Which organism does not cause disease in plants?

Herbivores; they are typically animals too large to exist within plant circulatory systems.

No, most micro-organisms and parasites are species specific. There are however, exceptions. Most of which you have probably heard of: equine encephalitis, swine flu, some tapeworms, avian (bird) flu and of course fungi (including yeast). There are other micro-organisms that use other animals as vectors, but since the other animal doesn't get sick, it doesn't count according to your question. An example of this would be malaria or rocky mountain spotted fever. There is a parasite that grows (infects) a betel nut grown in Guam that has been known to cause cancer in humans. But you cannot assume that any micro-organism that infects a plant will necessarily be harmful to humans. In any case, if you ate the gourd, the parasite would have entered your digestive tract, not your bloodstream.

Herbivore eats plants