Which disease is known as a very common bacterial disease affecting plants?

Crowngall is indeed recognized as a very common bacterial disease that affects a variety of plants. It is caused by the bacterium Agrobacterium tumefaciens, which induces the formation of galls or tumors at the roots and stems of the plants. These galls can disrupt the plant's nutrient and water uptake, leading to stunted growth and overall decline in plant health. The disease is particularly notable because it can affect numerous species of plants, making it a widespread concern in agricultural and horticultural settings.

The other options, while representing plant health issues, do not refer to bacterial diseases. Powdery mildew is a fungal disease that manifests as a white powdery coating on plant surfaces. Root rot is a result of various pathogens, primarily fungi, that affect the roots of plants, leading to decay. Botrytis blight, also known as gray mold, is a fungal condition that impacts various plant parts, especially in humid conditions. Therefore, the significance of crowngall as a bacterial disease distinguishes it clearly from the other diseases listed.