Resistant Pigweed Hits Seven Counties in Southwestern Ontario

WEED MANAGEMENT
Resistant Pigweed Hits Seven Counties in Southwestern Ontario
By Gabrielle Ferguson and Dr. Francois Tardif, Department of Plant Agriculture, University of Guelph
(reprinted from OMAFRA Crop Pest Newsletter)

Farmers in the counties of Haldimand, Oxford, Perth, Huron, Middlesex, Elgin and Lambton have a new weed problem to add to their list of targets.

Graduate student (MSc) Gabrielle Ferguson and Dr. Francois Tardif of the University of Guelph have confirmed imazethapyr (Pursuit) resistance in pigweed from at least 14 different fields throughout these counties. Some of the pigweed has cross resistance to flumetsulam, one component of Broadstrike Dual, Broadstrike Treflan, Fieldstar and Striker. Further studies will determine if any of these plants are resistant to other herbicides in the Group 2 herbicide family, which are found on page 33 in the Guide to Weed Control (Publication 75).

The resistant samples, collected last fall, were from fields where pigweed had escaped chemical control. In some of those fields the history of herbicide treatments showed that Group 2 herbicides had been continuously applied for six years. In other fields Group 2 herbicides had been used for four out of six years.

What can farmers do? At the minimum, any Group 2 herbicides should be tank mixed with different herbicide groups. Farmers who have used Group 2 herbicides for several years in the same field should rotate out to an alternate group of herbicides. This completely removes the selection pressure from the Group 2 family for that year.

For farmers who find themselves with escaped resistant pigweed, Reflex, Blazer and Blazer + Basagran should be adequate rescue treatments. The use of thifensulfuron (Pinnacle) and chlorimuron (Classic) is strongly discouraged because they are Group 2 herbicides. There is a strong possibility that they will have very little efficacy on this resistant pigweed.

The particular townships that have confirmed resistance so far include:

• Haldimand/Norfolk: Haldimand
• Oxford: East Zorra, Tavistock
• Perth: Elma
• Huron: McKillop
• MIddlesex: London, West William
• Elgin: Dunwich, Southwold, Aldborough
• Lambton: Moore

We believe there may be many more fields in these areas with resistant pigweed and what we are seeing now is only the tip of the iceberg. All counties were not surveyed so farmers should be scouting their fields diligently for weed escapes this season.

The study will be continued for 1998. Farmers who notice pigweed escapes in their fields over the season are advised to send samples of seed for analysis to the University of Guelph c/o Gabrielle Ferguson (Guelph, Ontario N1G 2W1) or to your local OMAFRA office for forwarding to the university.