First report of target-site resistance to glyphosate in Amaranthus hybridus L. in the Republic of South Africa

Abstract

Amaranthus hybridus L. is a major weed for summer crops. Although A. hybridus has been a known crop weed in the Republic of South Africa (RSA) for a long time, herbicide resistance has not been a problem. Nevertheless, A. hybridus populations from KwaZulu Natal (KZN) Province have caught the attention of farmers since glyphosate has been progressively becoming less effective in controlling the species. This study aimed to evaluate herbicide resistance in A. hybridus and the underlying mechanisms of resistance. Seeds from 50 glyphosate-resistant plants were collected from fields at Bergville and Winterton in KZN and compared with a susceptible population from Hendrina (Mpumalanga Province). Glasshouse screening was conducted where glyphosate (Roundup® PowerMax) was applied at 6-leaf stage, at doses 0, 540, 1080 (recommended dose), 2160 and 4320 g ae ha−1. Surviving plants were sampled for molecular analysis to establish any target site mutations in the EPSPS gene that confer glyphosate resistance. Dose–response assay indicated 100% control in the Hendrina population, variation in the control of the Winterton population and 100% survival in the Bergville population. Molecular analysis indicated a rare triple mutation (TAP-IVS) in the KZN populations. This kind of mutation endows a high level of glyphosate resistance, which explains why these populations survived even the 4× dose. These findings confirmed the first cases of glyphosate-resistant A. hybridus and established the mechanism of resistance as target site mutations in the EPSPS gene reported in the RSA. These findings will serve as a base for other herbicide resistance cases and the development of initiatives to control and minimize the spread of this weed in the RSA.