoxanil formulation (C50) which can be mixed with protectant fungicides mancozeb or fluazinam should a target spray date be missed or in periods of very high blight pressure. Cymoxanil has claims for 1-2 days curative activity.   A major concern of growers now is what to use as a replacement for fentin fungicides as they will not be given Annex 1 listing and will be revoked. Alternatives with tuber blight activity are required.

4. Other LB control measures (resistant cultivars, forecast systems, none)?

The most widely grown cultivars are susceptible or moderately susceptible, although some use is being made of cultivars with intermediate levels of field resistance.  Most growers take note of forecasting systems even if to demonstrate due diligence to their customers. Some will use PlantPlus forecasting kit together with on-farm consultancy. Others will have their own on-farm met stations providing Smith Period output.  Smith Period output has been available on the Blight Watch web site (www.potatocrop.com) for several years. It is based on interpolated met data provided by the met office via ADAS – and is free. In Northern Ireland growers receive advice on the occurrence of Smith Periods via the local radio, a recorded telephone service (Blightline) or the web-based DARD Blight-Net. 

Advice on fungicide programmes is updated annually taking account of changes in product availability and the pathogen population.

Resistant potato varieties cultivated: Valor, Verity, Winston

Source: Global Catalog of Late Blight Resistant Varieties: www.cipotato.org/gilb/Gilb_catalog/principal.htm

5. LB control effectiveness (fungicide or host resistance failures)?

No failures have been reported due to changes in fungicide sensitivity of the pathogen.  However, there are some concerns about QoI acting fungicides likely to be more widely available in the near future, and with the announcement this year at British Crop Protection Conference in Brighton of the discovery of another microtubule inhibitor. Vigilance is always needed but the range of products of differing modes of action would seem to suggest that fungicide resistance is not of great concern.

6. Pathogen strains (mating type, fungicide resistance, virulence factors, etc)?  

Details of the UK blight population are summarised below.  It should be noted that there have been no comprehensive surveys of England, Wales and Scotland since 1997. The British Potato Council have however organised a ‘Fight against Blight’ campaign for 2003 which should provide a means to bring the status of the UK population up to date.   Many isolates have been typed according to phenotype (race, mating type, and metalaxyl resistance) and genotype (isozyme, RG57, mtDNA) but increasingly new tools such as AFLPs and SSRs (simple sequence repeats) and SNPs (single nucleotide polymorphisms) are being applied.  These latter genetic tools are rapid and objective and allow a more detailed look at the movement of different genotypes and the extent of geneflow via sexual recombination.

England and Wales

Over the period 1993-98 almost seventeen hundred isolates have been examined and the majority (90%) of isolates are mtDNA type Ia.  The average A2 frequency remains at 5% and almost all A2 isolates are either sensitive or intermediate in response to metalaxyl.  There is a need for more population studies as no detailed surveys have been conducted since 1997.

Northern Ireland

The pathogen population is almost exclusively A1 mating type (no A2 mating types have been detected since 1995, so sexual recombination is unlikely) and monomorphic for Gpi and Pep (100/100, 100/100).  However, mtDNA haplotyping indicates sub-structuring.  Haplotypes Ia and IIa are common, IIb has been detected but not recently, Ib has not been found.  Metalaxyl-resistant and –sensitive strains occur in varying frequencies from year to year.  Haplotype Ia is strongly associated with metalaxyl-resistant strains, whereas haplotype IIa strains are more commonly metalaxyl-sensitive.  A few isolates have been tested for virulence in recent years and these have generally proved relatively complex.

Scotland

Amongst almost 500 isolates collected from 1995-1997 from farms and gardens 19% were of the A2 mating type.  AFLP fingerprinting defined identified three main lineages; the first included the majority of the A2 strains and was mtDNA type Ia and primarily metalaxyl sensitive, the second and third were of mtDNA type IIa and Ia respectively, primarily A1 mating type and a mixture of resistant and sensitive strains. The number of partially resistant strains increased over the three years of the survey and the molecular data suggests the occurrence of some sexual recombination.

7. Disease risks and/or major needs?

There is a need for agronomically acceptable cultivars with greater blight resistance, particularly to tuber blight and especially for organic production.  There is a continuing risk of import of new strains from mainland Europe where the P. infestans population is reproducing sexually.  The pathogen population is being monitored for such changes and also for the occurrence of fungicide-resistant strains (phenylamides and fluazinam), but there is a need to extend sensitivity testing to the much wider range of fungicides now on or coming on stream.  Ongoing monitoring for phenylamide resistance and appropriate advice to growers on anti-resistance strategies has allowed them to continue to make use of phenylamide fungicides.

8. Any publications on late blight in the country?  

Carlisle, D. J., Cooke, L. R., Wilson, D. G. & Deahl, K. L. (2002).  Natural occurrence of Phytophthora infestans on woody nightshade (Solanum dulcamara) in Ireland.  Plant Pathology 51, 392.

Carlisle, D. J., Cooke, L. R., Watson, S. & Brown, A. E. (2002).  Variation in foliar aggressiveness of Northern Ireland Phytophthora infestans isolates towards detached leaflets of three potato cultivars.  15^th Triennial Conference of the European Association for Potato Research/Global Initiative on Late Blight Conference, Hamburg, 11-13 July 2000 (abstract), p. 318.

