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Phosphoric Acid Fungicides can react with Copper Oxy Fungicides

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pagnr:
I recently read about this. Interesting because both are Fungicides with low off target problems, i.e. some of the "safer" fungicides to use.
Also various copper mixtures are residual, in the sense they stay on branches and leaves for a while.
I have encouraged this by adding milk powder to C Oxo and painting on trunks, or using a sticky version like Flo Bordeaux.
Also wondering how much these on plant residues could be affected by a later Phos Acid application, and for what period ?

Another warning was using Phos Acid fungicides on highly stressed trees.
Info came from here
https://www.daf.qld.gov.au/__data/assets/pdf_file/0020/72263/factsheet-citrus-and-wet-weather.pdf

Young and/or stressed trees are very sensitive to phosphorous acid-induced phytotoxicity (e.g. leaf spotting and defoliation). Experience has found rates exceeding 1.9g/L active ingredient to induce phytotoxicity in container grown citrus and stressed field trees. 1.3g/L active has been safely used for container grown citrus. This corresponds to a rate of 2.2mL/L for formulations of 600g/L phosphorous acid.
The best approach for stressed trees is to apply the lowest of the label rates, judging the need to reapply by inspecting root health after 2 to 4 weeks. Alternatively, the effectiveness of applications can be determined by analysis of phosphite levels in the roots (greater than 30ppm are required) – analytical companies such as SGS provide this service.

Phosphorous acid and copper fungicides
Phosphorous acid application in the presence of copper fungicide residues increases Cu ion release, increasing the risk of copper phytotoxicity – particularly if the pH of the phosphorous acid is not adjusted to >7.2. Risk depends on copper type (in order of decreasing risk): hydroxide > oxychloride> oxide.
Check thoroughly with the phosphorous acid manufacturer before attempting to tank mix with other chemical. As a general rule, phosphorous acid should not be tank mixed with any other fungicides.

HalcyonJon:
I rely on Copper fungicides instead of phosphoric for those very reasons.  I also use a 50:50 mixture of Captan powder+indoor latex paint on diseased trees.  There is a lot of leucostoma/cytospora in the soil here, and this is the most effective remedy I've found besides aggressive pruning.  Really helps prevent borers from entering, too.  Happy to send a copy of the study that suggested it, if you like!

Piss P:

--- Quote from: HalcyonJon on September 27, 2022, 01:39:39 PM ---I rely on Copper fungicides instead of phosphoric for those very reasons.  I also use a 50:50 mixture of Captan powder+indoor latex paint on diseased trees.  There is a lot of leucostoma/cytospora in the soil here, and this is the most effective remedy I've found besides aggressive pruning.  Really helps prevent borers from entering, too.  Happy to send a copy of the study that suggested it, if you like!

--- End quote ---

Not sure bout pagnr but I'd like to see the study. If you have a link to the paper it'd be great to post that here for posterity

pagnr:
Happy to send a copy of the study that suggested it, if you like!
Not sure bout pagnr but I'd like to see the study. If you have a link to the paper it'd be great to post that here for posterity.

Yes the link would be great.

Also mixing the Captan with the Latex paint, how long does the application last / how long it is effective for ?
Did it send any plants over the brink ? I have noticed that with other fungicides, they can sometimes give up and die after a fungicide  application.

HalcyonJon:
The study is a PDF file, so no link to a public site.  I think I'm only allowed to post images here...will try to send it direct.
As far as effectiveness, I have not noticed any negative effects or stress to the tress I've applied it to.  Once it's dry, the barrier basically lasts until the bark grows enough to create gaps in the paint mixture.  I reapply every 1-2 yrs, usually about a week after a copper application to keep fungal activity at bay.  Any cankers on branches I don't want to prune away get painted, as well as a thick ring around the base and any graft sites.

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