University of California

Some factors affecting the susceptibility of plants to fire blight


H. Earl Thomas
P. A. Ark

Authors Affiliations

H. Earl Thomas was Associate Plant Pathologist in the Experiment Station; P. A. Ark was Junior Plant Pathologist in the Experiment Station.

Publication Information

Hilgardia 12(4):299-322. DOI:10.3733/hilg.v12n04p299. January 1939.

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Abstract does not appear. First page follows.

Students of fire blight (caused by Bacillus amylovorus) for a century or more (4)5 have made observations which suggested that the environment of the inherently susceptible plant has a marked influence on the degree of susceptibility in any given situation. Certain elements of the environment have been studied exhaustively in more recent times, while others are still somewhat neglected.

In the work reported in this paper, consideration has been given to some of the factors which might be expected to affect the degree of susceptibility of a given plant, and more particularly those which might give some clue as to the nature of resistance to this disease. A few of the points dealt with have to do rather with escape than true resistance.

Morphology of the Plant in Relation to Infection

Apart from wounds made by insects and other agencies, the entrance of the blight organism into the plant seems to be limited largely or entirely to natural openings, particularly in the nectaries and to a less extent in certain other flower parts (31), the stomata of the leaves (14), (30), and probably also on the very young fruits (26) and shoots. It now seems probable (8), (17), (36) that penetration from the nectary into the receptacle, at least in the pear and apple, is through openings similar to or identical with the stomata.

There is a persistent impression, however, among pear growers and others, that infections may develop in the older bark of trunks, crotches, or large branches in the absence of any discernible shoot, spur, or wound. With this in mind, in three experiments, pear branches of varying ages up to about seven years were cut into convenient lengths and kept in a highly humid atmosphere until the whitish parenchyma tissues began to be exserted at various places on the surface. The blight organism was then applied to these tissues with a camel’s-hair brush. No infection followed.

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Thomas H, Ark P. 1939. Some factors affecting the susceptibility of plants to fire blight. Hilgardia 12(4):299-322. DOI:10.3733/hilg.v12n04p299

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