Resistant Weeds In California Orchards

California nut growers use glyphosate (eg. Roundup) to control weeds under their trees, similar to orchard growers in Ontario, and have generally been satisfied with the wide weed spectrum controlled at a reasonable cost. However, since the discovery of glyphosate-resistant ryegrass in 1990 in a few orchards, the possibility of selecting this problem in more orchards has been a concern.

Recent surveys have shown a low population of resistant weeds in 5000 acres of almond orchards. This is a fraction of their total acreage, but this shows a huge spread in just over a decade.

The recent reports of field crops with glyphosate tolerant horseweed (we call it Canada fleabane) from Maryland to Mississippi across to the mid-West are also of concern. Canada fleabane is a common weed in many Ontario orchards, and has actually developed resistance to Gramoxone (paraquat) in two orchards in Ontario.

These Roundup resistant weeds should be a red flag to orchard growers in Ontario. Although we have not identified glyphosate tolerant Canada fleabane or ryegrass in Ontario orchards, it can develop quickly if resistance management strategies are not used.

How can we prevent this problem? Orchard growers should continue to use a weed resistance management strategy:

1. Alternate glyphosate herbicides with herbicides with other modes of action, eg. tank-mix Roundup + Princep, or use Roundup for spring burndown, followed with Gramoxone or Ignite in June. It is best to rotate within the same year, but if this is not possible, alternating herbicide modes of action between years will help.
2. Use mechanical forms of weed control where possible. Cultivation, mulching, hoeing, or flaming have been used commercially with good results. In California, they are also using geese to eat weeds under trees (geese were effective weed control machines in Ontario strawberry fields before herbicides were developed).
3. Scout for weed escapes throughout the summer to identify which weeds are surviving your control efforts. Mapping these escapes over the years can help pinpoint problem areas and possibly the source of new weeds.
4. Practice good weed sanitation. Clean equipment before it comes into your orchards. Work in clean fields first, then move to weedy fields. Mow weed escapes and field edges before weeds go to seed.
5. Keep accurate and detailed records of herbicide use. If resistant weeds are identified, this can help alter your program to prevent further problems.
6. If suspicious weed failures are observed, collect samples to be tested for resistance.
7. Above all, be aware of the potential for weed resistance and watch for it in your fields.

Glyphosate is still a good choice to control weeds safely, efficiently and effectively, but like all pest management methods, it needs to be managed properly to maintain it as a long-term tool.

For more information:
Toll Free: 1-877-424-1300
Local: (519) 826-4047
E-mail: ag.info.omafra@ontario.ca