As I am writing this post, I learned some important research work on microbes that resides within the rizosphere of the roots.
It seemed at this stage of growth, the soil has to be fungi dominant, rather than bacteria dominant. When the soil is fungi dominant, its metabolic activities decreases the soil pH. This should helps the roots to absorb iron.
Upon learning this, I added brown mulch to increase the population of this beneficial fungi in the soil.
The mulch (brown) that is made of dry matter has to be added on top of the soil and not mix into the top soil. If it is mixed into the soil, it allows more bacteria to compete with the fungi to decompose the mulch. Fungi tends to thrive better in drier and brownier matter. Whereas bacteria works better on greener, fresher matter and more moist condition. This is not to say bacteria is not good, we need a balance of both, but for this treatment is meant to support fungi dominant.
And I also learned that I should avoid putting fertilizer that has N-P-K values higher than 10, as it may disturb the microbes in the soil. This isn't an issue to me, as I was already using organic fertilizer that is less than 5 N-P-K value.
Another thing that I did to increase the good microbes in the soil, was to collect the residue from my most healthiest rose of similar maturity (from water collector tray), and pour it into this problematic rose. Hopefully this transfer the good microbes and brings along the benefits.
As of now this is the current condition. I need to wait for another 4 weeks to confirm the impact. At least I have the assurance that I am on the right track. Just recently my Nozomi rose gave me this bloom. Despite its miserable condition, it still able to produce a flower. You can see the contrast of the nicely developed flower against the rusty spotted leaves at the background. There is still hope!
Nozomi rose is quite a hardy plant. It is a good ground cover. It grows horizontally. Hopefully in 4 weeks I get to see a more fuller and healthier ground cover.