(Marcelo L. Moretti, California State University, Fresno; Bradley D. Hanson, UC Davis; Kurt J. Hembree, UCCE, Fresno; Anil Shrestha, California State University, Fresno)

Weeds have always been a concern in crop production and in the protection of aesthetic value of non-crop areas.

Weedy plants of the Conyza genus are an example of such problematic weeds in crop and non-crop lands. This genus includes two important weedy species in California, horseweed (C. canadensis) and hairy fleabane (C. bonariensis).

Both these species are found on canal banks, roadsides, in orchards and vineyards, and other sites.

Plants of these species are able to produce thousands of seeds that are easily dispersed by air movements. The preferred sites for these species seem to be relatively less disturbed which explain their prevalence in orchards, vineyards, and non-crop areas.

Several herbicides registered in California provide efficient control of these weeds; however some herbicides use may be regulated in certain areas such as Ground Water Protection Areas. Among the herbicides applied for control of Conyzas, glyphosate is one of the most common.

This is because glyphosate is an excellent postemergence product that provides broad-spectrum weed control, is considered an ‘environmentally-safe’ product, and is more cost effective than several other herbicides.

Therefore, it is the most sold herbicide around the world and in California. However, these same reasons have resulted in repeated applications of glyphosate. Inevitably, glyphosate resistance evolved, and the first cases of glyphosate-resistant (GR) biotypes of Conyza in California were reported in 2005 for horseweed and in 2007 for hairy fleabane. This evolution necessitated the control of GR weeds with alternate herbicides such as glufosinate or paraquat.

Burn-down applications of paraquat is considered as an alternative for control of glyphosate resistant weeds in perennial crops. However, in recent years, poor control of hairy fleabane was reported in Fresno County.

Glyphosate-resistant populations of hairy fleabane have already been reported in California and in many other countries. Similarly, paraquat-resistant populations of hairy fleabane have been reported from Japan and Spain. However hairy fleabane populations resistant to both glyphosate and paraquat have not been documented in any part of the world. Therefore, a greenhouse dose-response study was conducted to ascertain whether the populations collected in the Central Valley had evolved resistance to both glyphosate and paraquat.

Our results showed that eight putative resistant populations of hairy fleabane from east Fresno county were resistant to paraquat and or glyphosate. All populations were exposed to increasing rates, up to 16-fold the label rate, of either glyphosate or paraquat. The plants were treated at the 5- to 8-leaf stage (approximately two month old plants), during late spring and summer of 2009. For either herbicide, the rates were based on the label recommendation for weed control in almonds. The label rate was considered as 0.45 lbs a.i. ac-1 and 0.98 lbs a.i. ac-1 for paraquat (Gramoxone Inteon, Syngenta Inc.) and glyphosate (Roundup Weathermax, Monsanto Inc.), respectively.

Some populations of hairy fleabane showed resistance to only glyphosate or paraquat and in some cases, a single population showed resistance to both herbicides. This is the first known case of resistance to paraquat and glyphosate in the same biotype of hairy fleabane worldwide.

Unfortunately this finding is not pleasant news because it has direct impact on weed management, and a consequent increase in production costs. As previously cited, one of the reasons that made glyphosate the most sold herbicide worldwide was its low cost, and it is also one reason why paraquat is a good herbicide for rotation with paraquat. Such resistance to both herbicides, if not properly managed, may lead to a serious problem and in a worst case scenario result in the loss of effective control of both products.

A secondary study was conducted to test the response of these glyphosate and paraquat resistant hairy fleabane biotypes to glufosinate (Rely, Bayer) and saflufenacil (Treevix, BASF). We found that both these herbicides, at the recommended rate, provided good control of the resistant biotypes. Therefore, in the short-term, these herbicides may be good options for control of glyphosate- and paraquat-resistant and susceptible biotypes. However, relying solely on chemical control may lead us to evolvement of new resistance to these herbicides as well.

Therefore, an integrated approach with planned herbicide rotations is necessary to avoid similar problems in future.