Question
My diffenbachia plant has some mushrooms growing in the soil.  I have had this plant over 5 years and have not done anything new to it.  Should I be concerned.

Answer
Mushrooms growing in house plants are caused by a fungus. The mushrooms are the fruit of that fungus. One of the common mushrooms found growing in house plants is the Leucocoprimus birnbaumii. This is a light yellow mushroom with either balled, or flat cap depending on how mature they are.

The spores that are the cause of mushrooms growing in house plant soil are normally introduced by contaminated soilless mix. But, occasionally, they can be introduced through other means such as airborne movement, or spores brushing off clothing, etc.
House plant mushroom prefer warm, moist, and humid conditions.

Getting rid of mushrooms is no easy task. Once the soil becomes infected, its is very difficult to remove the spores and fungus that cause the mushrooms. But there are a few things you can try.

Start by taking the plant out of the pot. Either clean the pot with bleach water, or get a new pot.
Remove most of the soil around the root ball, being careful to not damage any roots.
Fill a bucket with warm water, and a good fungicide that is meant to dissolve in water. Let the soil soak up the fungicide.
Repot the plant in fresh, moist, new soil and the clean pot.
Let the soil dry before watering.

Getting rid of mushrooms in house plants is hard. But, mushrooms growing in house plants will not harm your plant, nor will they harm you unless you eat them. If you have young children, who put things in their mouth, then by all means, keep them out of harms way. But then, the leaves of the diffenbachia are just as harmful in that case.

You may want to consider just removing the tops so that the spores don't spread to the other plants. This way you don't risk harming your plant by the cleansing method. But, if you haven't changed your soil in the last five years, it is probably due.