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Beware the pitfalls of herbicide carryover

Syngenta and Ask the Industry talk herbicide carryover.

Residual herbicides may have plant-back restrictions that, if ignored, can cause significant damage to susceptible potato crops. These symptoms can be exacerbated by other elements such as unfavourable weather conditions. Syngenta Solutions Development - Technical Lead, Scott Mathew, outlines what growers can do to avoid the risk of herbicide injury to potato plants.

Planting potatoes into pasture country or into ground following crops such as cereals can pose significant risk. Growers need to be aware of any plant-back restrictions. Residual herbicides may persist in the soil for quite some time and if taken up by susceptible crops can cause real damage.

Herbicide symptoms can include plant stunting, yellowing of foliage, whitening or bleaching of foliage, malformed roots or tubers, leaf puckering, distorted growth, leaf speckling, and of course, in extreme cases plant death.

The unfortunate truth is that injury from residual herbicides is not as uncommon as it should be. As horticultural areas continue to expand into more traditional broadacre farming country, we really need to be on top of this issue.

Some of the more common herbicide families to cause plant-back issues in potatoes are the:

  • Synthetic auxin type plant growth regulators e.g. carboxylic acids or pyridine or picolynic acid;
  • ALS/AHAS enzyme inhibitors;
  • and photosynthesis inhibitors.

Carboxylic acid, pyridine or picolynic acid growth regulator type injury is similar to that from 2,4D based products. General symptoms are curling of young leaves. Tuber yields can be greatly reduced. Overseas information indicates crop exposure may carry over into seed tubers and affect the following year's crop.

Inhibitors of ALS/AHAS enzyme include the imidazolinones e.g. imazethapyr, imazaquin and imazamox and the sulfonylurea family e.g. chlorimuron, chlorsulfuron, metsulfuron and triasulfuron. Symptoms include a light green appearance of leaves especially new ones. Leaves can be cupped upward, and the leaf may disintegrate leaving the mid-rib. Leaves may also appear drought stressed. Severe injury results in stunting and purpling. Tuber yield and quality are greatly reduced.

The triazine photosynthetic inhibitors include atrazine, cyanazine, simazine and hexazinone. Carry-over injury can occur when high label rates of metribuzin is used in potatoes after triazine has been applied in the previous crop.

What makes the situation worse?

If injury is mild and the crop is actively growing, it often grows out of herbicide damage to yield a decent harvest. Unfavourable weather conditions such as heavy rain and subsequent waterlogging or cool temperatures can exacerbate the severity of herbicide injury by slowing early crop growth and plant metabolism.

A worst-case scenario would be if the crop dies or is so badly damaged that it won’t grow through to provide a viable harvest. In that situation you’ll likely have to do your research and replant a less susceptible crop. That’s costly, so it’s well worth taking the following steps to avoid the risk.

What you can do to reduce the carryover risk

  • Firstly, pay attention to any plant-back restrictions on herbicide labels. Plant back restrictions are more than friendly recommendations for you to consider. As part of the herbicide label, they are the law! If you are leasing land to grow your crop, get the herbicide records from the owner.
  • Wherever possible, you should be thinking and planning your crop and chemical rotations over the long term. Aim to manage herbicide applications to minimize persistence in the soil that can impact on subsequent crops. Consider such factors as product selection, application rate, application timing, expected rainfall, soil texture and pH and tillage in the preceding crop.
  • Where possible, choose herbicides without plant-back restrictions to give you the ultimate flexibility.
  • And finally, keep records of all chemical use and conditions. Records are your best tool for unravelling what’s really going on.

This article was published in Potatoes Australia, October/November 2018 as supplied by Syngenta.