PLANTING DISEASED TREES: AN ATTEMPT TO AVOID SHOCK SYMPTOMS OF APPLE PROLIFERATION

After having gone through a period of shock symptoms most trees will not show shock symptoms again. By planting young trees with shock symptoms we attempted to prevent a reappearance of such symptoms in the future. Red Jonathan, a very susceptible cultivar, was budded to healthy and diseased M 26 rootstocks. Diseased and healthy trees were planted in two locations. Diseased trees had less vigor, smaller fruit and lower yield than healthy trees. In one location, six and seven years after budding, three diseased trees showed witches broom shock symptoms again, but no diseased trees were observed in the other location. Apparently the reappearance of shock symptoms can not be prevented by planting diseased trees.

M 9 virus-tested rootstocks, with and without proliferation, were planted in pots in soils having high or low pH and kept in the greenhouse. There was no effect of pH on the appearance of symptoms. Similar observations were made in the field where the severity of symptoms was not associated with the pH of the soil.

In the experiment above with diseased trees planted in two locations, the trees showed a great difference in symptoms in spite that the pH of both soils was the same.

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