Gramoxone Resistant Eastern Black Nightshade Confirmed in Ontario

Eastern black nightshade (Solanum ptycanthum), was confirmed to be resistant to group 22 herbicides (bipyridiliums), specifically Gramoxone (paraquat) in Chatham-Kent recently. The total area infested is not known at this time; however, the resistant weed was found in a perennial horticultural cropping system.

Eastern black nightshade is a dicot weed in the Solanaceae family. In Ontario, this weed first evolved resistance to Group 2 herbicides in 2000 in corn and soybeans. Sulphonylurea (i.e. Devrinol) and imidazolinone (i.e. Pursuit) resistant populations exist in Bruce, Elgin, Huron and Middlesex counties. Cross-resistance to other Group 2 herbicides is likely to exist. Source: www.ontarioweeds.com.

Herbicide cross resistance refers to a weed or crop biotype that has evolved a mechanism or mechanisms of resistance to one herbicide that also allows it to be resistant to other herbicides. Cross resistance can occur with herbicides within the same or in different herbicide families and with the same or different sites of action. For example, after the extensive use of herbicide A in a field, selection of a weed biotype resistant to herbicide A is found to also be resistant to herbicide B, although herbicide B was never used in that field (Gunsolus, J.L., 2002).

Herbicide resistant Eastern black nightshade has also been confirmed in the U.S.A. The following states have confirmed resistance:

1. Wisconsin (1999) - Group 2 (ALS inhibitors)
2. North Dakota (1999) - Group 2 (ALS inhibitors)
3. Illinois (1999) - Group 2 (ALS inhibitors)
4. Michigan (2004) - Group 1 and 5 (Photosystem II inhibitors, i.e. Sinbar)
   

Gramoxone (paraquat) resistance is of particular concern to Ontario producers, due to the reliance of this product in perennial horticultural production systems. There have been no known cases of paraquat resistant Eastern black nightshade anywhere else in the world (International Survey of Herbicide Resistant Weeds http://www.weedscience.org/In.asp).

Eastern black nightshade occurs throughout southern Ontario in open dry woods, edges of pastures, waste places, and in cultivated land, especially in row crops. It is distinguished by being an annual plant with thin, ovate to diamond-shaped leaves, small umbels of flowers on short stalks from sides of stems (not from leaf axils), small, white flowers, and small, black berries that are not partly enclosed by their expanded calyxes. It is often confused with pigweed species.

Eastern black nightshade is known to be a hard to control annual weed. In perennial horticultural cropping systems, post spot or hooded sprayer applications of Roundup (any glyphosate product), Ignite (glufonsinate-ammonium) and Gramoxone (paraquat) are usually effective if applied before the nightshade gets too big. Chateau (flumioxazin) and Aim EC (carfentrazone-ethyl) are two newer herbicides (both Group 14 - PPO inhibitors) that have shown good control of Eastern black nightshade. Chateau should be applied prior to weed emergence, whereas, Aim EC should be applied as a post application. Please see the product labels for specific application directions.

Rotating between herbicide groups or mode of actions is essential to prevent resistance development. If you have any plants that are not being controlled by your current herbicide program and you suspect resistance please contact the Agriculture Information Contact Center: 1-877-424-1300. The University of Guelph "Weeds Lab" can test for resistance of suspected weed species.

For more information on herbicide resistant weeds in Ontario visit:
www.plant.uoguelph.ca/resistant-weeds

Special thanks to: Dr. Francois Tardif and Peter Smith, University of Guelph for confirming the resistance and review of this article.

References

Gunsolus, J.L. 2002. www.extension.umn.edu/distribution/cropsystems/dc6077.html