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Herbicide-Resistant Super Weeds

Are they in New York State? Yes!  Where are they?  We're going to find out! A statewide weed herbicide resistance screening project will start this year. Dr. Lynn Sosnoskie, specialty crop weed science, Dr. Bryan Brown, IPM weed management specialist, and Dr. Toni DiTommaso, soil and crop sciences, will find out. Help them to help you!

Weeds compete with crops for light, water, and nutrients, which can result in yield reductions. Weeds can also interfere with crop production by serving as alternate hosts for pests and pathogens, providing habitat for rodents, and impeding harvest operations. Consequently, growers employ a variety of control strategies, including the application of herbicides, to manage unwanted vegetation. Although herbicides can be extremely effective at controlling undesirable plants, failures can and do occur. Weeds may escape chemical treatments for many reasons including the evolution of herbicide resistance.

Worldwide, there are 512 confirmed cases (species x site of action) of herbicide resistance. With respect to the United States, 165 unique instances of resistance have been documented.

A chart showing the status of herbicide resistance cases globally from 1950 to 2020.
Current status of herbicide resistance, globally, over time according to the International Survey of Herbicide Resistant Weeds (

In New York, only four herbicide resistance occurrences have been formally reported:

  • common lambsquarters (Chenopodium album)
  • smooth pigweed (Amaranthus hybridus)
  • common ragweed (Ambrosia artemisiifolia)
  • common groundsel (Senecio vulgaris)
  • All described as insensitive to photosystem II inhibitors (e.g. atrazine and simazine).

This, however, does not reflect the current on-the-ground situation in the state. Work done by Drs. Julie Kikkert (CCE) and Robin Bellinder (Cornell) indicates resistance to linuron in some populations of Powell amaranth (Amaranthus powelli). Recent studies by Drs. Bryan Brown (NYS IPM) and Antonio DiTommaso (Cornell) suggest that horseweed (Conyza canadensis) and waterhemp (Amaranthus tuberculatus) populations may be resistant to one or more herbicide active ingredients.

Pennsylvania has nine reported cases of herbicide resistance including glyphosate resistance in Palmer amaranth (Amaranthus palmeri), which was recently identified here in NY. While it is tempting to believe that herbicide resistance is a hallmark of agronomic cropping systems, herbicide resistance can and has developed in orchards, vineyards, vegetable crops, pastures, and along roadsides.

Photo of two horseweed seedlings.
Horseweed (Conyza canadensis) seedlings.  Photo: L. Sosnoskie

Beginning in 2020, we will undertake a screening effort to describe the distribution of herbicide resistance in the state.

You can be a part of this important work. This coming summer and fall, growers, crop consultants and allied industry personnel who suspect they have herbicide resistance are encouraged to contact Dr. Lynn Sosnoskie (, 315-787-2231) to arrange for weed seed collection.

Indicators of suspect herbicide resistance:

  • Dead weeds intermixed with live plants of the same species.
  • A weed patch that occurs in the same place and continues to expand, yearly.
  • A field where many weed species are controlled but a previously susceptible species is not.
  • Reduced weed control that cannot be explained by skips, nozzle clogs, weather events, herbicide rate or adjuvant selection, and calibration or application issues.

Growers can take several actions to stop the spread of herbicide resistant weeds and to prevent the development of new ones.

  1. First and foremost is scouting fields following herbicide applications and keeping careful records of herbicide performance to quickly identify weed control failure.
  2. Pesticide applicators should ensure that their equipment is properly calibrated and that they are applying effective herbicides at appropriate rates to manage the target species.
  3. Whenever possible, diversify herbicides to reduce chemical selection pressures that result from the repeated use of a single herbicide or site of action.
  4. If possible, incorporate physical and cultural weed control practices into a vegetation management plan.
  5. Be sure to control unwanted plants when they are small and never allow escapes to set seed.
  6. Clean equipment to prevent seeds of herbicide-resistant weed species from moving between infested and non-infested sites and harvest areas with suspected resistant populations last.

Juliet Carroll, your friendly SWD blogger, says, "It's that time of year, you've put on your pre-emergent and post-emergent herbicides in the rows or between the rows, and you're keeping an eye on things to make sure those weeds are dying or not emerging. Continue your vigilance, flag suspects, and contact the "Super Weed Team" to collect suspect seeds to help them help you fight herbicide resistance."

This article was contributed by Lynn Sosnoskie,, Horticulture, Cornell AgriTech.

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