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Battling blight the organic way

An example of the blight-free potatoes
An example of the blight-free potatoes
Potato late blight is a serious disease in both conventional and organic agriculture, causing drastic crop loss as the Irish potato famine of 1845 most graphically demonstrated. Organic production of potatoes has long relied on copper based fungicides to control blight, but the negative impact of the accumulation of copper in perennial crops (e.g. grapevine and top fruit) led, in March 2002, to a ban on their use in organic agriculture across Europe. Organic potato producers must learn to control blight without copper while maintaining the yields required to remain competitive. A Europe-wide project, funded via the European Commission's Key Action Five programme, is underway that seeks to develop new strategies for the long-term control of potato blight in organic agriculture.

Europe's agricultural plague

Potato late blight is caused by the fungus-like Phytopthora infestans and has long afflicted European agriculture. The Great Irish Famine of 1845-1847 was caused by the whole-scale loss of potato crops to blight, resulting in the death of one million people and the mass emigration of many more. While Europe does not experience crop failures on such a scale today, effective control of potato blight is still elusive. Farmers may suffer significant economic loss as a result of infection when tubers rot in the field or succumb to post-harvest bacterial and mould infection.
The use of resistant cultivars and chemical sprays in conventional production keeps blight infection under control but the pathogen often becomes resistant to new sprays and plant resistance. Organic production faces a steep hill to climb as few potato cultivars have been bred for blight resistance in organic systems and no significant chemical control will be available once organic approval for copper is withdrawn.

Europe fights back

A 5 year, €6 million Key Action Five project called 'Blight-Mop' aims to develop a systems approach for the management of late blight in organic potato production, and brings together researchers from the across Europe .
Three clear phases exist within this project: identification of current practice across Europe; research into the efficacy of new technologies and practices; and the development and testing of a strategy for blight control across Europe.
The project's research phase has already generated information critical for developing effective blight control strategies in four study areas:

  • Resistance management
    Blight-Mop has studied new potato varieties, bred for blight resistance in conventional production systems and discovered that certain resistant varieties perform well under organic conditions while others show reduced resistance when grown in organic systems. Late-maturing varieties with resistance levels which may allow production without copper fungicides have been identified, including a purple potato from an Hungarian breeding programme. The consortium aims to include the Hungarian producer of this potato in Blight-Mop through a call that will extend eligibility to Newly Associated States to take part in EU research projects.
  • Agronomic strategies
    Crop management has an enormous impact on disease control and yield. Blight-Mop is investigating different management systems including: changing planting depth in loose soil to help plants dry out more quickly; and encouraging seed potatoes to 'sprout' before they are planted to accelerate crop growth and allow the plant to mature further before blight strikes.
    Blight Mop has discovered that the format in which nutrients are added has a significant impact on plant growth and thus the growth stage at which the disease hits. Composted manure delivered the same nitrogen input as raw manure but resulted in a 20-30% higher yield, meaning that crop plants were more mature at the point at which blight struck and better able to tolerate the disease.
    The project has already confirmed that potatoes planted at an early date tolerate blight infection better than those planted late. In conventional potato production, crops grown after a grass/clover ley are more susceptible to blight and researchers will be examining this process in organic production.
    Potato plants left over from a previous crop (volunteer potato) allow blight to over-winter in mild climates and are a primary source of early infection in a new crop. Blight-Mop has established that pigs provide an effective method of removing these potato plants as they grub out and eat the entire plant, tuber and all.
  • Diversification strategies
    Blight-Mop investigated several strategies and discovered that planting a barrier crop helped maintain blight resistance in potatoes.
  • Alternative blight treatments
    Bioassays (using compounds acceptable in organic agriculture) on individual leaves have identified promising compounds/bio-control agent and the next stage is to scale them up to field-level tests.

Fighting blight

The final phase of the project will draw together all information generated over the first three years of the project and draw up a blight management strategy that incorporates innate resistance, good management and use of inputs to delay blight strike as late as possible in the growing season and limit its spread to tubers. The strategy will then be tested over a growing season in sites across Europe to assess its efficacy in different climates and soil types.
The results promise to provide a lifeline to organic potato producers who must protect their livelihoods against disease.


The rate and severity of the late blight epidemics 2001 and 2002 gave a rigorous test of the different components of the integrated system. In general, effects of the individual components were very similar in both years. In 2003, very hot and dry conditions prevailed in summer across the EU (particularly in August) which severely restricted blight epidemics. In some cases, it was not possible to evaluate the effi cacy of treatments for late blight control because of low levels of infection. However, in others, where infection did occur the general trends were similar to those observed in 2001 and 2002.

  • ‘State of the art’ blight management surveys:
    There are large regional differences in the impact of late blight on organic potato production in the EU. Farmers use a high diversity of strategies to stabilise yields and income, but not all use available technology. There is a widespread view that a copper fungicide ban will have serious consequences for organic potato production unless effective alternatives are available.
  • Variety performance:
    Resistant varieties consistently gave most effective prevention of foliage and tuber blight compared with diversification and agronomic strategies and alternative treatments to copper fungicides. However, yields of resistant varieties were not necessarily higher than more susceptible varieties although they would decrease the source of inoculum. Copper fungicides gave limited benefit in resistant varieties. Acceptability of varieties is determined more by market demands than blight resistance. Diversification strategies: Usefulness of mixtures of potato varieties for reducing the impact of blight depended on the level of resistance of varieties in combination, but in some cases, yield was improved where ecological interactions were improved. Alternating rows of susceptible and resistant varieties had a relatively small impact. Intercropping potatoes with grass/clover or spring wheat reduced blight in small plots grown perpendicular to the wind but yields were unaffected. Wheat was more effective at reducing blight than either grass/clover or potatoes, but yields were decreased because of competitive effects and there would be practical difficulties with such an approach. Such approaches may be useful as part of a combined control strategy.
  • Agronomic strategies:
    Plant population and spacing had no effect on blight but total and graded yields were affected. Effects of planting date and chitting on blight were small and insignificant but had marked effects on total and graded yields in 2001 and 2002 (but not 2003 because of premature senescence). Defoliation mechanically and/or by heat treatment decreased the number of blight sporangia per plant but had no effect on tuber blight. Blight was unaffected by manurial treatments or N:K ratio, but fertility management influenced yield. Potatoes grown after grass/clover or Lucerne were more infected with blight than after spring wheat but yield was not affected because of improved fertility. Pigs effectively removed groundkeeper potatoes post-harvest and hence the potential problem of volunteers as a source of blight inoculum but caused soil compaction and establishment problems in the subsequent cereal crop.
  • Alternative treatments:
    Extracts of manure-based composts gave control of blight in potato leaf assays but not in the field. However, use of an adjuvant in combination with an autoclaved compost extract gave improved control and slightly higher yields in an experiment in 2003. Some micro-organisms, plant extracts and existing products showed promising effects on blight control. Efficacy of a range of commercial and novel anti-fungal compounds was unaffected by dose rate or formulation. Copper free compounds had either no or limited effects on blight compared with standard copper fungicides at normal rates but low doses of copper products were almost as effective.
Project Co-ordinator: Prof. Carlo Leifert
Telephone: 00 44 (0)1661 830 222
Address: Tesco Centre for Organic Agriculture, Nafferton Farm, Stocksfield, Northumberland, NE43 7DM, UK

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