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The Ditton Laboratory at East Malling was built in 1926 to conduct research on storage of tree fruits and develop cold storage regimes for apples and pears. It was designed to mimic a ship's hold. It started out as an independent laboratory and later became part of the East Malling Research Center. During the 1990s, the work was shifted to a new facility.

The Ditton Laboratory at East Malling was built in 1926 to conduct research on storage of tree fruits and develop cold storage regimes for apples and pears. It was designed to mimic a ship’s hold. It started out as an independent laboratory and later became part of the East Malling Research Center. During the 1990s, the work was shifted to a new facility.

Sir Ronald Hatton, one of the first researchers at England’s East Malling Research Station almost a century ago, took on the job of classifying and testing apple rootstocks that were distributed throughout Europe.

When Hatton joined East Malling, there was a “mixed-up potage of easily rooted clones” that had been collected by gardeners over the centuries, according to Dr. Jim Cummins, retired Cornell University horticulturist. These clonal selections had become hopelessly mixed up and, in many cases, their identities were lost.

Hatton worked to bring order out of the chaos and released a series of rootstocks under the Malling name. The rootstocks M.1 to M.16 were released by East Malling in 1913–1914.

In around 1920, East Malling and the John Innes Institute at Merton, England, began breeding rootstocks for resistance to woolly apple aphid by making crosses with Northern Spy. They eventually released the Malling-Merton series of rootstocks as well as the Merton Immune series, which includes Merton 793.

Some of the more widely used Malling rootstocks have been:

M.7: Semidwarfing. About 50 to 65 percent of seedling. From the 1950s to 1980s, this was the most widely planted clonal rootstock in the United States, as it was productive and adapted to a wide range of soil types and climates. However, it is more prone to burr knots and root suckers than other commonly used rootstocks. Bred in France in about 1688 and formerly known as Doucin Reinette or Doucin vert.

M.9: Dwarfing. Selected as a chance seedling in France in 1826. Previously known as Jaune de Metz or Paradis. Many subclones exist today, as a result of heat treatment to rid the original clone of viruses and selection for easier stoolbed propagation. A tree on M.9 is about 35 to 40 percent the size of seedling, although clones vary in the degree of dwarfing. M.9 is precocious and yield efficient. For the past two decades, it has been the most extensively planted rootstock worldwide.

M. 26: Moderately dwarfing. An M.16 and M.9 cross released in 1959. Produces a tree about 45 percent the size of seedling. M.26 is precocious but less so than M.9. Susceptible to fireblight and burr knots.

M.27: Very dwarfing. A cross of M.13 and M.9 released in 1975. This rootstock produces a tree less than half the size of a tree on M.9 EMLA (the heat-treated, virus-free clone). It is very precocious, but yield efficiency is typically less than that of M.9.

MM.106: Moderately vigorous. About 60 to 70 percent of seedling.  A cross of M.2 and Northern Spy released in 1952. Susceptible to crown rot.

MM.111: Vigorous. About 70 to 80 percent the size of seedling. A cross of Northern Spy and Merton 793 released in 1952. Adaptable to various soil conditions and tolerant of drought stress.

Sources: East Malling Research; Intensive Orchard ­Management by Dr. Bruce Barritt, 1992; The New Wave of Apple ­Rootstocks by Dr. Jim Cummins, 1994; Wikipedia.