We can all be thankful that resistance to our key agrichemicals hasn’t progressed nearly as quickly here in New Zealand as it has in countries like Australia and the United States, says Tim Herman, Senior Technical Services Specialist with BASF.
“Put some of it down to good luck and a lot of it down to good management,” Tim observes. “However, now there are signs that the development of resistance here is picking up pace.”
Tim explains that numerous weeds, pests and diseases are known to have developed resistance to one or more herbicide, insecticide or fungicide.
“Last year six weeds (four grasses and two dicots) were first recorded as being resistant to herbicides from Mode of Action (MoA) Group 2 ALS inhibitors. One of those weeds, lesser canary grass (Phalaris minor), was also resistant to ACCase inhibitor herbicides, which are in MoA Group 1.”
The total number of different weed species with recorded resistance is now 18, so the tally went up by one-third in the last year alone.
“That’s a significant statistic,” Tim says.
Another statistic1 from 2021 was that there were weeds with resistance to either or both the Group 1 or 2 MoAs on 50 percent or more of the arable farms in Southland, Waikato and Bay of Plenty which participated in the survey.
“All these developments underline concerns about the ongoing threat from the continuing expansion of resistance.
“The good news is that there is still no immediate threat to the viability of any of our crops. All we need to do is integrate the key strategies for resistance management into our crop protection programmes.”
Those strategies are straightforward and common to all three agrichemical groups, namely:
- Only use agrichemicals when there is a clear need for them. This does not exclude preventative and pre-emergence applications of fungicides and herbicides, respectively.
- Don’t repeatedly use products from the same MoA group. Alternating between MoA groups helps reduce the risk of resistance development.
- For herbicides and fungicides, tank-mixing formulations from different MoA groups is also a positive management strategy. In many cases, chemistry that’s already under resistance pressure can be ‘spiked’ with a resistance-free option that will ensure good control is maintained.
- In many crops there are numerous options to choose between or combine. Herbicides from eight different MoA groups are registered for weed control in maize and sweetcorn, for examples. The herbicides are commonly applied in three slots – burndown, pre-emergence and post-emergence. Some may be used in more than one slot. Sharpen®, for instance, can be used at both burndown and pre-emergence. Others can only be applied in one slot, e.g. Frontier®-P at pre-emergence and Arietta® at post-emergence.
Tim advises growers to ensure that they are aware of which MoA groups they use in each slot and are careful to rotate between groups.
A good example for fungicides is the powdery mildew programme for grapes. Fungicides from at least eight MoA groups are registered for control of powdery mildew. Sustainable Winegrowing New Zealand recommends a diverse rotation for
minimising selection for resistance and maintaining effective control. The programme advises tank-mixing fungicides with single site MoA like Belanty® or Vivando® with formulations that have multi-site activity like Kumulus® DF, as well as rotating between MoA groups through the grape-growing season.
“Again, the combination of different MoAs – especially when multi-site activity is included – helps make disease control more robust and reliable2, and makes it harder for the disease pathogens to mutate so much that they become impervious to all the
different ways in which they’re being attacked.”
As always, make sure you follow the resistance management guidelines on the label for each product you.
ACVM registration numbers: Arietta® #P008825, Belanty® #P009798, Frontier®-P #P007979, Kumulus® #P003493, Sharpen® #P008391, Vivando® #P007973.
1 Buddenhagen CE, James TK, Ngow Z, Hackell DL, Rolston MP, et al. (2021 article)
Resistance to post-emergent herbicides is becoming common for grass weeds on
New Zealand wheat and barley farms. PLOS ONE 16(10): e0258685.
2 Brent and Hollomon, 2007. Fungicide resistance in crop pathogens: How can it be
managed? FRAC Monograph No.1 (2nd ed).