Good Fruit Grower / August 2009 / Geneva ­rootstocks ­commercialized

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Geneva ­rootstocks ­commercialized

Two fireblight-resistant Geneva rootstocks are alternatives to M.9.

Cornell University has released seven disease-resistant apple rootstocks from its rootstock-breeding program at Geneva, New York, but the university is focusing its commercialization efforts on four rootstocks: Geneva 11, G.41, G.935, and G.202.

Many apple varieties are susceptible to fireblight, but the disease is most devastating when the rootstock is susceptible, too, Dr. Terence Robinson, horticulturist at Cornell, reported at the International Fruit Tree Association's annual conference in Germany.

Infections in the top of the tree can kill branches or part of the leader, but the real problem comes when the tree is on a susceptible rootstock, such as Malling 9 or M.26. The bacteria travel down inside the tree, without symptom, until they get to the rootstock where they cause girdling and death of the rootstock and tree.

If the rootstock is resistant, infected scaffold branches or leaders can be cut out in the winter and regrown, and the tree can survive.

"Hence, having a rootstock that's resistant to fireblight is important in maintaining orchard viability," Robinson said.

Disease-resistant Geneva rootstocks are being tested in Italy, Spain, Germany, France, and Switzerland, as well as the United States. Robinson said Cornell considers G.11 and G.41 to be potential replace­ments for M.9 in areas of Europe and the United States where fireblight is a substantial problem. They are productive and in some cases slightly better than M.9 in terms of production ­characteristics.

G.11

Geneva 11 is the rootstock most readily available in the United States. It is precocious, similar to M.9, and productive with good fruit size. About 150,000 trees on G.11 have been sold, and substantial new stool beds are being planted in 2009-2010. The rootstock will be released in Europe this year and partner nurseries are bulking up their stool beds as fast as possible, Robinson said.

G.41

Geneva 41 has the same vigor as M.9, but is resistant to fireblight, crown rot, replant disease, woolly apple aphid, and cold damage. Its one drawback is poor propagation. Robinson said Cornell is ­trying to improve propagation through the use of tissue culture plants to establish the stool beds, high-density plantings in the stool beds, and Apogee (prohexadione-calcium) sprays during the season to improve rooting. Production in the United States has been limited so far, but new stool beds are being planted in 2009-2010. Robinson said he hopes there will be 300,000 plants—enough to produce 1.5 million liners for sale in the United States. Release of this rootstock in Europe will be delayed for two more years until stool beds are established.

G.935

Geneva 935 is slightly more vigorous than M.9 but less than M.26, and is extremely productive. Robinson said it should be suitable for organic production or for use with a weak scion variety. "It has a little more push," he said. Availability is limited, but stool beds are being planted in 2009-2010. In three years, there should be 100,000 plants in the ground, producing 500,000 liners.

G.202

Geneva 202 is the most vigorous rootstock of the seven released. It is resistant to woolly apple aphid and is being planted at the rate of about 150,000 trees annually in New Zealand. There is interest also in other parts of the world, such as Mexico, Chile, Australia, South Africa, Brazil, and Uruguay, where woolly apple aphid resistance is critical and growers want a little more vigor in their trees, Robinson said. It would be too vigorous for high-density spindle orchards. Stool beds are being planted in the United States, Mexico, and South Africa.