No treatment is needed, as these are minor leaf spots and even if caused by an infection (not all appear to be), it does not threaten the health of the plants. Leaf yellowing is hard to diagnose since it can result from a range of factors, but over-watered roots (or those not getting good drainage), drought stress, nitrogen depravation, and improper soil acidity (pH) are all possibilities. The mulch appears to be fresh...did it have a strong ammonia-like smell when applied (was it sitting in a large pile for awhile, where it could have "soured")? Or, was any herbicide used nearby to kill weeds? Is there a lawn nearby that was limed in the past year or so, where runoff from its application could have affected the holly planting area? Hollies need an acidic soil to thrive and absorb the nutrients they need from the soil, so if the soil here has risen in pH over time, that may explain the yellowing.

Although fertilizer can sometimes correct a nutrient deficiency, a good first step is to make sure the soil pH is where it needs to be, otherwise adding more nutrients won't make much of a difference. A laboratory soil test is the best way to analyze the pH as well as major nutrient levels to determine if any are too low. If the pH is too high, adding garden sulfur will help lower it to a range the hollies can thrive in. Home soil test kits might be sufficient, but overall they are less accurate and less detailed than a lab test. We can help to interpret test results if desired. In order to know how much sulfur might be needed, we'd need to know what pH level the soil is at currently.

Another possibility is sunlight...are the hollies receiving more direct sun than they used to, such as from the removal of a nearby tree or large evergreen? Even though hollies can grow well in full sun and retain a deep green color, if they were used to some shade and then received more direct sun, they can fade in color for a while until new growth emerges that is better-adapted to the brighter light.

Miri