7 April 2000

Beware water quality if youre cutting pesticide spray rates

By Andrew Blake

PESTICIDE activity may be more affected by water quality than previously thought, but not all chemicals react in the same way, according to HGCA-funded work by SAC.

"If you stick to manufacturers doses, water quality probably does not matter much," says researcher Simon Oxley. "But it becomes more important as you reduce doses."

The £10,000 one-year project tested the glasshouse performance of herbicides, fungicides and aphicides applied in waters of different hardness assessed by calcium content.

Three mains sources were used: Morley Research Centre, Norfolk (hard), Yorks (intermediate) and Scotland (soft). Tests using pure, deionised water were also carried out.

Arelon (isoproturon) herbicide was less active on chickweed in soft and pure water and had less initial effect on blackgrass in the latter. But Ally (metsulfuron-methyl) was less active in the Yorks water. Dagger (imazamethabenz) was less affected by water quality, but seemed to work better in the soft.

Hallmark (lambda-cyhalothrin) insecticide killed aphids irrespective of water type, but was most persistent in the intermediate quality.

Mildew control from triazole fungicide Folicur (tebuconazole) was most consistent with the Yorks water. Indeed control of a resistant strain was badly affected by using pure water. Although tebuconazole is not normally considered a strong mildewicide, it was used to help identify subtle differences.

"In theory you might think the purer the water the better." But the finding suggests water high in calcium ions may improve disease control. French vine growers find they often get better results with alkaline waters, he notes.

But there is more to water quality than relative acidity. Some streams with high levels of calcium can be very acid because of the buffering capacity of other materials, he says.

Performance could be improved by using buffering agents and ionic exchange materials or adding key ions to the spray tank. But further work is needed, says Dr Oxley. &#42

SPRAY WATER QUALITY

&#8226 Affects pesticide activity.

&#8226 More important at low doses.

&#8226 Effect differs between products.

&#8226 pH not sole influence.