Dahlia tuber rot

Asked May 10, 2018, 10:53 AM EDT

Hello --- I have noticed some rotted tubers while checking my in-ground over-wintered dahlias. The dahlias were cut back to the ground in November, covered with black plastic, and a leaf mulch layer (about 3 inches) placed on top. In March the coverings were removed, and I waited evidence of new growth. Most of the tubers are showing growth now, but I started digging around those that had not appeared. I found mushy, bad smelling tubers with lots of earth worms. Taking a few samples for review, a nursery person suggested it was Iris soft rot, caused by bacteria. I have googled for some more information and would greatly appreciate input from a Master Gardener. Assuming the problem has correctly been identified (and I could provide a sample if needed), what steps should be taken now? Can I replant in the same hole? The mushy tubers have been thrown out, but should I also remove the soil that surrounded the tuber area? I don't know how widespread a bacteria might be. Surrounding tubers (about 24" apart) are intact and sending up lots of growth. Do I need to disinfect my garden tools used for digging the tubers? What preventative and proactive steps can I take to prevent this problem? Can you recommend any products and practices to reduce or eliminate the causes of tuber rot? Can you recommend any further resources to help me learn more? Thanks much! Cathryn

Benton County Oregon dahlias horticulture

1 Response

Thank you for your question, Cathryn. First, here is a link to an article with various problems dahlias suffer. Dahlia tuber rot is caused by a specific fungus – Botrytis cinerea. As you can read from these dahlia care instructions, removing your tubers, treating them with a fungicide, and storing them appropriately will go a long way toward preventing this problem, which flourishes in cool, wet soil. Here is another WSU Extension article on the same topic. Be sure to destroy the tubers that are no longer helpful. (Don't compost!)

The other major thing you need to change is putting plastic over your garden. If it doesn't exist in the forest, it doesn't belong in your yard! When you put down plastic, you not only keep the soil from getting needed winter moisture, but you cut down/out the oxygen in the soil and make it a paradise for fungi that need cool, damp environments. Of course, if your tubers are stored appropriately elsewhere, they won't mind.

Covering the soil with the leaves is fine. It not only protects the soil from compaction, as a mulch, but those same microbes (and many others) consume or break down the fiber and enrich the soil by replenishing nutrients. So, this is probably how the whole problem started. You probably don't need to dig all that soil out, because if it is turned (gently!) and exposed to the air, the pathogen will not be in the environment (cool, dark and wet) that they need. I'd keep turning it for a few weeks before planting anything else. You can clean your garden tools with alcohol, but you should be doing that anyway to prevent the spread of other pathogens. Also true of any pruning tools.

I hope this is helpful for you. Write back with any other questions. Good luck!