A GRASS-KILLING SLIME MOULD. 



By JOHN W. HARSHBERGER, Ph.D. 



(Read November 3, 1906.) 



It has been known for a number of years that the club root, or 

 finger and toe disease of cabbage, radish, turnip and other crucifer- 

 ous plants is due to a slime mould {Plasmodiophora brassicce Wor.) 

 which gains entrance to the root region of the host plant.^ Each 

 spore gives rise to an amoeboid cell which with other similar cells 

 lives in the roots of the host in a truly parasitic manner, finally 

 destroying the cells which it inhabits. These amoeboid cells cause 

 great stimulation of the tissues of the host which enlarge to form 

 large swellings, and the plant, as a result, fails to head out. The 

 roots soon decay and the spores which have formed in the cells of the 

 host are again set free. A somewhat similar disease described by 

 Toumey- is called crown gall and is found at the base of young 

 cherry, plum, peach and apricot trees throughout the United States. 

 The amoeboid cells that are formed from the spores enter the stem 

 tissues of the host at the crown (a point where stem and root 

 join) and produce there hypertrophied tissue known as crown gall. 

 Minced galls, if used to inoculate healthy trees, will communicate 

 the disease. This slime mould which produces true sporangia within 

 the dead tissues of the host differs from the one on cruciferous plants 

 and has been named Dendrophagus globularis. In both of these 

 cases, the plasmodium exists in symbiosis with the protoplasm of the 

 host plant as a mycoplasm, and this mycoplasm can be distinguished 

 after treatment with osmic acid, which differentiates it. 



On August II, 1905, my attention was called to a lawn in Cyn- 

 wyd, Montgomery County, Pa., the grass of w^hich had been de- 

 stroyed in spots by what the owner, Mr. H. P. Gardner thought 



'Frank, Dr. A. B., "Die Krankheiten der Pflanzen," II, 14-18; Sorauer, 

 Dr. Paul, "Handbuch der Pflanzen-Krankheiten," II, 64-72. 



" Tourney, J. W., " An Inquiry into the Cause and Nature of Crown 

 Gall," Arizona Agricultural Experiment Station Bulletin 33, April 13, igoo. 



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