Good Morning David,

Thanks for reaching out! Mummy berry can be dreadful!

I reached out to our state plant pathologist, Ann Hazelrigg, and here's what she had to say:

Mummyberry is sometimes a problem and sometimes not. It all depends on the weather during bloom and the amount of inoculum (spores) that are around.

It is a good idea to grow several different cultivars with different bloom times to hedge your bets. One of the best things home gardeners can do is to apply a 2-4 inch layer of mulch in the later winter/early spring before the forsythia bloom to cover up any overwintered berries that will provide the spores for the next year's infection.

Pruning bushes to promote air circulation and drying will also help. Pick any infected berries now to decrease the overwintering inoculum. Fungicides can also be used but timing has to be correct. I would start with the mulch, pruning and sanitation of infected berries.
https://ohioline.osu.edu/factsheet/plpath-fru-46

There's also this great fact sheet from the American Phytopathological Society: https://www.apsnet.org/edcenter/disandpath/fungalasco/pdlessons/Pages/MummyBerry.aspx

Hopefully these fairly simple interventions will help improve your yields and eliminate some of the headache of the dreaded mummy berry!