Hello Vasoula!

Thank you for you question regarding wilting/browning stems and flowers on your Hydrangea plants.

Typically, Hydrangea wilt problems are related to weather stressors like large temperature fluctuations (causing scorching or frost damage), or either an over-abundance or under-abundance of water.

Right out of the gate I think we can safely rule out two of those three conditions. The temperature, though hot at the moment, isn't all that abnormal for this time of year and it's not like day time temperatures were 50 degrees right before the wilting and dieback started. The abundant presence of the white threads of fungus in the soil (Mycorrhizae) are a good indicator that the soil is rich with organic material and moisture, which is "usually" a good thing; my point here being that it's presence indicates that your soil isn't too dry.

However, I say it's "usually" a good sign. That white fungus by itself is beneficial, but also means that conditions further down may be similarly moist, even to the point of being too wet. That causes problems, mainly that conditions could be prime for root rot to develop. The white lines of fungus themselves could also be a problem if they are forming a dense fan-shaped mat, as that indicates that it's not Mycorrhizae. 

The white ball-like fungal structures could be benign if related to Mycorrhizae (it can develop them), or could also be an indicator of the presence of a type of bracket fungus, which lines up with the symptoms we're seeing here. It's hard to tell from the picture, the kind we don't want to see won't be a ball shape on the bottom, but will instead appear more shelf/ear like.

In this case specifically I think it's a type of armillaria root rot.

Root Rot Info here: https://www.tnstate.edu/extension/documents/Root%20diseases%20of%20Hydrangea%20.pdf

and here: https://homegarden.cahnr.uconn.edu/factsheets/hydrangea-diseases-and-pests/

It is difficult to say for certain what the EXACT cause of the problem is, but here's a few things that will be beneficial at this time:

1. Dig up the first plant and inspect the entire root ball, use a mister or a moist paper towel to keep the roots from drying out while doing so. We are looking for roots with white or black lesions on them, mushy wood anywhere on the root ball, or any other signs of damage or disease.

2. Cut away all signs of diseased or infected roots with a sterilized knife and pruners, and remove the infected stems. Bag these up along with anything you find in the roots and take it to your nearest extension office to see if they can identify the exact cause. Your local extension office information can be found here: https://cuyahoga.osu.edu/. The larger the chunk of your plant that you can take to them intact, the more accurate their diagnosis can be.

3. Replant the now pruned Hydrangea in a different spot, ensuring that it has good drainage. The first few weeks after doing this will be very stressful for your plant, especially in this weather, so check daily to see if it needs watering and only water if needed; the fungus that I suspect is at play here thrives in overly wet conditions.

4. Remove the mulch and the soil from the area where the infected plant was initially, bag it up, and discard it. Do not compost it unless your local extension office says it's okay to do so after they get a better look at what we are dealing with.

5. Dig up, inspect, prune, and relocate any other impacted plants on your property. Take special note of any other wood shrubs in the same planting areas as the impacted hydrangeas, root rot, especially the root rots caused by certain types of fungus, don't particularly care about what type of wood they are attacking, this issue may impact more than just hydrangeas.

Hopefully this information is helpful and I hope your hydrangeas a speedy recovery.