For the future, we don't recommend applying iron unless an iron deficiency is revealed by a laboratory soil test. Not only would adding unnecessary iron potentially cause its own damage, but it may not be absorbed well (by roots at least; foliar absorption from a spray is limited) if the soil acidity level (pH) is not low enough for a holly to thrive as an acid-loving plant. It appears as though this specimen is relatively close to a home foundation or walkway...are the other specimens that look better also close, or further away. Could leaching from nearby concrete or limestone chips be raising the soil pH over time to a point the holly will be unable to tolerate? We can help to interpret soil test results if desired. If the soil pH needs lowing, you'd need to apply garden sulfur, though knowing what rate to use will depend on what exact pH the soil currently has.

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...is there a roof downspout outlet nearby?), drought stress, and nitrogen depravation are all possibilities. Was new mulch applied recently? (If so, did it have a strong ammonia-like smell when spread; was it sitting in a large pile for awhile, where it could have "soured"?) Or, was any herbicide used nearby to kill weeds? Some ingredients can be more readily absorbed by tree/shrub roots than others.

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

Given how widespread the yellowing is on this one plant, as opposed to being more patchy or limited to only new or old growth, this is probably an environmental issue rather than a problem caused by a pest or disease. Do you see any holes in the bark on its trunk? Yellow-bellied woodpeckers like to use holly and other species to peck its sap wells for feeding, and extensive bark damage could cause some stress or dieback, though normally the birds don't kill plants outright. Do you see any insects glued to the leaf undersides or twig bark? Scale insects, usually white or brown in color, can become abundant on stressed plants and may contribute to leaf yellowing and premature shedding, but in those cases the scale will be quite prominent when the plant is inspected. Did anything injure its roots? Root loss from root rot or digging can cause premature leaf shed.

Frustratingly, it's not uncommon for evergreen plants to manifest symptoms of stress or dieback weeks or even months after the damage occurred, making diagnosis difficult. A plant that got too dry too often sometime in winter, for instance, could drop lots of leaves and die in spring when it actually reached a point of no return earlier the prior season. While hollies can regrow well from heavy leaf loss (such as what happens with severe pruning), it remains to be seen if a plant that denudes for reasons of stress can recuperate. If new growth is not evident by later this spring (say, mid-May or so), this particular plant might be too weakened to keep. Even if root rot was responsible here, you would not need to replace the soil since such pathogens are ubiquitous in the environment and only attack plants successfully once they become stressed and the soil is too wet for too long, depriving roots of needed oxygen.

If you find additional symptoms you're unsure of or suspect you've found an insect pest, feel free to send additional photos for feedback.

Miri