Assessing Alternative
Methods for Managing
Algae - 2009



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Project Leader and Principal  Investigators

David Spencer, ecologist, USDA/ARS, Exotic and Invasive Weeds Research Unit, Dept. of Plant Sciences, UC Davis




Research continued in 2009 on methods to control colonial and filamentous algae in California rice fields. Laboratory and field experiments examined the effectiveness of a commercially available algaecide and phosphorous fertilizer applications.

Algaecide studies

Hydrothol® 191 is a commercially available herbicide for control of submersed weeds and algae. Twelve field experiments were designed with different concentrations of this product to control Nostoc, the “black algae” plaguing many grower fields. Although the highest concentration listed for use on the Hydrothol® 191 label is 5 parts per million (ppm), it showed no effect on Nostoc until concentrations exceeded 6 ppm. The effect was noticeable for two to three days. At lower concentrations, however, Nostoc recovered after a week.

In six additional experiments, Hydrothol® 191 was tested against green algae. “Water net” (Hydrodictyon genus) was collected from a rice field where it had been previously observed. Water net showed a similar decrease upon exposure to the product, especially at concentrations above 5 ppm. Water net also rebounded by the end of the

In laboratory experiments, Hydrothol® 191 was quite toxic to Nostoc. However, when rice straw was added to a growth medium, the toxicity of the product declined. The straw may be affecting the product’s efficacy either chemically or physically.  It is also possible that the straw introduced bacteria that can break down Hydrothol® 191.

The results of these outdoor and laboratory experiments indicate that Hydrothol® 191 did not consistently kill Nostoc – even at concentrations greater than the maximum labeled use. Its effect on the green algae “water net” was more pronounced and lasting. It also appears that water quality, including the abundance of bacteria, may affect this product’s effectiveness in field conditions. This algaecide is not currently labeled for use in California rice fields and it is not clear how it may fit into control strategies.

Phosphorous management

Previous research indicated that phosphorous management is an important factor in controlling Nostoc. Field studies in 2009 compared two phosphorous fertilizer application methods – a conventional liquid phosphate fertilizer followed by a roller or a delayed phosphate application of up to 30 days after flooding.

Results showed clearly that delaying application reduced phosphate water concentrations and algal abundance in most fields. Delaying phosphorous fertilizer applications until rice seedlings have emerged from water may offer an alternative management strategy for some growers.


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