Hello everyone,
after I quickly received a diagnosis here in the forum (algae), but no explanation for my questions as to why they appear exactly at this spot, and only at this spot, I have since found a very nice article by an expert elsewhere on the internet, whose key statements might also interest you - at least my questions (not only "what", but also "why") are fully answered here. Consequently, I have completely abandoned the theory of the rust stain and the garden water.
1. Algae and molds preferably occur on synthetic resin plasters/ synthetic resin paints. Rarely on purely mineral plasters (like the one I have) - several unfavorable conditions have to come together for this.
2. I do have about 70 cm roof overhang all around, but the bay window with the stain protrudes under the roof and is sprayed with water. In addition, this wall faces north.
3. Algae are pioneer plants that only need light and water to settle. Through photosynthesis they produce organic matter; the required carbon is taken from the CO2 in the surrounding air. In contrast, mold arises where there is already organic matter (e.g. on a surface previously covered with algae), water must also be present, light is dispensable. This is the long-sought explanation why the algae grow precisely on the one spot on the house wall that is NOT obstructed by plants: light and water.
Since unfortunately the missing roof overhang at this spot cannot be remedied, the renovation now looks as follows: mechanical cleaning (already done), then disinfection with an algae remover. Next summer (when the renovation of the windows and the plaster cracks/ flakes is also done) pressure washing of the entire house facade, priming, repainting with a mineral, hydrophobized paint matching the mineral, hydrophobized plaster; cutting back the vegetation (to promote drying of the facade by wind exposure), above all repeating the facade cleaning and repainting every 10-12 years.
Best regards from Havelland,
Marco