What is the visual cue of an oak leaf blister infection?

The visual cue of an oak leaf blister infection is characterized by the puckering of leaves. This distinctive appearance results from a fungal pathogen that causes the leaf tissue to bulge and deform, leading to a bumpy or blistered surface. As the infection progresses, the affected area may appear raised or swollen, which is a clear sign of the disease.

Other options may describe symptoms associated with different plant issues but do not specifically relate to oak leaf blister. For instance, yellowing of leaves typically indicates nutrient deficiencies or environmental stress, while curling of leaves can suggest issues like insect infestations or environmental conditions affecting the plant's health. Fall coloration refers to the natural seasonal change that occurs as chlorophyll breaks down, resulting in vibrant colors in the leaves but is not related to the blistering caused by this specific fungal infection. Thus, the distinct puckering serves as the definitive sign of oak leaf blister, making it the correct answer.