My first suggestion would be hydrogen peroxide. Get the topical strength (3%) from the walmart pharmacy section, should be able to find it already in a handy spray bottle. Just spray it on the plant directly, no need to mix or water it down. Even young seedlings can withstand up to about 10% concentration of H2O2, but it will kill molds and fungi quite readily.

We call them puffballs. I looked online and came up with Lycoperdon but have no idea if that's correct.

I have the same problem, and with calamondin, I am sure this is not due to sunburn, I have been growing calamondin, kumquat and other citrus for many years and this is the first year that problems occur. At first, I thought that birds tried to eat the fruits but they are sour inside and so birds don't want to finish, I plan to go to the information desk of Teas nursery to ask, I would appreciate if any of you could help. K

if you have wood in your garden ... as in.. your mulch ... in the process of its decomposition ... all kinds of molds and fungus attack it ... and as far as i am concerned.. its a free show of ma nature... not a single one of them require any action on your part .... and unless you get rid of the wood.. there is nothing you can do about it anyway ... that said.. getting an ID.. and learning is highly approved.. knowledge is power ... ken ps: it should not even affect that green plant... since the green plant.. isnt dead wood ... it may be encircling it ... but it should NOT be attacking it [that said.. it could make a liar out of me.. lol]