Herbicide Resistant Weeds

Environmental effects on MSMA phytotoxicity to wild-type and arsenical herbicide-resistant common cocklebur (Xanthium strumarium).

Nimbal, C. I., D. R. Shaw, G. D. Wills, and S. O. Duke. 1996. Environmental effects on MSMA phytotoxicity to wild-type and arsenical herbicide-resistant common cocklebur (Xanthium strumarium). Weed Technology. 10: 809-814.

The effects of temperature, relative humidity, and light on MSMA phytotoxicity to wild-type and arsenical herbicide-resistant biotypes of common cocklebur (Xanthium strumarium) grown from seed collected in Mississippi were evaluated. In growth chamber studies, 20-day-old plants were sprayed with 0.5 kg a.i. ha-1 MSMA, and after 7 days, fresh weight and chlorophyll content were measured to estimate phytotoxicity. All three environmental factors influenced the level of weed resistance. In studies with day/night temperatures of 30/25øC and 500 æmol m-2 s-1 photosynthetic photon flux density (PPFD) light with a 14-h photoperiod, the phytotoxicity of MSMA at 40% relative humidity was minimal, but was significantly greater at relative humidities of 60 and 90%. The resistant (R) biotype was affected less than the susceptible (S) biotype at the higher relative humidities. The level of resistance estimated by fresh weight reduction was greatest at 30/25øC, but was most pronounced at 35/30øC using chlorophyll measurements. Resistance was lowest at 25/20øC by either measurement. MSMA phytotoxicity occurred at light intensities of 75, 500 and 750 æmol m-2 s-1 PPFD; however, biotypic response was greatest at 500, followed by 750 æmol m-2 s-1 PPFD. The resistance of the R biotype to MSMA was not significant at 75 æmol m-2 s-1, suggesting that differentiation between the biotypes for MSMA sensitivity would be difficult under low light.