In addition to grfrazees answer I would recommend treating the piece liberally with ethylene glycol, what can be bought for little money in the form of concentrated automotive antifreeze. This will kill any fungal mycelia in the part of the wood that still appear healthy. Additionally ethylene glycol is hygroscopic and thus mix with the water contained in the wood all the way through.
As ethylene glycol evaporates at a far lower rate than water it will stabilise the wood, similar to polyethylene glycol (what is non-toxic but doesn't kill fungi). If applied in sufficient amounts this can stop the cracking altogether. It may affect the colour of the wood to a small degree however, what is probably not an issue if the round is kept outside.
Ethylene glycol is moderately toxic to all eukaryotic lifeforms (plants, fungi, animals/humans). However after it is absorbed into the wood you would have to eat the wood itself to induce any of it's toxic effects. It got a bad reputation because of it's sweet taste what makes it likely to be ingested in large quantities by pets and children, resulting in death. For that reason automotive antifreeze is treated with denatonium these days in most countries, what makes this accidental ingestion far less likely.
You may want to look at this article for further information on the subject, including the use of borates to prevent fungal growths.