From what I hear, this is a public garden planted with donated plants in unamended soil...don't get involved in soil testing and micro examinations of the soil as you might if you were growing a display rose garden. You sure can broadcast epsom saolt on the top of your soil and water it in...or just wait for rain. Also remember too much rain on heavy clay means pooling water....and yellowing leaves. Start now abd beg all the free compost youc an find....grass clippings, leaves, shredded wood, coffee grounds, saw dust ( but not from treated deck lumber) and pile it on between the plants. You will be surprised at the improvement in a year....and a second year will bring even more improvement. Good luck! This is a good thing you are doing...but don't drive your self crazy over it. Compost and organic additions will make all soil difficulties better....even shredded paper is good. Linda C

Need to see image of plant overall. Seeing just one leaf can lead one astray in determining the problem.

Using chlorosis patterns in leaves to determine a nutrition deficiency is a bit of a crap shoot because there are many overlapping/vague chlorosis patterns. You really should take a soil sample & submit to a reputable lab for analysis. Applying too much of one nutrient can mess up the availability of other nutrients to your plant. So again my advice is, do a good quality soil test before adding a bunch of nutrients, especially the secondary & micro nutrients.

Just went out to check all my leaves. The disease-resistant roses with glossy foliage all have 7-leafets: Kordes Flower Carpet, Pat Austin and Tchaikovsky. Kordes Flower Carpet doesn't have mildew in shade, but Knock-out (5-leaflet) has mildew. Other DISEASE-RESISTANT with 7-leaflets: William Shakespeare 2000, Duchess de Rohan, Excellenz von Schubert, Annie L. McDowell, Blue Mist, Poseidon, Cloudert Soupert, and Crown Princess Mag. ... all have leaves in set of 7. These can take wet soil well, like multiflora rose thriving in wet lands. But the blackspot-prone roses: Comte de Chambord and hybrid teas have leaves in set of 5, and much larger & round leaves. These prefer well-drained soil, and tend to blackspot with prolonged wetness & acidic rain. Multiflora-leaves are clearly a set of 7, see below pic: http://na.fs.fed.us/spfo/invasiveplants/factsheets/pdf/multiflora-rose.pdf "Each multiflora leaf as 5 to 11 one inch-long oval leaflets with toothed margins. The undersides of the leaflets have tiny hairs and are paler than the upper surface. The base of each leaf stalk has a characteristic stipule (green, leafy structure) with hairs or a comb-like fringe along its margins. Flowers. As indicated by its scientific name Rosa multiflora, this plant has abundant, showy clusters of flowers which typically are white, though sometimes slightly pink." Below is Austin rose William Morris, which did terrible in slightly acidic wet & peaty potting soil, then finally died when I put in my wet clay made acidic with cracked corn. Note the leaves are in a set of 5, which means it prefer well-drained & loamy soil, and CANNOT take acid & wetness like those of set-7 leaves. Folks complain about WM being stingy and rust-fungal-prone. The drought-tolerant & disease-resistant Rugosa has rounder leaves in set of 9, plus very bristly canes full of thistles, see below: