Influence of water quality on pesticide efficacy at reduced doses

Abstract

The influence of three sources of mains water on the efficacy of herbicides, aphicides and fungicides was investigated. The water samples were categorised as soft, hard or intermediate, dependent on calcium ion content, and compared to a pure deionised water. The three natural waters had a neutral pH, but differing buffering capacities.

The activity of some of the herbicides was affected by water type, but this was not only related to calcium ion content. Isoproturon (IPU) was less active on chickweed in soft and deionised water, IPU also showed least initial activity on black-grass in deionised water. Metasulfuron methyl was less active on chickweed in the intermediate water. Imazamethabenz showed less response to water type, but there was indication of improved activity in soft water. HBN gave varied results. It was initially less active against chickweed in the soft water, but this improved later on when it was the hard water which appeared to be least active. Further tests are needed with fenoxaprop-p-ethyl because of black-grass resistance. These results suggest that other ions may also be important factors at affecting herbicide activity, as well as the buffering capacity of the water.

The pyrethroid insecticide lambda-cyhalothrin was effective against aphids in all water samples at the doses tested (down to 25% of full dose). Differences were however observed in the persistence of activity. The hard water reduced the persistence of the aphicide compared to the deionised, soft and intermediate waters.

The fungicide tebucanazole was tested against three wheat powdery-mildew strains (Erysiphe graminis). Disease control was most consistent with the intermediate and hard water sources, and more variable with the pure and de-ionised waters. This was most pronounced with a mildew strain known to be insensitive to triazole fungicides. This result suggests that water quality may have an influence on fungicide activity, and water sources with high levels of ions may in fact improve disease control.

It can be concluded that the quality of water can influence the efficacy of pesticides, but the differences varied depending upon the pesticide used. The herbicides and fungicide showed a similar pattern where the soft and deionised waters reduced pesticide activity compared to the intermediate and hard waters. Water quality did not affect activity immediately with the insecticide, but persistence of activity was reduced with the hard water. Further research is required to understand the influence of the water quality further, and to exploit these effects to improve pesticide efficacy and reduce pesticide usage.