APPLE DISEASES 



459 



ant on slender stems. The galls may 

 also interfere with sapflow and the decay 

 of soft galls usually involves adjacent 

 healthy tissues producing serious wounds 

 or in some cases girdling the stem with 

 death of the plant as a result. One of the 

 worst effects of the disease is the frequent 

 prevention of normal root development 

 resulting in the failure of a young tree to 

 establish itself or in the retarding of 

 its growth. 



Cause 



Crown gall has been known and recog- 

 nized as a serious disease for many years. 

 Until comparatively recently, however, 

 the cause was unknown. The careful 

 experiments begun in 1904 by Smith, 

 Brown and Townsend. of the United 

 States Bureau of Plant Industry, have 

 proved conclusively that the disease is 

 caused by a bacterium, under the name 

 Bacterium tiimefaciens. 



The bacterium causing crown gall is 

 an organism which can exist in the soil. 

 It seems to be widely scattered in many 

 soils, but appears especially abundant 

 in nurseries and in land where plants af- 

 fected with crown gall have previously 

 been grown. Plants in a young, tender 

 and rapidly growing condition, are most 

 subject to infection, whereas older and 

 more mature plants are not so frequently 

 attacked. The wounds made in root- 

 grafting and budding offer a favorable 

 point of entrance for the germs, and in- 

 juries to the underground parts of plants 

 by cultivation, attacks of borers, etc., 

 render them susceptible to infection. In 

 fact, the organism may be considered a 

 wound parasite. 



The Effect of tlie Organism 



In order to cause infection the bacteria 

 must enter into some part of the plant 

 where new cells are in the process of 

 formation. They make their way into the 

 living cells and stimulate them to abnor- 

 mal and very rapid multiplication. Cells 

 near those containing bacteria may per- 

 haps be stimulated in the same way. The 

 number of bacteria in any one cell is not 

 large. They multiply slowly and do not 

 appear to injure the cell to any great ex- 

 tent, but merely to stimulate rapid di- 



vision. When a cell containing bacteria 

 divides into two, each of the daughter 

 cells contains some of the bacteria and 

 hence will be stimulated to rapid multi- 

 plication. Thus, as a result of the pres- 

 ence of the bacteria, there is formed an 

 abnormal mass of rapidly growing tissue 

 in which the elements become distorted 

 and twisted. It is an unnatural, wasting 

 growth, in no way adapted to the needs 

 of the plant. Such galls or tumors vary 

 greatly in form, size and consistency, de- 

 pending apparently upon the virulence of 

 the particular germ causing the infection, 

 the kind of plant or variety which has 

 been attacked, the particular tissues 

 which were first Infected, etc. Even in- 

 dividual plants of the same variety show 

 great differences in susceptibility to this 

 disease. 



The Soft Gall 

 One of the commonest forms of crown 

 gall especially on herbaceous plants and 

 cane fruits, is the fleshy form in which 

 the outgrowth is somewhat soft. These 

 grow rapidly and are not protected by a 

 bark or corky layer. Growth usually 

 commences in the spring (sometimes 

 earlier). After a few months of develop- 

 ment the outer layers of the gall begin 

 to die and are attacked by various bac- 

 teria and fungi. This generally ends in 

 the decay and sloughing off of nearly 

 the whole tumor by the end of the sea- 

 son. The margin of the gall, however, 

 usually remains alive and during the 

 next season the tumors grow out again, 

 followed as before by decay. When the 

 gall dies some of the healthy substance 

 of the plant is usually killed and a 

 wound is formed which renders the en- 

 trance of other diseases easy. Galls are 

 known to die off completely in some cases 

 at the end of the first or second season, 

 while the plant recovers; but commonly 

 the tmnors reappear from year to year. 



The Hard Gall 



Not all galls are of this soft type, but 

 many are very hard and woody, in which 

 case the growth is apt to be slower and 

 the gall persists longer — often for many 

 years. The exterior may develop a bark- 

 like covering and instead of a rapid decay 



