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.pdfYoung 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.
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