The virus status of a tree or rootstock simply refers to whether the plant material in question is free of known viruses. The original Malling rootstocks, such as M.7, M.26, and M.9, contained some viruses. Many viruses are latent and do not produce visible symptoms, so it is difficult to know whether a tree is infected with a virus. Most of the Malling rootstocks have been "cleaned up," and the virus-free rootstocks carry the suffix "EMLA" (East Malling and Long Ashton); examples include M.7 EMLA, M.26 EMLA, and M.9 EMLA. The virus-free rootstocks are generally a little more (about 10%) vigorous than the virus-infected material. Some fruit growers prefer the less vigorous rootstocks, so some nurseries sell M.9, as well as M.9 EMLA.
Richard Marini, Penn State University