Welcome to NGA, JayGardner84. There are some leaf injuries that may be caused by pests like slugs but you need to catch them in the act so you can then apply appropriate counter measures. Monitor the leaves' tops and bottoms, the stems, the soil and under rocks during the day and at night. If you catch something, you can then take it to your local plant nursery for identification purposes.
The bulk of the leaf injuries however are caused by a severe case of Cercospora Leaf Spot. As the fungal lesions enlarge and grow, the leaf tissue decays and forms a hole. You can use clean sanitation guidelines, Bonide Liquid Copper Fungicide, keep the grass away, mulch and never water the leaves.
The leaf lesions are a fungal infestation called Cercospora Leaf Spot, a fungal disease. It is usually promoted either by a normally very humid local environment, by often over-watering the soil and-or by directly 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 or purple spots first appear on leaves near the base of the plant. The spots are usually about one-eighth to one-fourth inch in diameter and grow larger as the disease ages/spreads up the canopy. It becomes visible either in summer or fall but can occur in spring. Heavily spotted leaves turn yellow-green or yellow and may fall to the ground, helping 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 in old wood hydrangea cultivars. Therefore, in the following year, less blooms or smaller blooms might occur if the problem is not quickly addressed. Fallen diseased leaves and foliage that rubs the soil are the primary source of spores of the fungus. These spores are spread to the healthy lower leaves by splashing water. 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 (do not remove them all as still green leaves are still producing food for the roots in winter); when fertilizing during the year, 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. Clean sanitation practices should help control the problem greatly: remove all forms of plant debris (blooms, foliage, stems, etc.) at all times, especially when 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; use fungicides cleared for treating Cercospora Leaf Spot; 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 when spotting of the leaves is first seen and continue applying per product directions. Next year, start spraying in the spring after leaf out or new stem growth.
Fungicides registered for the control of Cercospora leaf spot include the following (the fungicide's active ingredient is listed first and some commercial product names follow in parenthesis): azoxystrobin (Heritage; has the smallest application rate and the largest repeat interval); chlorothalonil (Bonide, Daconil); copper hydroxide, copper octanoate/copper soap (Kocide, Bonide Liquid Copper, etc.); mancozeb (Dithane; Protect; other names); myclobutanil (Immunox); thiophanate-methyl (Cleary's 3336).
Feel free to get two fungicides with different active ingredients so you can rotate the applications and prevent the fungi from becoming resistant to a single fungicide. The products may have different application frequencies (every 30 days, every 2 weeks, etc. etc. etc.). As an example, you may end up applying product ABC for 30 days first. Then after 30 days, you would apply product DEF for 2 weeks. Then you apply product ABC again for 30 days and so forth.
