Which condition does NOT cause mosaic virus to occur?

Mosaic virus is a plant disease caused by various viruses, and understanding the transmission routes is crucial for disease management in agriculture. The correct choice indicates that runoff water does not contribute to the spread of mosaic viruses.

Contaminated tools and equipment, infected seeds, and insects are all well-documented avenues for the transmission of these viruses. For instance, plant viruses can easily be transmitted when tools used for pruning or harvesting come into contact with infected tissues. Additionally, seeds can serve as a vehicle for the virus, leading to new infections if they are not tested or treated properly. Insects, particularly aphids, are notorious vectors that can transmit mosaic viruses as they feed on plants, facilitating the spread from one plant to another.

In contrast, runoff water, while it can carry various pathogens in a broader sense, is not a direct method for the transmission of mosaic virus. The virus requires specific biological vectors or infected plant material to spread, and water runoff lacks the necessary mechanisms to facilitate the viral transmission effectively. Therefore, recognizing the limited role of runoff water in the lifecycle and spread of mosaic viruses is key to understanding plant disease management strategies.