Here is the info. As usual it is a mixed answer on the subject but I think we have the answer needed.
http://www.dnr.sc.gov/water/aquaff/pithalgae.html S.C. DNR
Ohio State University Fact Sheet
School of Natural Resources
http://ohioline.osu.edu/a-fact/0003.html This is a great link on FA
Copper-Resistant Algae in hard or high alkaline water:
One form of filamentous coarse hair algae, Pithophora, can be especially troublesome because it is resistant to normal applications of copper compounds. Although it is not widespread, scattered reports of Pithophora in Ohio ponds are received every year. If, after a normal treatment with copper sulfate, there is algae remaining that does not appear to be affected, it may be Pithophora. Positive identification can be made by sending a sample to the Plant and Pest Diagnostic Clinic at Ohio State University. Samples can be submitted directly to the clinic or through the county offices of Ohio State University Extension.
Pithophora is extremely difficult to control. Its unique cell wall structure and the tight clumping of filaments inhibit the penetration by copper. Additionally, large numbers of resilient spore-like bodies, called akinetes, germinate and provide a continuous source of new plants. Partial, short term control can usually be achieved with either of the following herbicide mixtures:
Ratio Application
Rate of Mixture
Mix 1. Cutrine Plus Liquid
and Diquat/Reward 1:1 2 gallons per acre-foot
Mix 2. Cutrine Plus Liquid
and Hydrothol 191 Liquid 2:1 1 gallon per acre-foot
Additionally, Cide-Kick, a nonionic spray adjuvant, should be added to the mixture at the rate of 1-2 gallons per surface-acre. This material acts as a cell wall penetrant to increase the effectiveness of the herbicides.
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