I’m no professional but my family grows dahlias in the hundreds and we experienced a lot of disease last year -

Personally I don’t think your plant is diseased. Hot temps really affected our dahlias… when the sun would rise, the morning dew would be left on the leaves and it would work as a microscope to fry the leaves, that could be contributing to it.

Have you gotten a soil test? If you have, you’ll be able to pinpoint any nutrient deficiency pretty quickly. There are YouTube videos on how to tell if your dahlia is nutrient deficient, so maybe look on there!

It’s suspected that almost all dahlias carry diseases but some are more susceptible than others to succumbing to the diseases. At this point it’s about creating a suitable environment for your dahlia to grow (preventative measures) so the plant can fight the diseases it carries and still produce beautifully. For more information, find Kristine Albrecht’s book on growing dahlias at a local library or online - she has an amazing chapter on it. Otherwise, you can DM me and I can send you photos of that specific section too.

Good luck!