Hello Daniel,

This looks like a leaf spot infection, possibly caused by the fungus Cercospora, a group with can infect a wide variety of plant species. Conditions of prolonged foliage wetness, such as from sprinkler irrigation, can promote disease development. A fungicide is not recommended (nor would it be able to cure existing disease), but if any spotted leaves fall prematurely, they can be collected (or blown out of the garden bed) and disposed of so the spores they harbor don't sit at the base of the plants to promote re-infection. Plant disease severity and spread can be highly tied to weather, so won't necessarily be the same from year to year. Vulnerability also depends on the genetics of the plant variety itself.

Nandina is an invasive species in our region, and even though this particular cultivar (looks like 'Firepower') might not produce berries and therefore probably won't spread from seed, we have limited information about their disease vulnerabilities as they are usually tough enough to prevail. Don't apply any more fertilizer, as ailing plants should not be fertilized, lest the nutrient boost spur more tender new growth that the plant can't defend from infection (or insect attack, when they are the cause of damage, which is not the case here). Shrubs like Nandina would rarely need nutrient supplementation, even when unhindered by pests or disease. Mulch can help to reduce the number of viable fungal spores that splash off of the soil surface and onto leaves, though since foliage cannot heal from damage, its use would not have changed the appearance of the shrubs for foliage that was already infected by the time mulch was applied. (Symptom development sometimes lags behind when infection actually took place.)

During drought, regular watering (only as needed, which is when the soil a few inches deep becomes somewhat dry to the touch) does help keep the roots from being stressed. To avoid contributing to the spread of infections, try to wet only the root area (soil/mulch), not the foliage. If wetting leaves can't be avoided when watering, then just try to water early enough in the day so that foliage can dry by nightfall.

Overall, the plants pictured look normal and healthy, so the pictured symptoms are not too concerning and should not cause plant dieback unless a separate issue like drought or over-watering were damaging roots. Mulch gradually breaks down into organic matter (compost, essentially), which will be good for the soil since it reduces compaction over time. If for any reason you need to replant shrubs in this bed in the future, mixing-in a bit of compost upon planting can help to improve soil conditions around the roots for the short term, and over the long term, annual mulch top-dressing (as needed, depending on how fast each year's layer degrades, to maintain about a 2- to 3-inch layer) can also help by continually supplying small amounts of organic matter over time, slowly improving the soil.

Miri