Dear Master Gardeners, I started some dahlia bulbs in February. They are about a foot tall now but still in their posts. I have had them outside on the deck, hardening off for a few weeks. There have been nights below 10 degrees. Their tips have become burned-looking. I am concerned that they are diseased, or that the cold has given them frost bite. If it is a disease, I do not want to put them out in the garden. Could you please help me? Many thanks for this help.
Thank you for contacting the Toronto Master Gardeners concerning your dahlias. At first glance it would appear that your plant is showing signs of frost damage rather than a disease. When I look at your photo, it appears as if only the young tender leaves seem to have been affected.
Do keep an eye on your plant. If stems begin to die back and leaves curl, wilt and drop, these are signs of a fungal disease. In this case, you will need to get rid of the plant and be sure that you do not re-use the soil for dahlias, or other plants, that are susceptible – Verticillium wilt is a soil-borne fungal disease that remains in the soil and can re-infect other plants.
Penn State has an excellent article on Dahlia diseases. The chart describes the symptoms of various diseases along with how to manage the disease.
Good Luck with your dahlias