A few years ago I noticed a few Hostas were not doing so well, but figured the weather was getting to them. After a few days I went out and they were nearly all dead, leaves and stems just laying there. I picked them up and at the base were what looked like little insect eggs that I assumed were some sort of spores. After a little research, I discovered they were Sclerotium rolfsii.
With surgical precautions and precision, I cut the Hostas back, dug them and the surrounding soil out, disposed of the foliage and soil, soaked the rootstock and tools in a 10% bleach solution, poured the bleach water in the hole, rinsed, potted and placed the rootstocks in quarantine until I was sure they were safe to replant. A lot of work, but it proved successful.
This year it came back and I found it in multiple gardens, perhaps imported in topsoil. To begin with, I successfully treated it similarly to before. I began finding it in more plants though and was getting discouraged. Digging up plants is hard, bleaching soil destroys more than the spores, and it seemed never ending. Fungicides are mostly ineffective except for a few which are not available without a license. It was time to begin my own research and experimenting.
Since Neem oil is anti-fungal, I searched the internet to see if there was any information regarding it and Sclerotium rolfsii and found that it did show some promise. I mixed 1 1/2 teaspoons of 70% Neem oil in a quart of water and sprayed Sclerotium rolfsii spores with little results. I then had an idea - Oregano oil, which amongst other uses, is anti-fungal. I could find nothing on the internet regarding its use in killing Sclerotium rolfsii, so I was on my own.
I dropped a few drops of Oregano oil directly on a few Sclerotium rolfsii spores. They dried up immediately and never spread. Using straight Oregano oil is not practical, so I experimented until I found a dilution that was effective.
This is the method that ultimately worked successfully for me. I have not only used it on Hostas, but on Ajuga, Phlox and Siberian Iris. Cut the plant back as far as possible and clean out all debris, carefully putting everything in a trash bag to be thrown away. Sclerotium rolfsii spores can easily hide, so doing this makes inspection and spraying much easier and more thorough. Fill a quart spray bottle with water and add one dropper full of Oregano oil. Spray to the point of drenching the affected plant and surrounding soil, shaking the bottle frequently to assure the Oregano oil is thoroughly mixed. Inspect and repeat daily as needed as the spores are so insidious that one missed spore can quickly re-infest when the conditions are right. Individual crowns may be especially difficult, and at one point I poured a little of the mix into some of them to insure coverage. It has been several weeks since I have found any spores, and as quickly as they spread, I am hopeful they are under control.
California Natural Wild Oregano Oil with 70% Carvacrol. I cannot guarantee results, but when all else has failed or the situation seems hopeless, it is certainly worth trying.
Prevention is important when it comes to Sclerotium rolfsii. Closely inspect, some even go so far as to sanitize, any plants you import or transplant. Frequently sanitize tools as well. Inspect your garden daily if possible, even the veggies. If foliage is turning yellow or dying, check the base of the plant for spores. Sclerotium rolfsii strikes fast, so finding it early is key. Throw infected plant material away, do not compost it. Do not mulch up against plants; leave an air space around them. Monitor plants around newly disturbed soil as spores can live indefinitely underneath, waiting to be exposed. It is the spores that are in the top few inches of soil that cause the damage.
For more information on Sclerotium rolfsii ...
UPDATE: I hoped that an exceptionally cold winter had killed any potentially remaining spores, but on June 15, 2014 I found a few on one Hosta. I had about half a quart of last year's mixture in the garage, so after carefully disposing of the affected leaves, I shook it up and poured it straight on the area. I carefully inspect daily, and despite perfect conditions for Sclerotium rolfsii, I have not found any more.