2-17474 p145-150 APMdj

Floating algal mats are a significant problem in California rice fields. Cyanobacteria fix atmospheric nitrogen and are thus considered beneficial in some rice production systems. California rice fields are planted from seeds not transplants, and rice seedlings are adversely impacted by floating algal mats. The algal mats cause two significant problems that lead to yield reduction: floating mats can entrap and uproot seedlings; and large thick mats may accumulate in windward areas and sink, smothering seedlings. These problems are most critical during the 30 days following initial flooding of the fields because seedlings are less impacted after they emerge from the water. California rice fields typically have shallow water depths (<15 cm) and relatively high levels of nitrogen and phosphorus (Spencer et al. 2006). The high nutrient levels are believed to improve rice seedling establishment and survivorship, but they provide ideal conditions for growth of algae as well as rice (Chapman et al. 1972). The most problematic cyanobacterium was identified by Spencer et al. (2006) as Nostoc spongiaeforme , described by Desikachary (1959). Nostoc spongiaeforme has been very difficult to control using currently accepted methods (i.e., aerial applications of copper sulfate at 1 mg L -1 ) during the critical 30-day period following spring flooding of fields. Two herbicides commonly used in rice production are Londax (bensulfuron-methyl (methyl 2[[[[[(4,6-dimethoxy2pyrimidnyl)amino] carbonyl]amino]sulfonyl]methyl]benzoate)) and Shark (carfentrazone-ethyl (ethyl α ,2-dichloro-5[4-(difluoromethyl)4,5-dihydro-3-methyl-5-oxo-1H-1,2,4-triazol-1-yl] 4fluoro-benzenepropanoate)). They may be used in combination to increase the spectrum of rice field weeds controlled with a single application (Hill and Fisher 1999). Discussions with pest control advisors indicated that less severe algal problems were observed in fields with combinations of these herbicides. The purpose of the experiments described here was to evaluate the effect of a Londax-Shark mixture on N. spongiaeforme . MATERIALS AND METHODS