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Skin Spot Management - fungicide application methods

Publication Date: 
1 December 2010
Author/Contact :
Author/Contact: 
Stuart Wale (SAC) & Glyn Harper (SBCSR)

 

A number of methods of seed tuber treatment have been investigated with the aim of improving the coverage and hence the efficacy of fungicide treatment. Surface coverage,more than residue level is important for disease control. Conventional hydraulic spray applicators on roller tables were more effective than a spinning disc applicator. Four potential ways to improve tuber coverage and residues have been identified for conventional roller table spray applied equipment. These include swopping to a more effective nozzle, avoiding using a downward fan, improving the configuration of spray nozzles over a roller table and using an adjuvant in the spray solution to improve wetting and spreading. A range of alternative methods of applying seed tuber treatments were evaluated. Cold and hot fogging gave inconsistent coverage on different surfaces of tubers. Dipping of tubers was very effective at achieving good surface coverage and an optimum dilution for the dipping solution has been identified. However, there are potential drawbacks for dipping and these are described. Different ways to dry tubers using an air knife were evaluated and optimum use of this system of drying tubers is described.
 
A key component of the project was to determine the biological efficacy of a fungicide, in preventing skin spot, under the various application methods. Field trials over two seasons have been completed and include a repeat evaluation of specific treatment methods undertaken in the first year. The tubers from harvest 2010 require further storage until visual analysis of skin spot symptoms can be satisfactorily carried out and the results will be reported as an annex to this report in May 2011.   Results to date have provided indications where improvements in existing equipment can be made. These include:
 
• Exchanging a Delavan hollow cone nozzle for the superior Lurmark Mint Green nozzle.
 
• Adding an adjuvant to the spray solution to improve coverage. This was effective in year 1 with Maris Piper but not King Edward but improved residues in year 2 with Desiree. By testing three contrasting adjuvants it is clear that they are not all effective
and correct selection is important.
 
• Improvements in configuration can increase coverage and residue. Many roller tables rely on a single spray nozzle but increasing the area of table on which spray mist is applied by increasing the number of nozzles used improved application. To ensure that the spray volume is minimised, selection of lower output nozzles will be important. In this project the hollow cone nozzle with the least output was the Lurmark Grey
 
• Switching off a downward fan. The results in the first year of experimentation suggested that a downward fan would not improve coverage or residue. The seed tubers used in year 2 of experimentation were larger than in year 1 (35-55mm cf. 35-45mm). Without any other changes, the levels of coverage and residue were higher in the second year. This demonstrates that on a standard roller table larger tubers offer a greater target for the spray. Very few growers have variable pitch roller tables that can be adjusted to
permit changes in pitch to present more tuber surface when smaller tubers are graded. As a result, those treating seed potatoes on standard roller tables can only optimise coverage and residues by keeping the roller table as full as possible. To achieve this, more growers should utilise hoppers to collect seed and release in a uniform way. Throughout the two years of experimentation, there was no evidence that dose or coverage affected seed tuber vigour or growth.

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