Does this look like DMV? I have been comparing my dahlia leaves too many pictures online but the pattern of yellowing does not look similar to pictures of DMV. I could use some expert help. I have checked the relevant question on your website already but I’m still seeking answers.
Thank you for contacting the Toronto Master Gardeners concerning your dahlias..
Dahlias are susceptible to a number of fungal and viral diseases. Upon close inspection of your photograph I do believe that your dahlias show signs of a viral infection. Viral infection first appear as yellowing on the leaf in some form. It can appear in a number of different patterns: yellow spots or blotches, vein yellowing, leaf margin yellowing and even necrosis. Your photos clearly show signs of dahlia mosaic virus (DMV): vein yellowing and spots on the leaves. The vector for this virus is usually aphids but can be any another insect that pierces plant tissue and sucks the sap eg. thrips and mites.
Unfortunately there is no cure for DMV. Once your plants are infected your best course of action is to remove and destroy the plants showing these symptoms (not in your compost), then monitor the remaining plants closely for insects, and treat any quickly (blasts of water from your hose will usually remove aphids). DMV affects different dahlia cultivars in different ways (including asymptomatically). It is spread by insects and by gardeners, so it is important to control insects and to sanitize hands and tools when working with infected plants.
For more information about growing :
Fungus Control for Dahlias and the Garden
Dahlias are beautiful plants but can be high maintenance. Best of luck with yours!
July 1, 2022