There are likely different fungi (or bacteria) causing the symptoms on those three different plants. Black-eyed Susans tend to get Septoria Leaf Spot and a bacterial angular leaf spot, for example, though some cultivars or species are naturally more resistant to infection than others. (Variety 'Goldsturm', the one most gardeners use, happens to be very susceptible in our experience to one or both of the above.) Hydrangea leaf spots also vary but include Cercospora and anthracnose fungi. Hibiscus can contract Cercospora (not necessarily the same species of fungus) as well as Phyllosticta and bacterial spots. By name, "black spot" is a rose-specific fungal disease, though as a category it is lumped-in with other leaf spot diseases.

If you are spraying fungicide once symptoms appear, it may be too late since fungicides are not curative; they can only help protect healthy growth from infection, and sometimes an infection has already begun but is still asymptomatic when a treatment is applied. Other reasons for a failure of good disease suppression includes weather (rain too soon after application, or which interrupts a re-application schedule, can allow spores to enter leaves) or certain fungicide active ingredients not being the best choice for that particular disease. Copper-based fungicides are among the broadest-spectrum options, able to suppress a wide range of plant disease spores (mainly fungal and a few bacterial) while they are still outside of leaf tissue. However, it has a limit as to how many times per year it can be applied (it will say this in the instructions) since you don't want copper building-up in the soil where it becomes toxic to roots. Neem oil can be labeled for disease control but is mainly an insecticide and generally not as effective for fungi or bacteria.

Although an eyesore, some degree of leaf spot infection is not a serious threat to a plant's long-term health and does not need to be treated. As you have been doing, removing the worst-affected leaves is usually sufficient to help suppress re-infection during the growing season, as is a thorough cleaning-up of plant debris after frosts kill foliage. If you irrigate during drought, avoid wetting leaves and instead only water the soil, since prolonged leaf wetness encourages infection; if unavoidable, water early enough in the day so that leaves can dry by nightfall.

Bear in mind that fungicides and their residues may pose a risk to pollinators and other organisms, even though they target pathogens. On plants attractive to pollinators and for flowering native species, like Black-eyed Susans, we discourage the use of fungicides. It also might be cheaper and easier in the long run to replace a plant often heavily infected with a variety that is more resistant, especially given the cost of yearly fungicide applications. Using the same fungicide for several years in a row might also encourage resistant strains of the disease in question, though we don't have data as to how often this happens with these infections and this copper type of fungicide in particular.

Other conditions that predispose plants to leaf infection include anything that reduces air circulation around its leaves, like crowding with other plants and dense branching or a location close to a house wall or solid fence. Hydrangeas, depending on type, can be thinned in early spring or midsummer to remove some dense stem growth to allow for better airflow.

Miri