On Jun 11, 7:29 pm, Frank Booth Snr wrote:

Thanks for your opinion but above link confirms that this is Pelargonium
rust or at least the start of visible signs of the it. I would have
expected orange pustules as in other rusts, but maybe they develop later on.

It would appear from that link that is is one of the rusts, although
I've never known those symptoms to develop into the devastating
disease that cause so many problems in the 70's & 80's. At the time I
used to grow several thousand seed and cuttings raised regal, zonal
and ivy-leaved Pels every year. Occasionally such markings would
arise on the oldest leaves of a few plants, but it was easily
eradicated by picking them off and rarely recurred. It always happened
when we had warm days followed by rather cool nights. We did have a
major rust problem during one year, but the symptoms were as I
described earlier. The disease spread exceptionally rapidly through
one greenhouse and all plants had to be destroyed.