First off is there continuous wetting of these areas? If so you will have the potential for mold.
Typically, if this is a one time event and it is repaired and properly dried out you should not experience any problems with mold.
Two things need to be present for mold to grow:
- The first is moisture. Moisture comes from leaks which develop and continually leak without notice or through soils which are open to the inside of the home that are not covered with a vapor barrier.
- The second is food. Food comes for contaminants which are brought into the home by the the moisture (water) infiltration such as organic material from a roof or gutter or even the paper on a sheet of drywall.
As for plaster this is a cement based product and can withstand events of wetting and drying without production of mold.
- However you have mentioned a key item...that is your house is 87 years old.
- In your case, plaster of that age usually contained horse hair in the base coat as a binding agent. This is similar to the use of fiberglass fibers in plaster these days.
- This hair is and can be a source of food for the mold since it is an organic product.
I would recommend if not already completed:
- Stop the moisture infilltration.
- Dehumidify the area of the leak, and
- Have a sample of the plaster tested for mold growth. If it is present remove or treat the area appropriately.