Research conducted by Albert Fischer and James Eckert, Department of Plant Sciences, UC Davis, has confirmed that some populations of smallflower umbrellasedge from rice fields in the Sacramento Valley have become resistant to propanil. Following is a summary of their findings.

Smallflower umbrellasedge seed from eight fields where resistance was suspected was collected and germinated in the greenhouse. Plants were grown in pots and, at the 2-3 or 4-5 leaf stage they were sprayed with SuperWham, UltraStam or RiceShot at half field rate, recommended field rate, and twice field rate. Both spray timings produced similar results, but the earlier application produced the most striking results.

Table 1 shows the percent control of smallflower umbrellasedge when sprayed with 3 formulations of propanil. For simplicity, I'm showing only the results of the application with 6 lbs a.i./a. Percent control of smallflower umbrellasedge from fields 4, 6 and 8 was very poor, between 6 and 45%. Control of smallflower umbrellasedge from field 3 was mediocre, between 62 and 74%. And control of smallflower umbrellasedge from fields 1, 2, 5 and 7 was very good, between 68 and 100%.

Table 1. Percent control of smallflower umbrellasedge with 3 formulations of propanil applied at 6 lbs a.i./a.

Smallflower umbrellasedge from fields 1, 2, 5 and 7 can be considered susceptible to the formulations of propanil applied. They were tested because control failure had been observed in the field. Results from the greenhouse tests suggest that something went wrong with the field application. For example, the application could have been made too late, coverage may have not been appropriate, there could have been incompatibility in the tank mix, wrong application rates might have been used, etc. Populations from fields 3, 4, 6 and 8 can be considered resistant. This prompts the question, what are the options to control these populations?

To answer this question, another set of greenhouse tests were conducted to evaluate herbicide options for propanil-resistant smallflower umbrellasedge. Results (Table 2) show that propanil-resistant smallflower umbrellasedge was also resistant to Londax, resistant or partially resistant to Granite SC and Sandea, and susceptible to Shark H2O when applied as a foliar.

Table 2. Percent control of smallflower umbrellasedge with alternative herbicides.

Results suggest that in fields with propanil-resistant smallflower umbrellasedge, Shark H2O could be used to achieve control. In fields where resistance is not a problem yet, the best approach is to alternate modes of action whenever possible. If you suspect you have propanil-resistant smallflower umbrellasedge, collect seeds at the end of the season and take them to the Weed Science project at the Rice Experiment Station in Biggs for screening.