Dahlia’s weather stressed or diseased?



I am growing Dahlia’s for the first time in midtown Toronto, they were doing quite well until heat spell and a recent deluge of rain. Now one plant has some yellowing along the leaf veins, looks mottled (possible DMV? plant type is bride-to-be). On a few others I see some small yellow spots (cafe au lait). Other Dahlia’s planted in the area look fine. I’ve looked for aphids but haven’t found any. I have seen some examples of either slugs or earwigs chewing on leaves but I’m not using pesticides as I have a young child who plays in the backyard. I am wondering if I should rip out the plants that have the yellow on the leaves as it is likely a disease or if this was more likely related to the recent weather stress. Photos attached.

Thank you so so much!


Thank you for contacting the Toronto Master Gardeners about your dahlias. Many plants have been stressed by the extreme swings in the weather conditions this spring, so it’s quite possible that your dahlias have also. However I think the symptoms you are seeing on your plants are not caused by the weather, but that the weather stressed your plants and caused them to be susceptible to whatever has attacked them. And I agree that this looks like symptoms of Dahlia Mosaic Virus, in particular the chlorophyll loss that has resulted in the mosaic pattern 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.

I think 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.

Here are some links with information about growing dahlias that might be useful :

Dahlia Mosaic Virus symptoms

Growing dahlias

Dahlia diseases

Dahlias are beautiful plants but can be high maintenance. Best of luck with yours!