Brown spots on oleander
We are in the U.K. and the plants are brought into the porch in winter. They are base watered never over head unless it rains.
Certified GKH Gardening Expert
Unfortunately, they have received too much water. This is the start of a fungal infection.
I can't see the container in the photo, so I don't know whether there are drainage holes in the container or not. If there are then the treatment will be much faster. If not, there will be a lot of waiting involved.
Treatment will be as follows: The first thing to try will be a simple water/peroxide solution. Mix 1/4 peroxide to 3/4 water. Water this in AFTER the soil dries almost completely to the bottom.
Wait until the soil is almost dry again, completely and do this one more time. After a week or two, you should see new growth that is free from spots. The old spots will not go away, but they will not spread after this.
Should this treatment not work, you will move on to a little more harsh of a treatment. DOLOMITIC lime and WETTABLE sulfur will be your next line of treatment. You are going to have a very hard time finding this in your country, unlike here in the US. Your best bet will be online.
Rest-assured, dolomitic lime is much safer than the hydrated lime that you might find in your area. Hydrated lime, or Calcium hydroxide is corrosive and quite toxic, where Dolomitic lime is mined and a natural source of Calcium carbonate, AND Magnesium carbonate.
If that treatment doesn't take care of it (which is very unlikely) then your option from there will be a copper fungicide... I like to avoid these.