Biotyping resistance testing

Weeds are rapidly becoming resistant to some of the newer herbicides, and it is important to realize the consequences. For example, in Missouri, Bader et al. (1995) conducted a study on corn-soybean rotations in which they used only ALS-inhibitor herbicides, that is, imazethapyr in soybean and primisulfuron-methyl in corn. Within 4 years, a common waterhemp biotype resistant to 5-fold higher rates of ALS inhibitors was flourishing. Greenhouse tests confirmed that ALS-resistant common waterhemp biotypes were present in several plots in the experiment. Within the same state, Bader et al. (1994) reported one case of atrazine-resistant common waterhemp, which developed where a farmer grew continuous com and used only atrazine for more than 10 years. Table 11.1 shows trends in the numbers of herbicide-resistant weeds. 

In some cases, even before a weed is confirmed in laboratory tests to be resistant to an herbicide, the farmer has already changed his weed control program and the resistant weed may no longer be easy to find. There have been also cases where the resistant weed was identified as one species, but was later confirmed by a taxonomist as another. In those cases where no qualified weed scientist was available to conduct confirmation tests, the resistant biotypes are not listed as resistant, even though they may no longer be controlled with the once effective herbicide. 

Tissue cultures of both biotypes of Eleusine can be initiated by a simple combination of plant hormones. Surprisingly, when the cultures were tested for resistance, there was no difference in the susceptibility (as measured by growth) even though the regenerated plants retained the characteristics of the parental line... 

Biotypes with target-site-based resistance to ACCase inhibitors were also selected in wild oat species [Avena fatua, A. sterilis). The resistance patterns were found to be variable. For example, the resistance factors for ACCase from the Canadian A. fatua biotype UMl were 105 for sethoxydim, 10 for tralkoxydim, and 10 for didofop and fenoxaprop, whereas for the Aoena fatua biotype UM33 from Canada the ratios were 10.5 for fenoxaprop, 1.2 for diclofop, 5 for sethoxydim and 1.7 for tralkoxydim. It was proposed that this was due to different point mutations, each being assodated with a characteristic resistance pattern [37]. Another reason could be the frequency of homozygote and heterozygote resistant and susceptible plants within a tested population. 

Different to the cases described above, the herbicide propanil is detoxified in rice and weed species by the action of an aryl acylamidase (aryl-acylamine amido-hydrolase). High activity of this enzyme in rice confers crop tolerance. In Colombia, a biotype of Echinochloa colona was found that is resistant to propanil. Enzyme tests with extracts from this biotype revealed an about three-fold higher activity of aryl acylamidase in the resistant than in a susceptible biotype. It was concluded that resistance of the E. colona biotype is based on enhanced propanil detoxification [74]. 

In a recent paper ShuHa attempted to identify molecules that target herbidde-sensitive and -resistant forms of ACC [80]. Among several experimental and commercial compounds, all the tested substances inhibited ACC from sensitive biotypes of Setaria viridis (green foxtail) and Eleusine indica (goosegrass). The I50S of... 

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