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Information - The FFC Bulletin - 2017 V3 July

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Mesh crop covers for non-chemical potato pest & disease control: Final results from the 2016-17 Field trial of mesh vs. agrichemicals

By Charles Merfield

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Summary

  • A large-scale field trial, under commercial conditions, was conducted to compare three meshes with different hole sizes (0.3, 0.4 & 0.7 mm) with a ‘full monty’ fungicide & insecticide regime and a null control, on their effects on tomato potato psyllid (TPP, Bactericera cockerelli) aphids, and potato blight (Phytophthora infestans and Alternaria solani). 
  • Mesh practically eliminated TPP (total of 12 individuals across all three mesh treatments) compared with chemicals (total of 1,614) and the control (total of 1,250).  From this result, added to previous years results, it is concluded that mesh is effectively a 100% means of controlling TPP on potatoes. There is also a very low chance of TPP developing ‘resistance’ to mesh, and so it can be considered a permanant and complete solution to the TPP problem, i.e., the TPP problem on potatoes has been solved if growers use mesh crop covers. 
  • As mesh prevents TPP even landing on the potato crop it is believed that it will also achieve close to 100% prevention of Candidatus Liberibacter solanacearum (CLSo) infection of potatoes.  Due to insufficient funds, only 20 tubers from the control and 20 from the mesh 0.3 mm were tested with all control tubers infected and zero infection from mesh tubers. 
  • Yield was significantly increased by mesh, with a bulk yield of 94.53 t/ha for the best mesh compared with 84.47 t/ha for agrichemicals and 74.97 t/ha for the control, a 12% increase over chemicals and 26% increase over the control.  Mesh marketable yield for tubers >60g was 86.5 t/ha, and 68.2 t /ha for >125g tubers, a 24% and  60% increase over agrichemicals.  Average tuber weight and maximum tuber weight from mesh both increased 67% over agrichemicals.  This yield increase considerably exceeds the industry target of 12% yield increase over ten years, providing more than double the target in a single year not a decade. 
  • The best two mesh yields also exceeded the modelled / theoretical maximum yield of 90 t/ha. 
  • Mesh is cheaper than agrichemicals which coupled with higher yields means that mesh increased the field gate returns by between $4,531 to $21,110 (27% to 75%) from a lower input - lower return to a higher input - higher return scenario.  This considerably exceeds the industry target of a increase in returns of $1,500 over ten years, but achieving that increase in one year. 
  • Mesh also impacted microclimate with an increase in temperature, giving a 19% increase in growing degree days, as well as reducing relative humidity, at temperatures above 15°C and also considerably reducing wind damage to the haulm. 
  • As in previous trials, aphids got under the mesh and large populations started to build so were controlled by Chess.  All other means by which the aphids could be getting under the mesh are now considered exhausted and it is hypothesised that winged adults are alighting on the mesh, producing nymphs which can then penetrate the mesh.  Due to the very small holes that aphid nymphs can get through, coupled with mesh inevitably getting damaged / holed in real-world use, it is considered impossible to have an aphid proof mesh.
  • Therefore, a biocontrol program, based on existing glasshouse practices, needs to be developed to control any aphids that get under the mesh, along with the residual TPP.  This should then achieve as close to zero insect pests in potatoes as it is possible to attain. 
  • Due to low blight levels this year, this trial has produced little information on blight, apart from an indication that mesh and agrichemicals achieved similar control of blight which in turn had statistically lower levels than the control. 
  • A range of future research is considered vital to create a fully farm-ready mesh system for potatoes. 
    • Solving the aphid problem with biocontrol, particularly for the seed industry. 
    • Control of blight both early and late needs to be causally proven. 
    • Direct growth / yield benefits of mesh need elucidating. 
    • The ability of mesh to control all other potato pests needs to be established.