An update on fungicide resistances in arable crops in New Zealand

There has been much concern lately within the arable industry over the risks of disease resistance to commonly used fungicides in New Zealand. Of particular interest has been Septoria leaf blotch (Zymoseptoria tritici) where the disease has become a major threat to winter wheat crops over the past number of seasons.

Fungicide resistance occurs when a disease naturally develops a mutation that allows the disease to grow and reproduce in the presence of a fungicide. Once resistance is developed, a disease can get out of control easily even with high doses of fungicide applied.

Understanding the status of various fungicides and fungicide groups is very important as well as the trends from season to season, not only to aid in fungicide selection but in order to design responsible fungicide strategies going forward.

From samples sent in for resistance testing by various chemical companies including Bayer New Zealand over the past two seasons, it has been concluded that the strobilurin group are ineffective in controlling Septoria leaf blotch in most parts of New Zealand. The DMI group, which includes Prosaro® and Opus®, has been on the centre of attention as this group has been the primary control method for Septoria leaf blotch in New Zealand over many seasons. The new SDHI group, which includes Aviator® Xpro, has been shown to have a high sensitivity to Septoria leaf blotch.

Earlier this year Bayer New Zealand sent another batch of samples from the 2014/15 season taken from a range of sites surveyed throughout New Zealand to the Resistance Testing Centre in Monheim, Germany. The results suggest that Septoria leaf blotch is still highly resistant to the strobilurin group as expected. The triazole group has a similar level of sensitivity and there is no change in the baseline level of the SDHI actives treated (that is, there is no evidence of resistance).

While the results are good news, it is still very important that growers are vigilant over how fungicides are being used, especially the DMI and the SDHI group. 

Bayer suggest for the coming season:

• Make sure that a maximum of two applications of Aviator Xpro are made to a crop in one season and that the label rate of 1.0 L/ha is observed.
• Use the SDHI group protectively. Use in a highly curative situation increases the chance of resistance development.
• Keep the DMI rate up. For Prosaro, this means a rate of 1.0 L/ha.