Answer: Excessive soil moisture from overwatering may contribute to the problem. However, there can be mushrooms in potting soil even if you don't overwater.
Most commercial potting soils contain compost. The compost may be from many sources, but in every case fungi are part of the composting process. If the compost was properly made, the fungi should be no problem. The fungi in the potting soil should not infect your tree. The fungi are just continuing the process of decomposing the organic material in the soil to form humus and humic acids. If the soil was not properly heat treated during composting or afterwards, the quantity of active fungi will be very high. When the potting soil bag was opened new fungi could have been introduced to the soil. It would then reinfest, and fungi can grow in the potting soil.
You can reduce the growth of fungi and development of mushrooms in your potted plants by allowing some drying of the soil, or you can add a layer of sand or gravel to the top to exclude air from reaching the top of the potting soil. Best wishes with your lemon tree!
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