Many of these plagues were unleashed because of man's disobedience to God.....Same with syphilis. It existed naturally in sheep with no harm to the sheep, until someone had sex with a sheep against God's command, unleashing a disease God never wanted unleashed.
Syphilis came to Europe in the 1500's from the New World, where it had been endemic for centuries. Since sheep did not exist in the New World prior to contact with Europeans, I doubt it was possible for the locals there to be having sex with them. More importantly, the organism that causes syphilis,
Treponema pallidum, has no known host besides humans, so it could not have "existed naturally" in sheep, or been unleashed on humanity through intercourse with them.
As for plant evolution, I accept the challenge of discussing the divergence of plants from animals, though I hope someone else chimes in, as my primary specialty is bacteria, not plants/animals/eukaryotes.
First off, lets say that, broadly speaking, there are three branches of eukaryotes
(organisms with nuclei and other cell compartments) - plants, animals, and fungi. The last common ancestor of all three existed somewhere around 2.4 to 1.6 billion years ago, and was single-celled and very simple. Somewhere around that time, the common ancestor of animals and fungi became distinct from the ancestor of plants. Finally, fungi and animals began to diverge, with the first modern fungi appearing by 1.4 billion years ago.
Now that we have outlined the timeline of events, lets focus on what made plants distinct from animals and fungi. The most salient change, and one that probably happened first, is plants' ability to photosynthesize, or make sugar from carbon dioxide and sunlight. There were small bacteria, likely similar to the cyanobacteria of today, that were photosynthetic long before plants arose. The first plant ancestor either preyed on or was parasitised by these bacteria, and eventually came to rely on the bacteria living inside them, similarly to the process by which eukaryote ancestors gained mitochondria. This reliance on photosynthesis for survival was likely what separated early plants from animals and fungi, and caused them to evolve distinctly from one another.
So that's the basic shape of the evolution of plants as understood by science. If anyone wants me to discuss any specific aspects, let m know and I'll try to explain them to the best of my ability.