What causes black stuff coming from oak tree?

The black substance on oak trees is often caused by sooty mold, a fungal growth that thrives on honeydew produced by sap-sucking insects like aphids or scale. When these insects feed on the tree's sap, they excrete honeydew, which provides a nutrient-rich environment for the mold. Additionally, environmental factors like high humidity can promote mold growth. While sooty mold doesn't harm the tree directly, it can block sunlight and photosynthesis if it becomes too thick.