I currently have a beautiful garden full of dahlias, but unfortunately over the last couple days, the tips have turned black like they were burned and the leaves have developed some marbling of some thing I do have pictures I can show you I’m really worried I don’t know what to do do I cut them? Do I pull them from the ground or what? Please if you can help me, I would really be grateful. They are all different sizes from 12” to 4’ tall I planted in late April. It would only let me upload 1 picture
Thank you for contacting the Toronto Master Gardeners about your dahlias. We have consulted with dahlia experts and long-time growers about what could be happening with your plants. When shown the picture you provided, no one has seen this before. Everyone agrees that this is not a good year for dahlias because there has been so much rain. Dahlias do not like wet feet which can lead to rot, and humid conditions are favourable for the growth of fungi. It’s likely that your plants have been quite stressed by the weather and fluctuations during the last few months, which has increased their susceptibility to disease. Below is what the experts said about your plants.
The mottled discolouration on the leaves could be the result of Dahlia Mosaic Virus. The tubers that you planted could have been infected when you bought them, and the infection is spread to healthy plants by aphids (you should check your plants for these insects). Infected plants should be destroyed (not put in your compost) and aphids should be controlled on your remaining plants (quick blasts from your garden hose is usually all that is needed). There is no cure for this virus. There is more information about Dahlia Mosaic Virus in the article at at this link :
DMV symptoms, disease cycle and management
Another possible cause is Verticillium Wilt disease. Dahlias are among the many plants that are susceptible to this disease. It is soilborne and remains in the soil for many years, so only plants that are not susceptible should be planted in soil where affected plants were previously grown. This disease gets into the tissue that transports water throughout the plant, causing leaves to wilt, dry up and fall (or they could remain attached to the plant). It progresses from the bottom up, so lower leaves are affected first. Infected plants and roots should be removed and destroyed. There is no cure for this disease.
Overwatering (lots of rain) and not enough heat or a rot of some sort are other possible causes. If rot is the cause, it could be cut away (affected areas on the plant would be mushy). You could check your plant’s tubers as well as the parts of your plants above ground for rot.
None of the dahlia experts and growers knows what has caused the burnt-looking tips on your plants.
If the symptoms of mottled leaves and burnt-looking tips are spreading among your plants, I suggest that you pull up and destroy the affected plants and see if this slows down or stops the spread. If all of your plants are infected you will need to consider that it is likely that your soil is infected with Verticillium Wilt and plan for changes to what you are growing and where in future years. There is a list of of plants that are susceptible to this disease included in the article at the link provided above.
If you purchased your tubers or plants recently prior to planting them in late April, you could bring pictures and plant samples to the garden centre where you made this purchase and ask them about what might be happening with your plants.
For definitive diagnosis, you could try providing plant tissue samples to the University of Guelph Agriculture & Food Laboratory. I don’t know whether they provide this service to homeowners or just to commercial growers, but you could contact them to find out.
Below is some further reading that might be helpful :
Dahlias are beautiful plants but can be high maintenance. All the best with your current challenges!