- development of a systems approach for the management of late blight in EU organic potato production
The EU-FP5 project blight MOP started in March 2001 and included 13 partners. The project is due to be completed in October 2005, but has already achieved considerable success in identifying principle strategies to improve the control or minimise the impact of late blight in organic but also conventional potato production in Europe. The effect of alternative treatments and agronomic strategies evaluated in the project and the impact of reduced copper fungicide use on Cu-levels in soils and potato tubers are described below. The effect of within field diversification strategies and socio-economic impacts of late blight in organic potato production (which were also assessed under the blight MOP project will be presented in future articles in the QLIF newsletter. The final report from the Blight MOP project will be published on the QLIF website towards the beginning of 2006.
Alternative foliar treatments
A wide range of alternative treatments (plant sea weed and compost extracts and biological control agents were tested for their activity against foliar blight in in vitro and in excised leaf bio-assays. Some of the alternative treatments (e.g. some compost extracts, biological control agents and on plant extract made from rhubarb) showed activity in bio-assay and laboratory tests. However, of the >60 different treatments evaluated in field trials the levels of control, only the Cu-controls and a rhubarb extract showed significant activity levels against foliar blight. However, the levels of control achieved by the rhubarb extract were significantly lower than those obtained with Cu. This, the relatively high concentrations of rhubarb extract required for activity and the cost of the product make the use of the product unsuitable for commercial potato production.
Since in both of these extensive studies no commercially viable alternative treatments for Cu-fungicides, that would be acceptable under organic farming standards could be identified, it appears to be unlikely that alternative treatments with activity similar to Cu-fungicides will be identified in the near future.
However, it should be noted that Cu-oxychlorite gave higher levels control than Cu-sulphate when applied at the currently permitted Cu-input level of 6 kg/ha in field trials in the UK.
||In should also be noted that in pot trials (see section 2.2) application of sea weed extracts significantly increased the yield of some varieties of potato. This was thought to be a nutritional or growth promotion effect of the sea weed extracts, which are well documented in the scientific literature.
Use of novel resistant varieties can significantly reduce blight incidence and development and increase yields (up to 50%) in all 4 European regions included in trials (CH, F, N, UK). However, in the UK trials it was shown that even in the most resistant varieties application of copper fungicides still has a significant effect on yield (up to 10%), even when foliar blight levels are very low (<10%) at the time of defoliation. This may be related to a nutritional effect of foliar Cu applications in resistant varieties (the trial sites used had low soil Cu-levels), but this will have to be confirmed in future trials.
Changes to fertility management practices (within the range permitted under organic standards) are unlikely to affect the level of foliar blight. However, fertility input trials carried out in 2001 strongly indicate that in long term organically managed soils a switch to composted manure may result in a more rapid tuber development and up to 40% higher yields (when inputs were applied at the same N-input level) at the time when blight levels require defoliation of crops.
However, in recently converted soil there was no difference in yield between fertility input types, and in sand based pot trials and field trials carried out on recently converted sites with very sandy soils chicken pellets resulted in slightly (15%) higher yields than compost (when inputs were applied at the same N-input level). The interactions between (i) fertility input types and levels, (ii) soil type, (iii) biological activity (and N-mineralization potential) and (iv) water availability are still poorly understood, and require further work before accurate re-commendations with respect to optimum fertility management practices can be made.
Introducing chitting treatments was also shown not to affect foliar blight development. However, it was confirmed to result in more rapid tuber development higher tuber yields (up to 15%) been produced at the time when blight levels require defoliation of crops. However, this was only significant for the determinant variety included in trials and when late planting dates were used. For indeterminate varieties and early planting dates chitting treatments were shown to have no significant effect on yield and are likely to reduce the gross and profit margins of potato crops due to the additional costs associated with chitting treatment.
Reductions in planting density were shown not to have a significant effect on foliar blight development and crop yields in the UK trials, which compared various crop densities and configurations in bed systems. However, parallel trials carried out in the Netherlands showed that a reduction planting density resulting from an increase in the distance between ridges/rows from 70 to 90 cm will increase tuber yields, but again had no effect on late blight incidence and severity (see ANNEX 2, section 5.1).
There was no significant difference between different defoliation treatments (flailing, burning and a combination of flailing and burning) with respect to the main tuber blight risk indicator (sporulation on stems after defoliation) that could be assessed in trials. However, since tuber blight levels were low in both seasons in which trials were carried out, future trials (in seasons with higher tuber blight pressure) should be carried out to confirm these results.
Effect of copper fungicide treatments on potato tubers and soil
Copper fungicide-use in potato production at the levels (6 kg/ha currently permitted in organic farming in many EU countries; Cu use is completely banned in DK and NL) did: (i) not increase the Cu content in tubers, (ii) not cause major changes in the composition of tubers and (iii) not increase in soil Cu-levels to levels that are of environmental concern (especially if potato are grown only every 5-6 years and are the only crop treated with copper in the rotation).
However, some results of trials reported here indicated that the copper applied with Cu-fungicide sprays may (a) increase potato yields on Cu deficient soils via a direct nutritional effect on the crops and/or (b) contribute to reducing soil Cu-deficiencies which are common in many areas of the UK and can have negative effects on crop yields and quality and livestock health.
Over the 4 field trial seasons yield losses due to potato blight were relatively limited for most varieties included in trials and very low levels of tuber blight were recorded. Due to the high yield potential of the more resistant varieties, it is likely that organic main crop potato production without Cu-fungicides would be possible without loss in productivity, if (a) the more resistant new main crop varieties (e.g. the Balfour and Sarpo range) were introduced more widely and (b) greater market acceptance for the new range of varieties could be achieved
The greatest potential for improved control of late blight in organic potato production was clearly the use of resistant varieties. It is therefore strongly recommended that a continuous breeding efforts will me maintained in Europe to allow a continuous supply of new resistance to be introduced into the UK potato industry. Even varieties with more polygenic/horizontal type resistances were shown to be eventually overcome by the development of more virulent strains of the fungus. For example, Sante showed relatively low foliar blight resistance in most regions of Europe, but was originally classified as having high tolerance/resistance to foliar blight.
The greatest potential for increasing the yield levels in organic potato production was clearly associated with improved fertility management practices. However, further research will be required to identify the complex interactions between organic matter based fertility inputs, environmental conditions (especially soil water levels) and soil biological activity
There are clearly significant interactions between the different agronomic factors affecting yield and blight development in organic potato crops and these should also be investigated further in the future.
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