I've had something that looked like an infestation too. I sprayed with an organic insecticide, and it was strongly suggested (by a couple of people that have been showing dahlias for years) that I follow up the insecticide with fish fertilizer. With the leaves looking as bad as they were (and at least as bad as your picture,) I can't imagine that the two (insecticide and fertilizer) will make that much difference, but we'll see. They indicated that the fertilizer would strengthen the plant to enable it to fight off whatever was bugging the plant. It's a fairly easy fix, it wouldn't hurt to try. I did, and of course now my hands smell like aged fish . . . Good luck and return to the posting to let us know what you tried and what worked! It's a bad year for a lot of us!

Thank you for the response. I have sprayed my dahlias with soap mixed with water, fertilized & diligently taking bugs out, right now we have japanese beetles & earwigs around. I have taken out most of the brown leaves & everything is looking better. Hopefully this works... good luck with yours!

~ Karen from Ontario, 6A

Hi Karen03, is hard to make a diagnosis with out knowing the exact growing conditions ie how much direct sun/day, how often do you water plants, is your plant growing in potting mix or garden soil do you have pets which use that part of garden? etc.,
Your dahlia plants look to me as though they have root damage hence the foliage is dying off as a response to lack of nutrient & water getting to the leaf ?
A similar effect can be the result of magnesium or other element deficiency causing breakdown of green chlorophyll organs with in leaf. Over use of lime etc. can cause this.
I would take plant out of soil (if in pot) & examine roots to see that they are healthy first though if in garden soil, would check drainage in garden first, if ok then cut the top 1/3 of plant foliage off to reduce stress on roots, hopefully new growth will make up for lost foliage soon after. Good luck allan

Hello Karen! Perhaps you could do some reading about some bugs on dahlias & other common diseases of dahlia flowers.

Here is a link that might be useful: Common Diseases of Dahlia Flowers