Straw mulch in organically grown potatoes - evaluation and optimisation for virus vector control

1. In order to evaluate effects of straw mulch applied at 2.5 – 5 t ha-1 in organically grown potatoes (Solanum tuberosum L.), 21 field experiments were conducted over five years at two locations Northern Hessen and Southern Niedersachsen, Germany). The experimental sites were characterised by temperate climate conditions (635 – 709 mm precipitation year-1; 8.1°C mean air temperature) and loamy silt soils. The main focus of the study was on aphids and the aphidtransmitted Potato virus Y (PVY). This disease is a main problem in seed potato production. In addition to virus and vectors, associated agronomic effects of straw mulch were studied. 2. Straw mulch significantly reduced the incidence of PVY. It was most effective as a protectant for young plants against PVY, thus when a high vector pressure occurred early in the year. Combined mulching and presprouting (chitting) had a synergistic effect on the reduction of PVY incidence, with mulching affecting early vectors while the chitted plants exhibited adult plant resistance earlier, thus protecting from late occurring vectors. 3. Straw mulch reduced aphid infestation on potato leaves and populations of potato-colonising aphids, but did not affect population growth rates. Scaling up the area mulched stepwise from 100 m² to 900m² consistently kept aphid infestation at reduced levels. 4. In a small scale experiment, straw mulch resulted in a reduction of the number of winged aphids landing in green water traps, compared to traps placed on bare soil; this effect was significant with amounts of 200 g straw m-2 and ≥ 400 g m-2, but increasing the straw quantity beyond 200 g m-2 did not cause a further significant aphid reduction. 5. In two further field experiments in 2003, aphid landing in green water traps placed on various backgrounds was tested, including differently coloured plastic sheets, straw and uncovered soil as backgrounds. Aphid catches were highest in traps on uncovered background (soil), and lowest in traps on white or silver backgrounds. For seven aphid species there was a negative correlation between UV-reflectance (320 – 400 nm) of backgrounds and log(N+1)-transformed number of individuals. However, the effect of straw mulch (reduced aphid catches with straw compared to soil), could not be attributed to differences in UV-reflectance, as the UV reflectance was almost identical in soil and straw. 6. Tuber yield and tuber size distribution were not influenced significantly or in a uniform direction by straw mulch application in eleven field experiments, conducted over four years. 7. There was no consistent effect of straw mulch on weed parameters as number of weeds, weed cover and above-ground biomass of weeds. 8. The fact that yield and weed development were not significantly affected by straw mulch is largely attributed to the relatively low amounts of straw applied, which were chosen for the primary purpose of vector control. 9. The risk of undesirable post harvest N-leaching was reduced by straw mulch due to the immobilisation of nitrate-N after harvest at 6.8 – 7.0 kg N t-1 straw in two experiments (18 – 34 kg NO3–N ha-1). 10. Soil erosion was greatly reduced (by >97 %) in a rain simulation experiment on a 8 % sloping potato field with 20 % crop cover. 11. Severity of late blight (Phytophthora infestans) was estimated in five of the experiments at 3 – 7 dates per experiment. Straw mulch had no significant effect on late blight severity, measured as relative area under the disease progress curve, in any of the experiments, but a trend reducing late blight by straw mulch was observed in all five experiments. 12. Infestation with sclerotia of black scurf (Rhizoctonia solani) on harvested tubers, assessed on 100 – 220 tubers per plot, was not influenced consistently by straw mulch, with effects being nonsignificant in eight out of nine experiments. 13. Effects of straw mulch on microclimate, measured in one field experiment, were dependent on the time of the day, with the air in mulched plots being moister and cooler at night and dryer and warmer during the day. This effect was less marked in the period 4 – 6 weeks after mulching than in the fortnight directly after mulching. 14. Prospects and constraints of straw mulch application in organic potato production are discussed and parameters for optimisation are suggested.