Weeds serve as reservoirs for which plant disease?

Aster yellows is transmitted by leafhoppers, which feed on infected plants and then spread the disease to healthy plants. Weeds can serve as alternative hosts for the pathogens responsible for aster yellows. These weeds may remain in the environment, providing a reservoir for the disease and facilitating its transmission to cultivated crops.

When considering the other options, while cherry X-disease and corn stunt disease are associated with specific hosts (e.g., certain fruit trees or corn), they do not have as significant a connection to weeds in the same way that aster yellows does. Powdery mildew fungi, although they can affect a wide range of hosts, do not typically use weeds as reservoirs in the same manner. Thus, aster yellows is the option that best fits the description of weeds serving as reservoirs for the disease.