|Authors: ||J. Viseur, Tapia M. Figueroa|
A new method has been tested to determine the degree of fire blight resistance in pears and apples by artificial inoculation of plantlets produced in vitro.
Shoots, 2–3 cm long, obtained through micropropagation were inoculated by cutting an apex leaf with scissors dipped in a suspension of virulent Erwinia amylovora.
Symptoms obtained in vitro were similar to those observed in the field: blight, necrosis of shoots, petioles and leaves, ooze production.
A strain of E. amylovora which was avirulent under natural conditions did not induce any symptoms when inoculated in vitro.
The experimental conditions used (culture medium, light, temperature, inoculum) influenced the level and uniformity of symptoms.
After optimalisation of the test, we compared the resistance levels of 15 pear and 7 apple cultivars.
Our method permitted to establish a clear distinction between highly resistant and very susceptible clones, which correlated well, as for pear, with the fire blight resistance of the corresponding cultivars under field conditions.
Twelve days after inoculation of pear plantlets with E. amylovora, the field-sensitive cv "Durondeau" and cv "Doyenné du Comice" reached mortality levels of 92% and 88%, respectively, whereas the resistant cv "Old Home X Farmingdale N°51" showed only 9% mortality.
For apple under similar conditions, more than 90% mortality was observed for cv "Golden delicious" and rootstock "M.26", while values less than 20% were obtained for cv "Perpetu R Evereste", "Jubile R Delgollune" and rootstock "M.M. lll".
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