Environmental Effects on Junglerice (Echninochloa colona L.) Susceptibility to Postemergence Herbicides.

Junglerice is a problematic weed in annual and perennial cropping systems in California. The problem has been further exacerbated with documentation of glyphosate-resistant populations in the state.  As such, alternative herbicides are being sought for their immediate control.  However, the efficacy of postemergence herbicides is often observed to be influenced by shade, soil moisture, and temperature conditions existing in orchards and vineyards.  Therefore, studies were conducted in 2015 to assess the efficacy of glyphosate, glufosinate, and sethoxydim on potted junglerice plants growing under various soil moisture [100% field capacity (FC), 75% FC, and 50% FC)] and shade (Full Sun, 70% of Full Sun, and 50% of Full Sun) conditions. Label rates of the herbicides were applied at the 4- to 6-leaf stage of junglerice and the pots were immediately put under the abovementioned conditions for an additional 4 weeks. Plant mortality was evaluated at 28 days after treatment and the plants were harvested, oven-dried, and their aboveground biomass was recorded. Plant mortality was affected differentially by light intensity, moisture level, and herbicide type. Among the herbicides compared, glufosinate provided 100% control of the plants under all levels of shade and moisture. Control with sethoxydim was as low as 25% under 70% shade and 50% FC compared to 100% under 50% shade and 100% FC. Control with glyphosate was 100% under 50% shade and 75% FC and under 30% shade and 100% FC but was lower under the other environmental conditions. Therefore, both shade and soil moisture conditions should be taken into consideration during the use of these herbicides for control of junglerice. The experiment is being repeated in 2016.