Pictures may have helped as it is particularly difficult to accurately diagnose symptoms of brown splotches and not bright green leaves without knowing the type of hydrangeas. The following are thus general comments with a slight tilt towards Big Leaf Hydrangeas (h. macrophyllas).
There are several diseases that can produce browning in the leaves but the most common is Cercospora Leaf Spot. After it infects the foliage, the disease may cause splotches and leaf discoloration. The leaves turn yellow, drop and help spread the fungal spores. But some discoloration (typically without no brown splotches) can also be caused by too much sun, overwatering, normal autumnal changes in preparation for dormancy (leaves turn yellow, red/burgundy or orange) and a high soil pH reading. Spindly growth may be common in some species and with new stem growth. Or it could indicate that the shrub needs more sun. “Neglected” can mean several things. In Baton Rouge, you can remove leafless stems that still remain leafless by the end of May (or later) all the way down by then. Depending on the type of hydrangea, you could do some pruning. The ones that bloom on new wood, can be safely pruned or deadheaded now without impacting blooming in future years. The ones that bloom on old wood already have dormant spring 2024 flower buds near the ends of the stems so any pruning done could reduce spring 2024 blooming; this you have choices: prune now at a cost of less or no blooms in 2024 or prune after they stop opening new blooms in spring 2024.
The leaf lesions could be an infestation of Cercospora Leaf Spot, a fungal disease. It Is usually promoted either by a typically very humid local environment; by over-watering the soil; and-or by directly watering (overhead watering) the leaves. Fungal spores for this disease exist in most locations world-wide so this problem is very common; it tends to become visible only after the Summer Solstice –though not exclusively- when chlorophyll production tappers off. Scattered, small somewhat circular brown, red or purple spots first appear on leaves near the base of the plant. The spots are at first small, usually about one-eighth to one-fourth inch in diameter and grow larger as the disease ages/spreads up the canopy. Sometimes, heavily spotted leaves turn yellow-green or yellow and fall to the ground, helping to spread the spores. Larger/older leaf lesions may decay the leaf tissue and form leaf holes.
While Cercospora leaf spot rarely, if ever, kills the target plant, heavy spotting of the leaves and premature leaf shed is unsightly and may reduce plant vigor and flower bud set for next year. So, next year, less blooms or smaller blooms might occur if the problem is not quickly addressed and the infestation is extensive. Fallen diseased leaves and foliage that rubs the soil are the primary source of spores of the fungus. The spores are spread to the healthy lower leaves by splashing water or by shrubs that are planted very close. Once the fungus is introduced into a planting of hydrangea, yearly outbreaks of this disease are likely to occur. Fungicides do not cure the infestation but help control it, especially if they are applied starting in the spring, before symptoms are visible (spray as soon as there is leaf out or new stem growth). Frequent late summer rain showers will not only greatly increase the rate of disease spread, but also intensify the level of leaf spotting and defoliation. On the other hand, extended periods of heat/drought will usually suppress disease development and spread.
To help slow disease development and spread, remove the worst of the diseased leaves (green leaves are still producing food for the roots); apply just enough nitrogen to maintain a moderate growth rate; and water the soil, never the leaves. Since the appearance of symptoms is usually delayed until late summer to early fall (when foliage is ready to fall as the plants enter dormancy), protective fungicide sprays are rarely needed for the control of this disease on hydrangea in the landscape but you have a heavy infestation so I recommend starting fungicide applications soon to gain control of the problem. Clean sanitation practices should help control the problem greatly: remove all forms of plant debris (blooms, foliage, stems, etc.) at all times, especially once the plant goes dormant; do not water over-head (use soaker hoses or drip irrigation) but water the soil instead; if using a sprinkler system, activate the station that waters the hydrangea around 6-8am so the foliage will not spend a lot of time wet; improve air flow inside and around the plant; replace the mulch with new mulch; increase the amount of direct sun that the plant receives if it is practical/possible; etc.
Fungicides should only be a part of an overall disease management program that includes the previously mentioned clean sanitation practices. For effective control of Cercospora leaf spot with a fungicide, begin applications now when detecting the problem and continue applying per product directions until you gain sufficient control. After removing foliage (cut the string that attaches a leaf to the stem) and deadheading the blooms (cut above the first pair of leaves) once dormant, make one last spray application until spring and re-start spraying in the spring after leaf out or new stem growth. Since green foliage is still producing food for the roots in winter, leave some green leaves and only cut the worst ones. The worst ones should yellow out but sometimes they just remain green.
Fungicides registered for the control of Cercospora leaf spot include azoxystrobin, chlorothalonil, mancozeb, myclobutanil, thiophanate-methyl. You can also get two fungicides with different active ingredients and rotate them continually so the fungi does not become tolerant of the fungicide.