In a three years series of on-farm field experiments in organically grown potatoes in Germany the influence of straw mulch on the incidence of Potato virus Y (PVY), on the infestation with Colorado potato beetle (CPB, Leptinotarsa decemlineata), and on tuber yield was investigated. The effect of straw mulch on PVY incidence appeared to be dependent on the overall level of PVY infection, which differed between years, locations and varieties. There was a consistent PVY reduction, when the overall level of PVY infection was high. In contrast to this, no consistent trend could be observed, when there was a very low general PVY infection level. Straw mulch did not significantly affect the infestation with Colorado potato beetle in the field experiments, where three levels of straw were applied (nil, moderate amount and high amount). There was a consistent trend with the highest infestation in the nil amounts and the lowest CPB infestation at the highest amount of straw. In five out of six field experiments there was no significant effect of straw mulch on tuber yield.
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