The tobacco ringspot virus—an RNA virus infecting many different kinds of plants—has jumped from the plant kingdom to the animal kingdom, infecting the European honeybee. Various genera of rhabdoviruses are able to infect vertebrates, invertebrates, or plants, implying that both cross-kingdom and cross-phylum jumps have occurred within this virus family. Both groups are RNA viruses that—in their plant-infecting forms—use animals to spread from one plant to another...I imagine (but have no evidence) that this combination helped facilitate the jump between kingdoms.

There was also one documented case of plant-to-fungus virus transmission, where the cucumber ringspot virus was found to infect Rhizoctonia solani, a fungus that causes cucumber belly rot. The linked paper shows that the virus can actually move back and forth from plant to fungus to plant, making it a single multi-Kingdom virus.