Bigleaf Hydrangeas (the type pictured) are very commonly afflicted with one or more types of leaf spot; most fungal, some bacterial. They can be an eyesore when heavy, but are not serious enough to cause long-term plant damage. Fungicides will not cure existing infection, and while its use might suppress new infections on still-healthy leaves, bear in mind that these chemicals risk harming pollinators or other organisms if they come into contact with spray residues. While cutting back stems might temporarily improve a plant's appearance, it's not the best solution for several reasons: it can force new growth late in the season, when it will have less winter hardiness; depending on the timing of pruning, it can remove many of the flower buds for that year; it can weaken plants over time by forcing them to replace foliage that isn't getting enough time to replenish root energy stores, so if done over a few seasons, may cause plant decline. Individual leaves that are heavily spotted can be clipped off if desired, but otherwise the rest of the leaf tissue is unaffected and still feeding the plant, so is best to leave as-is. (Granted, this also leaves some of the pathogen spores in place, but they can splash or blow around anyway, so their removal won't necessarily make the infection slow or stop.)

The foliage pictured has a minor degree of leaf spot infection so far, but it is also chlorotic. Chlorosis is a type of nutrient deficiency that results from low levels of either iron, manganese, or magnesium. It does not necessarily mean that the soil itself is deficient, however, and only a laboratory soil test can determine that. Instead, it might indicate that the soil is not acidic enough, since hydrangeas are acid-loving plants. Soil acidity (pH) plays a role in how easily roots can absorb the nutrients they need, and a soil rich in nutrients can still cause chlorosis if the plant growing there isn't in the right pH. A lab test will measure pH as well, and provide recommendations for adjusting it to meet the needs of acid-loving plants if it's outside the range they prefer.

If the shrub is growing near a house foundation or concrete walkway, for example, leaching from those sources might be raising the pH beyond the range the plant prefers. In that case, adjusting the pH might be enough to get new growth to look normal (nothing can necessarily fix the color on existing leaves), so fertilizer might not be needed.

If you water plants using a sprinkler or anything that wets the foliage in the process, try to only wet the soil instead. Prolonged wetness on leaves can make them easier for pathogen spores to infect. If you can't avoid wetting the leaves, then try to water in the early half of the day so that foliage can air-dry by nightfall. This will help reduce the spread and severity of leaf spot, though weather of course also plays a role.

If you choose to use a fungicide, follow all of its label directions carefully, and re-application several times over the course of the growing season maybe needed to maintain a degree of protection. Make sure the product label includes "leaf spot" diseases as conditions it treats, and realize that it won't necessarily work perfectly, nor can it reverse any existing symptoms, but it may help to keep younger growth clean. (Note that neem oil in particular, while sometimes labeled with fungicide benefits in addition to its primary role as an insecticide, is nonetheless not very effective on most leaf-spotting diseases.)

Miri

Thanks for all the information, Miri!

You're welcome!