luis_pr said:Welcome to NGA, DSwish. The leaf spots on the leaves are an extremely common fungi called Cercospora Leaf Spot. Like black spot is for roses, this is a very common infection in hydrangeas as the fungi is almost all over the world. Something that makes it shift into high gear is overhead watering... for example, getting the leaves wet when you hand water or letting the sprinkler system water the leaves early in the evening. There is no cure for this but this is usually an aesthetic problem that causes some leaf spots mostly although some plants drop badly infected leaves, something that may reduce plant vigor. The leaves always have the leaf spots but, when the plant leaves produce lots of chlorophyll, the green stuff tends to hide the spots unless you look closely.
The first thing you need to do is stop watering the leaves. To minimize the problem, never water the leaves if you can help it. Water the soil if you water by hand or water using drip irrigation. If you use a sprinkler system, you need to set the station that waters the hydrangeas to water as close to sunrise as you can so the leaves will be wet as little time as possible. There are no fungicides that will cure this. The ones on the market only help control infestations and work best if applied early. To various degrees, the leaf spots will reoccur again most years. They become visible after the summer solstice, June 20th. That is when chlorophyll products starts going down, which helps make the leaf spots more visible. 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. Extended periods of drought will usually suppress disease development and spread.
Try some of the clean sanitation practices and use some fungicides. Anything that will make the environment difficult for the fungi can help.
• Do not overwater. Water the soil when a finger inserted into the ground to a depth of 4" feels dry or almost dry. A single watering is enough if the water reaches down to a depth of 8" or so. To check if you watered enough, use a similar finger method test to a depth of 8".
• Pick all plant debris "as soon as you can" and throw it into the trash. That includes things like any dead wood (stems), dead leaves and spent flowers. Must do in the Fall when the plants shut down and go dormant. Remove the leaves by cutting the petiole string that connects the leaf to the stem (do not cut the stem). Remove the flowers by cutting the peduncle string that connects the bloom to the stem (do not cut the stem).
• Improve air flow inside and around the hydrangeas. If a plant has a lot of foliage, cutting a few stems (1 or 2) strategically in the middle can increase sunlight and air flow in there, things that will make life difficult for the fungi.
• Do not let the hydrangea be in contact with or be blocked by other plants. This helps increase air flow and minimizes the chances of spreading leaf spot from one plant to another. While there are some plants that can catch the fungi too, this is not necessarily easy. For example, when I moved into my existing home, the hydrangeas by the front door were infested with Cercospora. I first switched the time on my sprinkler system closer to 6am but, the system then broke so, I replaced the sprinkler in that area with drip irrigation. That was almost 20 years ago. The plants still have some leaf spots but nothing like they originally did. And they have not spread the fungi to other nearby hydrangeas that I planted in their vicinity.
• Replace the mulch with new mulch with bad infestations.
• You can use fungicides to control bad episodes. Using fungicides before the leaf spots become visible seems to help tone things down. Lack of moisture in some years also seems to help. I never used fungicides because, until recently, they were too expensive and, the problem showed up in late Summer, just as the leaves are going to brown out and fall down so why waste money. Now I have seen products available for less than $10. I you decide to use fungicides, get two fungicides with different active ingredients and apply as follows: the first application, use the first fungicide only. The fungicide will have to be reapplied after "x" days. After "x" days, for the second application, use the second fungicide instead. That fungicide may have to be reapplied after "Z" days. After "z" days, the third application should use the first fungicide again for "x" days. And so forth.
• On an infected plant, you can also cut the leaves that look the worst. But just remember that the leaves normally produce food for the roots so do not cut all the leaves. Cut the petiole string that connects the leaf to the stem.
• For effective control of Cercospora leaf spot with a fungicide, begin applications when spotting of the leaves is first seen and continue applying that treatment as needed. Next is a list of active ingredients (fungicide names in parenthesis) registered for the control of Cercospora Leaf Spot: azoxystrobin (Heritage; smallest application rate and largest repeat interval); chlorothalonil (Daconil); mancozeb (Dithane; Protect; othrers); myclobutanil (Immunox); thiophanate-methyl (Cleary's 3336). Use only fungicides whose active ingredient has been approved for Cercospora as the others may not work and may affect the plant's vigor and-or foliage.
• You can also replace the plants that are already infected with new ones. Like I pointed out earlier, I did not replace my infected plants but, I added other hydrangeas and they have not caught leaf spot (so far… and it has been 15+ years).
Applying all these chemicals (insecticides, fungicides, etc.) has caused some leaf damage. It is probably causing some –not all- of the yellowing and the crinkly leaves. I assume you were using insecticides because you thought there was a pest problem? If so, this use of chemicals not "designed" to affect the problem can affect the plant vigor and foliage. Leaves that are yellow may stay yellow for a while or they may eventually fall. Normally you get new foliage in 2-4 weeks in Spring but we are now warm enough where high temperatures might make the plant not wat to produce leaves unless absolutely necessary.
Aluminum sulfate is used to prevent chlorosis in alkaline soils as well as to promote blue flowers. I am not sure if that was its intended purpose. If it was, apply it per the product label directions and do not exceed the amounts unless you do it very carefully. Hydrangea roots are very tiny, fibrous roots that only grow in the top 4" of the top soil so, if you add a product with lots of sulfur, it could harm the roots.