Selection of bioassay method influences detection of annual bluegrass resistance to mitotic-inhibiting herbicides
Dinitroaniline-resistant annual bluegrass (Poa annua L.) has been reported in several states; however, there are no standardized screening methods for detecting resistance. Research was conducted to evaluate screening techniques (Murashige and Skoog [MS] media, filter paper, hydroponics, and soil based) to detect herbicide resistance to dithiopyr, prodiamine, and pendimethalin in a suspected resistant ecotype of annual bluegrass from Chattanooga, TN (Chattanooga). A senstitive ecotype from Fresno, CA (Control) was also tested. All the bioassays were able to diagnose the ecotype from Chattanooga as resistant to prodiamine and pendimethalin. However, the degree of resistance was highly variable between bioassays. In hydroponics, the amount of prodiamine required to inhibit Chattanooga growth by 50% was 26 times more than Control. Comparatively, in MS media the amount of prodiamine required to inhibit Chattanooga growth by 50% was 80 times more than Control. Minor dithiopyr resistance from the Chattanooga ecotype was detected by the hydroponics, filter-paper and soil-based bioassays. Hydroponics provided the most rapid diagnosis of resistance, accessing resistance for a mature plant in 10 d. The MS-media bioassay had the least amount of confounding variables. These findings highlight the potential variation in results that can occur in mitotic-inhibiting herbicide resistance detection simply on the basis of how plant samples are assayed.