THE MONTHLY BULLETIN. 



CALIFORNIA STATE COMMISSION OF HORTICULTURE 

 Vol. V. June, 1916. No. 6 



CROWN GALL OR PLANT CANCER. 



By Clayton O. Smith, Instructor in Plant Pathology, Citrus Experiment Station, 



University of California. 



The disease called crown gall, black knot, plant tumor, or plant 

 cancer, has for manj^ years been well known to the horticulturist and 

 the plant pathologist. It is only within a few years that the cause of 

 the trouble has been thoroughly investigated by Dr. Erwin F. Smith 

 and his associates of the United States Department of Agriculture, and 

 their bulletins are available to any who may be especially interested in 

 this trouble. 



Crown gall is nearly world-wide in its distribution, being found in all 

 parts of the United States, in Canada, South Africa, Asia, the countries 

 of Europe, New Zealand, Australia, Mexico and probably also in South 

 America. 



The malady is a serious one on many of our orchard trees, as it not 

 only decreases their vigor and productiveness, but also at length is a 

 frequent cause of their death. A large number of different species of 

 trees are susceptible. In fact, the organism causing the disease is 

 almost omnivorous as to its choice of a host, but it is especially severe 

 on the more important economic trees, such as the apple, almond, 

 apricot, cherry, grape, peach, plum, prune and walnut. 



GALLS OR KNOTS. 



The disease, when once seen, is easy to identify, as it is characterized 

 by an enlargement or growth of soft, spongy tissue of a more or less 

 spherical shape. These growths are usually situated just below the 

 surface of the ground on the trunk or body of the tree, often extending 

 downward some distance on the larger roots, and may be found on them 

 at some distance from the trunk. The galls continue to increase in size 

 from year to year until a large excrescence may be formed. The tissue 

 produced is abnormal and easily decays, making an especially favorable 

 place for wood-destroying fungi to gain an entrance to the tree. While 

 the galls are usually below the ground, occasionally aerial ones are 

 formed on the trunk and large branches. These are hard and cause 

 little or no serious effects on the tree, except when they may be numerous 

 as on grapes of the Vinifera varieties. 



Trees affected with the disease may not die for some years, indeed 

 will not until the trunk is entirely surrounded with diseased gall tissue. 

 The affected trees usually do not make as rapid growth as healthy ones, 

 but for a time produce a large amount of fruit. The cause of the abnor- 

 mal amount of fruit is the same as in twigs that have been ringed to make 

 them productive. The root of the tree suffers most and soon ceases to 

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