I have a Japanese grafted wisteria, which is about 18 years old. It has always flowered gloriously. but now I have noticed a considerable amount of rot near the ground at the base. What could this be, and can I treat it with anything to sustain the tree please?
I have attached some pictures for you to see.
Thank you for contacting Toronto Master Gardeners.
You are lucky to have a Wisteria that has flowered so well for you. The rot on the trunk is certainly a concern.
From the picture I can not make a definitive diagnosis. Trunks can be prone to bacterial rots as well as fungal rots. Bacterial rots will have a strong smell and often have a liquid that weeps from the wound. Fungal rots often do not and may have mushrooms popping up near by.
There are many reasons why this may have happened to your plant. Damage from garden tools, lawn mowers, animals etc. can cause an initial wound and an opening for infection. If the ground and surrounding debris are too damp or waterlogged the bark can break down and create an opening for infection. Has there been any change or construction in the vicinity that has changed water drainage near the plant? Wisteria have also been known to reject their grafts up to 20 years of age which can lead to this problem.
Unfortunately there is nothing that can bring dead wood back to life. At this point you need to protect the plant as best you can and hope that there is enough live wood to maintain the plant moving forward. Keeping the area clean of any debris and possibly infected material is essential. Good cleanups help to make sure nothing transfers to other potentially vulnerable plants as well. If you use any tools on the wisteria make sure they are washed well before and after to contain any risk of spreading infection.
If you can take a good look at the area and assess if there is any odour or if the wood is soft you can take that information to a local nursery and discuss with them what you have found. They will want to know: how did the plant grow through the summer, did it have thinner growth than normal and were there shoots sprouting from the trunk close to the ground. Once you know what is causing the problem hopefully they will have something that may help you treat it and/or stop it from spreading.
I am attaching some links below that maybe of some use to you.
Good luck and I hope you can enjoy blooms for many more years.