Menu
Archive of Past Issues
Ask an Expert about Resistance Issues
Information for Libraries
Submission Information
Pest Management Laboratory
Mission Statement
Contact Us
Site
designed by:
Erin A. Gould
Research in Resistance Management
Quantifying the Incidence of Herbicide Resistance in the Winter Rainfall Regions of South Africa
P.J.
Pieterse (corresponding author) & J.L. Kellerman
Department of Agronomy
University of Stellenbosch
Private Bag X1
Matieland, 7602
Republic of South Africa
ABSTRACT Resistance of weeds to herbicides has developed in most countries in the world and South Africa is no exception. The incidence of herbicide resistance in South Africa is increasing, but little data is available to quantify the increase in occurrence of resistance. In this study the incidence of suspected and confirmed herbicide resistance is investigated by making use of literature reports, surveys and databases of relevant research institutions. Apart from confirmed cases of resistance reported on in the literature, more information on resistance was obtained by making use of questionnaires and by greenhouse testing of suspected resistant weed populations. The questionnaires were distributed in 1999, 2000 and 2001 to obtain a clearer picture of the incidence of suspected herbicide resistance in the winter rainfall area of the Western Cape Province. Seed samples obtained from suspected resistant weed populations were germinated and the resulting seedlings were subjected to spraying with three different herbicides at four different doses. The treatments were replicated three times and the dry mass production and mortality of the seedlings were evaluated four weeks after date of application. Results from the questionnaires indicated that there was a steady increase in the number of suspected resistance cases in the area surveyed and that more than 230 suspected cases occurred in 2000. Results from the greenhouse tests indicated that, from a total of 49 weed populations tested up to now, 8 populations exhibited no resistance, 12 exhibited single resistance, 14 exhibited cross resistance and 15 populations exhibited multiple resistance. These results are a clear indication that herbicide resistance are present and spreading like wildfire in this particular area and that everybody in the industry should implement strategies aimed at curbing the rate of spread of herbicide resistance.
KEYWORDS herbicide resistance, South Africa, winter rainfall area
INTRODUCTION Towards the end of this century the International Survey of Herbicide Resistant Weeds recorded 235 herbicide resistant weed biotypes of which 150 unique species can be found in 43 countries (http://www.weedscience.com). In the winter rainfall region of South Africa, the first case of resistance was recorded in 1985 involving Avena fatua and diclofop-methyl (Cairns & Laubscher, 1985). Subsequently resistance was recorded in Lolium spp. (Smit & de Villiers, 1998; Smit, Smit & de Villiers, 1999), Phalaris minor (Smit & Cairns, 2000), and Raphanus raphanistrum (Smit & Cairns, 2001).
Although these weed species were proven to be resistant and various warnings about the threat of herbicide resistance have been issued (Lochner, 1999; Khorombi, 2000; de Villiers, 2001) the extent of the problem has not been quantified. This project attempts to quantify the incidence of suspected herbicide resistance in the winter rainfall wheat producing area of South Africa. Furthermore, samples from suspected herbicide resistant weed populations are tested under controlled conditions to verify resistance if present.
MATERIALS and METHODS
Suspected herbicide resistance
Questionnaires were distributed to agriculturalists in the winter rainfall wheat-producing region of South Africa. The questionnaires requested information on the occurrence of suspected herbicide resistance cases, the area involved, crop species and weed species involved. The area was divided into two climatologically distinct areas, viz. the Swartland, a strict winter rainfall area and the South Coast, where precipitation is distributed more evenly throughout the year (Figure 1). The data was arranged accordingly.
Confirmed cases of resistance
Investigations to confirm resistance in suspected resistant weed populations have been carried out at the Department of Agronomy at the University of Stellenbosch since 1999. In most cases, if enough material was available, three herbicides from two different modes of action were applied to each suspected resistant weed accession. Data obtained from the tests gave an indication of the type of resistance present, i.e. simple-, cross- or multiple resistance.
RESULTS and DISCUSSION
Suspected cases of resistance
In 1999 only 13 questionnaires were returned. According to these questionnaires, the number of suspected herbicide cases has increased from less than 10 in 1995 to about 80 in 1999, with most of these cases in the South coast region (Figure 2). In 2000 the number of questionnaires returned was 55, and the number of cases was about 240. In 2001 only 26 questionnaires were returned and they reported about 100 cases of suspected resistance (Figure 3). This indicates that resistance is quite common in these regions. It is difficult to make an accurate estimation of the areas involved in resistance but Figure 4 indicates that the areas involved may well be in the excess of 100 000 ha. The weed species involved in these cases of suspected resistance are given in Figure 5. It is obvious that Lolium spp. and Avena spp. are the most important species involved in resistance. Lolium spp. is more important in the South Coast than in the Swartland, probably because there is more pastures crops in that area.
Confirmed cases of resistance
Lolium spp. has developed resistance to about every ACC-ase herbicide that are registered on it, as well as all the ALS inhibitors (Table 1). Although no cases has been observed yet, it is highly probable that some Lolium populations could have developed resistance to most or all herbicides registered against it. Avena spp. and Phalaris minor also developed resistance against a few ACC-ase inhibitors and ALS inhibitors (Table 1). From Figure 6 it is obvious that more than 80% of suspected resistant weed populations were tested positive for resistance. It is alarming that such a high percentage of populations exhibit multiple resistance.
Resistance is not a simple problem, as can be seen from Figure 6. If resistance has been confirmed on a farm, it does not necessarily mean that all the fields are infested with resistant biotypes. From Figure 7 it is clear that resistance to different herbicides can differ from field to field.
REFERENCES
CAIRNS, A.L.P. & Laubscher, E.W., 1985. Differential tolerance of Western Cape wild oat biotypes to diclofop-methyl and mixtures containing diclofop-methyl. Final Report, Dept Agronomy and Pastures, University of Stellenbosch, Stellenbosch.
DE VILLIERS, B.L., 2001. Onkruid: Weerstand verg nuwe benadering. Landbouweekblad, 10 Mei 2002, 14-18.
KHOROMBI, G. 2000. Mode of action: The key to herbicide resistance management. SAGrain, November 2000. 68-70.
LOCHNER, H., 1999. SA sit op weerstand tydbom. Landbouweekblad, 3 Julie 1999, 10-13.
SMIT, J.J. & CAIRNS, A.L.P., 2000. Resistance of little seeded canary grass (Phalaris minor Retz.) to ACC-ase inhibitors. S. Afr. J. Plant Soil 17, 124-127.
SMIT, J.J. & CAIRNS, A.L.P., 2001. Resistance of Raphanus raphanistrum to chlorsulfuron in the Republic of South Africa. Weed Res. 41, 41-47.
SMIT, J.J. & DE VILLIERS, B.L., 1998. Lolium spp. resistance to ACC-ase inhibitors in wheat (Triticum aestivum L.) within the RSA: a preliminary study. S. Afr. J. Plant Soil 15, 158-161.
SMIT, J.J., SMIT, H.A. & DE VILLIERS, B.L., 1999. Differential efficacy of tralkoxydim and diclofop-methyl on a suspected resistant ryegrass. S. Afr. J. Plant Soil 16, 169-1172.
Supported By:
Editors:
Mark
E. Whalon
Robert M. Hollingworth
Area Editors:
Plant Pathology
Margaret Tuttle McGrath
Herbicide
Jonathan Gressel
Maintained
by:
Theresa A. Baker