Dahlia Problem, new to gardening

[HeidiLikesFlowers]

Spring came late here were i live (which is when I planted my dahlia) and it sure got too much water, even if i planted it in a pot and thought that it was well drained and all.
It started to wilt on the lower leaves at first, but then the problem spreaded on almost every part of the plant.
I thought that giving it a bigger pot would help it heal but things still went worse after just a couple of days.
Some searches on google point to some disease and fungi but Im not really sure how to manage

 

[HeidiLikesFlowers]

 

[HeidiLikesFlowers]

Spring came late here were i live (which is when I planted my dahlia) and it sure got too much water, even if i planted it in a pot and thought that it was well drained and all.
It started to wilt on the lower leaves at first, but then the problem spreaded on almost every part of the plant.
I thought that giving it a bigger pot would help it heal but things still went worse after just a couple of days.
Some searches on google point to some disease and fungi but Im not really sure how to manage

Also, i tried asking for help in r/Gardening on reddit, but I only got 1 answer telling me to cut to soil level the plant.
Since almost 8hrs passed from any other responses i did so

 

[DirtMechanic]

Tuff stopping fungi. Systemics work but they sort of need a diagnosis because they do not work on everything, everywhere, all at once. The cultural practice of dryness helps. Soil looks ok- did you repot it? Roots ok? Pick the dead and dying, you do not want more spores flying about. Topical antifungals can help depending on what it is. Is it getting a lot of heat and sun there? No lighting in the picture. Good growth up top so maybe its just wet like you say.

 

[HeidiLikesFlowers]

Tuff stopping fungi. Systemics work but they sort of need a diagnosis because they do not work on everything, everywhere, all at once. The cultural practice of dryness helps. Soil looks ok- did you repot it? Roots ok? Pick the dead and dying, you do not want more spores flying about. Topical antifungals can help depending on what it is. Is it getting a lot of heat and sun there? No lighting in the picture. Good growth up top so maybe its just wet like you say.

My knowledge on plants and fungi is very limited, whats tuff?
Since ive started growing and buying plants ive had lots of problem involving fungis, my mint have small white spots everywhere, my gladious have what i saw is called "rust", a small rhododendron could have "powdery mildew" and my dahlia has what you saw in the picture.
Im not really sure whats causing this?

As for the dahlia, im not sure how dahlia roots should look like, but when ive repotted (in the pot that is in the picture) the roots seemed ok.
It gets LOTS of sun, i did cut everything at almost soil level and is now regrowing a bit more healthy it seems.
How do I understand what kind of fungi i have on my plants?

 

[DirtMechanic]

Tuff is American Slang for strong willed.

You can use google lense or plant id apps and software to speed up identification of plants and fungi. Bacterial and viral issues that have visible symptoms as well. Just focus on one problem at a time and the solution thereof. Over time patterns of cultivation that work for you in your location will emerge in what you are doing in support of the plants. Lots to read about.