This is happening on 3 out of 4 plants I have. No difference in how they are planted, or soil.
Are you watering overhead? Looks like water damage.
No, not at all. I'm watering with a can directly onto the soil.
Looks like dahlia smut from the photos a few postings down. I gathered some info from different sites. Here is the link for a photo. http://plant-disease.ippc.orst.edu/image.cfm?RecordID=141
Initial symptoms are a pale spotting on the lower leaves of the plant, this enlarges and the symptoms moove
up the leaves to higher leaves on the plant. Eventually holes appear in the centre of the spots leading to
necrosis and leaf die back. This causes brown, round markings on the lower leaves. To avoid problems, do not grow dahlias in the same place each year. The disease is prevalent in moist humid conditions and can be spread by water splash and contact.
Controls; Bordeaux mixture, Copper ammonium sulphate or any copper based fungicide, Fosetyl aluminium, Propicanizole.
Seriously doubt if it's dahlia smut. Doesn't appear the same to me.
I suggest it's a combo of heat and a water shortage.
So, a question. Just how much water do you apply with the can? Also the frequency?
I water them well with the can, and sometimes with a hose.
However, now the whole plants of 3 of my dahlias have turned almost completely brown.
Any ideas? this is very frustrating.
Thanks for the feedback.
But please understand I still don't know what, precisely, you mean by water them well. As in how much -- 1 cup? 2 gallons? or what?
As an example of how much detail a long-distance diagnosis requires, consider this when I tried to troubleshoot a veggie problem over the phone. The description sounded like a water shortage but the gardener said that "It can't be; I water daily." Then, after further questions from me, I learned it was with a drip system. Then, after still more questions, I found out the drip system was on for just 5 minutes a day. (Yikes!)
The diagnosis: insufficient water in spite of daily watering.
Okay, back to your dahlias. Here are some things to consider.
Are these new or established plants?
If new, the original rootball is most likely drying out,this because it is "missed" when you water. The water would have to be applied directly on top of the rootball, possibly drizzled rather than poured from a can or hose.
If established, perhaps you aren't adding enough water.
On the other hand, the roots might be drowning. Have you checked just how moist the soil is? (To tell the truth, I doubt they're drowning because the leaves don't show the correct symptoms.)
How hot and/or dry has it been recently? Also currently is?
Do you have burrowing critters? Moles can tunnel under plants, thereby redirecting water away from roots. And gophers just plain eat roots/plants
I suggest a water shortage from whatever cause because of the dried leaf edges and tips.
You will need to be our on-site detective.
Check the soil moisture content.
Perhaps even sacrifice a plant by digging it up so that you can see what's going on in the rootball.
Let us know what you find.