Cooke, D. E. L., Maughan, V, Birch, P. R. J., Toth, R., Gourlay, F., Carnegie S. & Duncan J. M. (2000). The status of Scottish late blight populations. SCRI Annual report. 1999/2000 p. 150-153.

Cooke, D. E. L., Lees, A. K., Young, V., Birch, P. R. J., Hussain, S., Toth, R., Gourlay, F., Carnegie, S. F. & Duncan, J. M. (2002). Phytophthora infestans populations in Scotland: The implications of mixed mating types on potato late blight management.  Abstract for the GILB 2002 Conference. Late blight: Managing the global threat 11-13 July 2002, Hamburg, Germany

Cooke D. E. L., Young, V., Birch, P. R. J., Toth, R., Gourlay, F., Day, J. P., Carnegie, S. & Duncan J. M. (2003). Phenotypic and genotypic diversity of Phytophthora infestans populations in Scotland (1995-1997). Plant Pathology (in press)

Cooke, L. R., Little, G., Quinn, M. & Wilson, D. G. (2001). Potato late blight population studies 1999-2000 and field results on the integration of cultivar resistance and fungicide programmes.  Fifth Workshop on the European Network for Development of an Integrated Control Strategy of potato late blight.  Munich, Germany, 6 - 10 September 2000.  PAV-Special Report no. 7, 245-254.

Carlisle, D. J., Cooke, L. R. & Brown, A. E. (2001).  Phenotypic and genotypic characterisation of Northern Ireland isolates of Phytophthora infestans.  European Journal of Plant Pathology 107, 291-303.

Cooke, L. R. & Little, G. (2002).  The effect of foliar application of phosphonate formulations on the susceptibility of potato tubers to late blight.  Pest Management Science 58, 17-25.

Cooke, L. R., Carlisle, D. J., Wilson, D. G. & McCall, R. D. (2002). Sensitivity of European isolates of Phytophthora infestans to zoxamide (RH-7281). Sixth Workshop on the European Network for Development of an Integrated Control Strategy of potato late blight.  Edinburgh, Scotland, 26 - 30 September 2001.  PPO-Special Report No. 8, 39-48.

Cooke, L. R. Carlisle, D. J., Quinn, M. & Wilson, D. G. (2002).  Sensitivity of Phytophthora infestans isolates from Northern Ireland to phenylamide and fluazinam.  Resistant Pest Management Newsletter [http://www.cips.msu.edu/whalon/rpmnews/] Volume 11 (2).

Cooke, L. R. & Little, G. (2002).  Responses of potato cultivars to Phytophthora infestans and fungicide programmes for integrated control of late blight in Northern Ireland.  15^th Triennial Conference of the European Association for Potato Research, Hamburg, 14-19 July 2002 (abstract), p. 221.

Carlisle, D. J., Cooke, L. R., Watson, S. & Brown, A. E. (2002). Foliar aggressiveness of Northern Ireland isolates of Phytophthora infestans towards three potato cultivars.  Plant Pathology 51, 424-434.

Cooke, L. R., Carlisle, D. J. & Mccall, R. D. (2002). Activity of zoxamide against European isolates of Phytophthora infestans.  Proceedings BCPC Conference - Pests and Diseases – 2002.

Hussain, S., Cooke, D. E. L., Lees A. K. & Duncan, J. M. (2002). Development of novel SNP markers for genotyping Phytophthora infestans in Scotland. Abstract for the GILB 2002 Conference. Late blight: Managing the global threat 11-13 July 2002, Hamburg, Germany.

Lees, A. K., Hussain, S., Cooke D. E. L. & Duncan, J. M. (2002). The development of Phytophthora infestans specific primers and their use in epidemiological studies. Abstract for the GILB 2002 Conference. Late blight: Managing the global threat 11-13 July 2002, Hamburg, Germany.

Shattock RC (2002) Phytophthora infestans: populations, pathogenicity and phenylamides. Pest Management Science 58, 944-50.

9. Is tuber blight an increasing problem?

Tuber blight can be a major problem and even crops with little foliage infection may sometimes be severely infected.

10. Other comments 

The likely withdrawal of all fentin-based fungicides from the UK market at the end of the 2003 season will limit the options available for treating crops where some infection is already present and for reducing tuber infection.  Currently, Northern Ireland growers still make extensive use of fentin, especially in tank-mixes with translaminars.

In the last four years, eight or nine new fungicides active against late blight have been developed with at least four new modes of action.  Several of these have yet to be commercialised, but growers do have access to a wider range of fungicides than ever before.  However, although claims are made in the literature for curative activity, options for treating crops in which blight is present are limited, since manufacturers are understandably reluctant to endorse such use on product labels.  The newer fungicides are now being investigated for effects on tuber blight, but as yet there is not a lot of information.

Compiled from reports submitted to David Cooke (Scottish Crop Research Institute) from Dr Nick Bradshaw (ADAS, Cardiff) and Dr Louise Cooke (Department of Agriculture and Rural Development, Belfast).