I think that the spores are 4-nucleate (the nuclei are difficult to see, globose transparent regions in the spore centre, about 2 µm diam.). This would be typical of S. sclerotiorum. 

What is the diameter of your apothecia? There is a problem with Dumontinia tuberosa which has also which has also 2-nucleate spores, and a close relative with 2-nucleate spores. Both are about 10-20 mm diam., more cup-shaped and with often very long stalks, while S. sclerotiorum around 5-15 mm.

Zotto

Thanks for your answer Zotto.
Yes, I know I should have to observe the nuclei when it was fresh, but I didn't, sorry. Maybe they are still visible in dry specimens with Giemsa or acetic carmine...?
Maybe I can try to return on the spot to seek for some other apotecia, the collection was made three weeks ago...
I know D. tuberosa well enough to exclude it. These apothecia were rather small, not more than 10-12 mm; I found the first ones under a hazelnut so my first idea was an amenticolous Ciboria but then I have seen the sclerotia...
Usually D. tuberosa is bigger, darker, less regular (in the mature ones) and the sclerotia are more tuberoid, in my specimens I noted all sclerotia were round and flat.
Anemone was present in the wood but it was close only to some apothecia, the most were spread in the area near rather different herbs so the fungus should not be so host-specific.
Regards
Mario

I suppose the spores are still alive when you have dried them recently without heat.

Staining methods on dead material are rather complicated, I have no experience. 

Then there  is only the possibility of Scl. minor which is binucleate.

Zotto

I made a mistake, sorry:

actually Kohn 1979 says that

S. sclerotiorum is 2-nucleate and
S. minor 4-nucleate

Therefore it is not easy to separate between sclerotiorum and the large, Dumontinia-like species on Ficaria and Corydalis.

Zotto

Hi Mario

I suggest to try first with IKI before trying with any aggressive media. Maybe you can mix it with a bit of water if your IKI has too much iodine in order to see them better. If not the colour will be saturated. Spores may still be alive if little time has passed and you have done a normal drying. Anyway maybe the empty space taken up by the nuclei can still be seen as an empty area so you can estimate how many nuclei they were.

Sclerotinia sclerotiorum: 2 nuclei
Sclerotinia minor: 4 nuclei

I attach spores of S. sclerotiorum in water and IKI. I agree that spores are a bit wide to fit in S. sclerotiorum, moreover if you have rehydrated and after taken the measurements. But also take in consideration overmature spores which are not uncommon in this genus. Then abnormal spores can be seen, though not in your microphoto about spores. Can you provide a scale for that one? In my collections I never found over 7 µm wide.

My idea of what S minor is:
http://www.ascofrance.fr/search_forum/15005

Cheers,
Raúl

PD: I see Zotto has corrected in the meanwhile

Just one question.

Dumontinia tuberosa has 2 or 4 nuclei ?
I thought it was 4 nuclei. I have some collections with 4 nuclei on Anemone nemorosa.
 
Regards,
Daniel

Thank you for your advice, Raúl, I cannot provide a scale for the photo because it was made with a makeshift tools (actually a compact camera with unspecified zoom and autofocus).
I tried some hours ago to observe the spores of the collection with Giemsa, and in my amazement I have seen almost all the spores were germinating in the asci, so I think no possibility to understand exactly how many nuclei they contained. I can try again with other apothecia and with IKI, maybe tomorrow.
Zotto if the only choice is between S. minor and S. sclerotiorum, the ectal excipulum clearly indicates the latter... No need even to see the micro, the macrophoto (central specimen) should be enough...
But Kohn's key, is not too simplified?!?!

Regards
Mario

Hi Mario

I can only speak about tuberosa/sclerotiorum. The excipular differences that Kohn reports and uses as diagnostic I could not really verify in several of the specimens that I had studied of both species.

Zotto

Today I returned to the spot after lunch. I spent one hour on my kneeds and I found some 10 further ascomata.
Maybe I have made a HUGE error. I have to work a little on that. 8-|

No error luckily!
I was afraid to have considered as a Sclerotinia, something different.
Returned to the spot, while I was searching for the apothecia, I tried to find some relations with the roots of the host to identify it. The sclerotia are always superficial, the ascomata are easy to collect without to break the stipe, and I was not able to find any relations with the roots. Moreover, they are usually in the spaces between the herbs and never close to their stems. So I considered the possibility that the sclerotia arose from some kind of plant remnants in the litter more than from living roots; and I tried to identify all the leaves present in the litter. In addition to the trees I listed above, I found some leaves of Acer, some fruits of Castanea and, close to a cluster of 4 apothecia, a pod of Robinia. A Robinia pseudoacacia tree was just over my head, in fact.
I found two sclerotia on the surface of the ground, without apothecia.
And seeking for some other ones, I realized that there were many Robinia seeds, those little dark brown "beans"... rounded... and flattened(!) of the same size of the sclerotia...! Were they really sclerotia, or maybe stromatized seeds? That should change everything.
OK, at home however I cut some sclerotia, and they are exactly... sclerotia with a black rind and a white medulla, without any remnants of plants at the inside.
So I returned to the observation of the nuclei, this time in the living spores they were not difficult to see (in Lugol). The spores are binucleate so I think Sclerotinia sclerotiorum can
be confirmed. Maybe I will measure some others spores and try a statistics.
Thank you all!
Mario

Good Mario! You will not be the last one to be cheated by beans when searching for sclerotia ;-))
Just to compare, my Sclerotinia sclerotiorum spores range: 11,5-15 x 6-6,7 

Cheers,
Raúl

Great exercise!

Zotto