ISA Certified Arborist Practice Exam 2025 - Free ISA Arborist Practice Questions and Study Guide

Over-watering can significantly impact tree health by leading to root rot and other fungal diseases. When the soil around the tree roots becomes excessively saturated, it limits the oxygen available to the roots. Roots require oxygen for respiration, and when they are deprived of it, they can become stressed and begin to die off. This unhealthy root environment creates an ideal condition for pathogens, particularly fungi, which thrive in moist, oxygen-deficient conditions. As root rot develops, it can severely impair the tree's ability to take up water and nutrients, leading to further decline in health and vitality.

Additionally, over-watering can cause other issues such as promoting shallow root systems due to the roots staying near the surface in search of oxygen, rather than growing deeper into the soil where they can access moisture and nutrients more effectively during dry periods. Overall, the effect of over-watering is detrimental, making option B the correct choice in this scenario.