Development of a systems approach for the management of late blight in eu organic potato production

Resistant varieties consistently gave most effective control of foliage and tuber blight compared with diversification and agronomic strategies and alternative treatments to copper-based fungicides. They did not invariably outyield susceptible ones but decreased disease inoculum and consequently the risk of infection within and between crops. Applications of copper oxychloride to both resistant and susceptible varieties, improved blight control and yield in the most resistant varieties, improvements were relatively small. Growing resistant varieties is the most effective strategy to reduce or eliminate the need for copper-based fungicides in organic cropping systems whilst maintaining production. Reductions of between about 16.5 and 50% of current levels copper fungicide applications might be achieved by growing them instead of susceptible varieties to the maximum extent that the market could absorb. This potential advantage will only be realised if the resistant varieties that are or become available, are acceptable for the organic market and substitute for the susceptible but more popular varieties currently grown currently without decreasing economic returns. Shifts in regional/national late blight populations towards increased pathogenicity with more resistant cultivars in production seems unlikely but virulence to new R-genes bred into new cultivars cannot be ruled out completely.

Reducing the impact of blight with mixtures of potato varieties depended on the level of resistance of varieties in combination. Occasionally, mixture yields increased independently of effects on disease where interactions between the constituent varieties improved resource capture and utilisation. Alternating rows of susceptible and resistant varieties had small effects on the disease. Intercropping potatoes with grass/clover or spring wheat reduced blight in small plots grown perpendicular to the main wind direction, but yields were unaffected. As an adjacent crop, wheat reduced blight more effectively than grass/clover, but this advantage was offset by competitive effects at the boundary between the taller wheat and shorter potatoes which decreased the yield of potatoes. Commercially, it would be more difficult to manage crops grown as mixtures or alternating rows of different varieties or in beds separated by intercrops than as a single variety occupying a large area on a field-scale. Compatibility of companion varieties regarding nutrient requirements, maturity, storage requirements etc.; rotational considerations with intercrops and marketing of variety mixtures for most purposes would need to be addressed. Diversification may be useful as part of a combined control strategy under low to moderate late blight disease pressure.

Site-specific, integrated strategies for late blight and crop performance management often improved the physical (marketable yield) and financial (cost/benefit) performance compared with Currently Used Late Blight Management Strategy (CULBMS). Blight control was generally lower than where copper-based fungicides were used, but copper did not always improve yields. Critical appraisal of CULBMS identifies deficiencies, but it may already be optimised in some cases. For others, simply adopting current state of the art technology would improve efficacy. There is no single blueprint: the integrated system must be designed for a specific situation. The systems approach at grower, country and EU level will narrow the production gap between crops grown with and without copper-based fungicides. Effective information dissemination is required to achieve this objective and further research to develop less advanced strategies; identify positive interactions between components and site-specific requirements successfully. Optimising systems could take several years in many cases and will need re-evaluation/modification as technological, political, climatic, environmental and economic conditions change. A lower-input programme of copper-based fungicides may be an interim solution. Less dependency on copper will sustain the production and profitability of organic potato growing in Europe, benefiting all stakeholders and meet an important EU organic farming policy objective.

Pigs effectively removed volunteers as a source of blight but the technique is of limited application. Rotational position: following fertility building grass/clover or Lucerne, potatoes were more infected than after spring wheat but yield was unaffected because of improved fertility. Manure type, level or N:K ratio had no effect on disease, but improved nutrient supply and crop growth and yield. This growth-promoting aspect of fertility management should be fully exploited. Planting date (early) and pre-sprouting (chitting) effects on blight were small and insignificant but by advancing tuber bulking, acceptable yields were usually achieved before the crop was killed-off. Within commercial limits, plant density and configuration had no effect on blight but large effects on total and graded yields. Optimised, scheduled irrigation used about half the water of traditional regime yet maintained yield without increased risk of late blight. Pre-harvest defoliation by flailing and/or burning to avoid tuber blight decreased blight sporangia per plant but had no effect on tuber blight or the safe interval between defoliation and harvest but it was not possible to conclude which method would be most effective because tuber blight was rare.

Some alternative treatments e.g. manure-based compost, plant and seaweed extracts, micro-organisms and existing products reduced blight infection to variable extents in the laboratory/glasshouse assays but much less so under field conditions. Efficacy of a range of these novel and commercial compounds was unaffected by dose rate or formulation. Low doses of copper products were almost as effective as full rates and far more effective than copper-free compounds. Underleaf spraying gave more uniform spray application but no improvements in blight control with either copper-based fungicides or alternatives. Timing of application is likely to be more important than method. Acceptable levels of blight control might be achieved by using lower doses of copper-based fungicides (i.e. of the order of 2 kg/ha/year of elemental copper) than those which are currently permitted, especially when applied according to decision support systems. Further research is needed to test this and the most effective application strategy (timing and amounts) where such reduced rates are to be used. Where current national legislation and organic standards allow, this approach may offer an interim solution to the late blight problem in organic potatoes until fully effective and acceptable alternatives become available.

Growth of EU organic potato production is likely because of increasing consumer demand, but crop profitability may decrease. Late blight is a major but variable problem for many growers. Diverse strategies are used to stabilise yields and income but not all growers use available technology. Where permitted, copper fungicides are relied upon for control to maintain yield and quality as alternatives are usually ineffective. Almost two-thirds of copper-fungicide users expected organic potato production to decrease with serious consequences if it is banned. However, region-specific optimization/integration of technologies should improve yields and yield stability with less dependence on copper. Some growers could achieve this simply by adopting existing production strategies i.e. state of the art technology or improving efficiency. Key factors identified in need of further development, exploitation and evaluation under various regional conditions were: - Crop resistance management strategies - Agronomic strategies - Soil fertility management - Novel crop protection strategies. Dissemination of the resulting know-how/ novel technologies from the research programme would increase success and future stability of organic potato production considerably with less reliance on copper compounds.

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