At times pear trees are severely damaged by frost. The trees are often planted on low, rich soil, and from their location are more subject to cold. A difference in elevation of only a few feet may often be sufficient to determine whether the trees will be severely affected or not. (For a fuller discussion of the question of Frost Injury, see under Apple) The pear, like most other fruits, is affected with galls both at the crown and at the tips of the roots. While orchard trees show the enlargements, there is a greater tendency for nursery stock to be affected. In any case the destruction to pears is less rapid, and therefore less extensive, than to peaches or apples. Hairy-root, another form of the Crown Gall disease, is less common on the pear than on other fruit-trees. Both the galls and hairy - roots may be induced by factors other than Bacterium tumefaciens; among such agencies may be noted (a) improper wrapping of grafts, (6) heavy applications of nitrogenous fertilizers, and (c) the woolly aphis. A discussion of the galls, the causal factor, and remedial measures are treated more fully under Apple, page 108.
Selby, A. D. Some diseases of orchard and garden fruits. 4. Root or crown gall of pear. Ohio Agr. Exp. Sta. Bul. 79: 127. 1897.
Swingle, D. B. Fruit diseases in Montana. Crown Gall of pear. Montana Agr. Exp. Sta. Circ. 37: 317 - 318. 1914.
Martin, G. W. The common diseases of the pear. Crown gall. New Jersey Agr. Exp. Sta. Circ. 52: 10 - 11. 1915.