The Malling series and clones have been standard rootstocks for apples for many years and remain the "workhorses" for the commercial industry in the United States and the UK. However, since most of them are susceptible to disease some Malling rootstocks are being replaced by new breeds, including the Cornell-Geneva series, which has resistance to the major problems preventing quality production of apples utilizing organic control systems. One of the newest rootstocks of the "CG" series, only released commercially in 2004, is CG5202(G.202) which adds resistance to the woolly apple aphid, and when combined with highly resistant cultivars such as "Liberty" it is showing great potential. Rootstocks based on Siberian Crab apple are being used in colder areas for more cold tolerance.