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Fireblight-resistant apple rootstocks developed at Cornell University in Geneva, New York, could secure a place in the Swiss apple industry, members of the International Fruit Tree Association heard during their annual meeting in last February in Germany.

Dr. Simon Egger, horticulturist with the Agroscope-Changins Wädenswil Research Station in Switzerland, reported on a trial he conducted with a range of apple rootstocks, including Budagowsky 9, six Geneva rootstocks, Supporter 4, and P.16, P.59, and P.60 from Germany. They were compared with the standard rootstock Malling 9 (337 clone), which is extremely susceptible to fireblight.

Egger said there are several ways that fireblight can infect rootstocks. Bacteria from blossom or shoot infections can travel down through the trunk into the rootstock, but they can also reach the rootstock through burr-knots, cracks in the bark, or open wounds on the collar or trunk. In addition, bacteria can enter the tree through root suckers.

Any new rootstock adopted by Swiss growers would need to be of about the same vigor as M.9-337 he said. It should have a strong root system and fit established systems with hail netting. It should be adaptable to a wide range of conditions and stimulate early, high, and regular production. It should be tolerant of fireblight, collar rot, nectria, and replant disease. In addition, it must be easy for nurseries to propagate.

M.9 replacements

Egger planted the trial at Wädenswil in 2003. The area has loamy soil that retains water well, has a humid climate, and has been planted to apples for 50 years. Trees were planted 3 feet apart with 11.5 feet between rows. The scions were Galaxy Gala and Topaz. B.9, G.11, and G.41 proved to be the most interesting rootstocks as potential replacements for M.9, he said.

Presenting the results for Gala, Egger said trees on G.11 and G.41 were 25 percent more vigorous than trees on M.9, in terms of the trunk cross-sectional area, while B.9 was about 10 percent smaller. Egger said the Geneva rootstocks might be particularly suitable for replant sites.

Packouts

Geneva 11 had the highest cumulative yield from 2004 to 2008, closely followed by G.16 and G.41. They all yielded more than 60 kilos (132 pounds) per tree, compared with around 45 kilos (99 pounds) for M.9 and a little less for B.9. The yield efficiency (kilos per square centimeter of trunk cross sectional area) for G.11, B.9, M.9, and G.41 was in the same range.

Trees on M.9 had the largest fruit, followed by B.9, although the differences between rootstocks were slight, and not enough to be concerned about, Egger said. Malling 9 also generated the best packouts in terms of fruit size and color, but again the differences were slight and Switzerland has a market for smaller Gala apples. No differences were found in fruit firmess or sugar content.

The Geneva rootstocks are resistant to fireblight, and Egger said there have been reports from the United States that B.9 has some field tolerance to the disease. However, that has not yet been proven in Swiss conditions.

A drawback of B.9 was root suckering, which was not a problem with G.11 or G.41. However, G.41 appears to be difficult for nurseries to propagate. Egger concluded that B.9 and G.11 are the most promising rootstocks overall but, because G.11 is not yet commercially available in Switzerland, growers who need to replant immediately will probably use B.9.

Egger is setting up on-farm trials in different regions of Switzerland to study whether G.11 is better than B.9 in replant situations.