There is a lot of bad geometry in this model, I opened it in Wings, selected the whole thing and clicked "cleanup". This removes very short edges (less than .001 long) and isolated vertices.

The result was 315 faces, 588 edges and 273 vertices were removed.

After this my renders looked no different to the original model.

The problem appears to be poor construction of the rose with the petals intersecting each other in many places and with gaps where there should be petals deep inside the model.