Mosaic virus can potentially stunt young plants that display a lot of symptoms, and may also cause odd, fern-like growth on young marijuana leaves. A plant virus can be hard to pin down, since many factors can cause similar symptoms. The main problem with TMV is that it may cause plants to grow slowly and produce poorly. If your plant is growing fast and healthy, with no other symptoms, and youāre not noticing it spreading from plant to plant, you probably shouldnāt get too worried.