It would be like the history of Easter Island...they used all the trees (the last tree was cut in 1680 a.c.e.) and had massive social consequences. No trees, no boats, no food. They resorted themselves to cannibalism. If a virus would kill all the trees on the planet...we...the human race would die. Slowly. Erosion, undrinkable water, and life on Earth would end. I believe there would be breathable oxygen but our global society would shut down.
No more photosynthesis, no more oxygen, we all die.
If there were no more trees we woun't live. Humans woudn't even be in this world.
If there was no more trees we woudn't live on this planet. Humans can not really even breathe!
Trees provide us with fruits, nuts, wood and paper. Trees are also home to many animals, and all plants produce oxygen. If we didn't have trees we would be mising a lot of things we take for granted, however, I seriously doubt that we wouldn't be able to live. It wouldn't be pleasant but it would be possible. We would also miss out on the beauty of a magnolia in full bloom and the awesomeness of a redwood.