88 A MANUAL OK BACTERIOLOGY 



Remedial measures against bacterial diseases are generally 

 unsuccessful. Crop sanitation and the use of immune or resistant 

 varieties are the only practices which have as yet afforded any 

 relief. The student should consult 



SMITH. Bacteria in Relation to Plant Diseases, Publ. 27. Carnegie Inst. of 

 Washington, 1905-1911. 



VAN HALL. Bijdragen tot de kennis der Bakterieele Plantenziekten. Amster- 

 dam, 1902. 



MARSHALL. Microbiology. Philadelphia, 1911. 



JORDAN. General Bacteriology. Philadelphia, 1910. 



DUGGAR. Fungous Diseases of Plants. Boston, 1909. 



CHESTER. A Manual of Determinative Bacteriology. New York, 1901. 



Exercise 128. The Blight of Pome Fruits Caused by Bacillus amylovorus 



1. Note the general condition of pear or apple trees in various 

 stages of blight. What parts of the trees are worst attacked ? 

 Look also for cankers on the bark of the trunk and larger 

 limbs. How does the appearance of the bark indicate the progress 

 of the disease ? If the weather is moist in spring or early summer, 

 look for brown, sticky beads on the twigs or blighted fruits. If 

 the. weather is dry, the only evidence of the beads may be a dry, 

 glistening spot. Place such material in a moist chamber with 

 the ends of the twigs in water. What is the consistency of these 

 beads ? 



2. With a sharp knife cut through diseased twigs. In what 

 tissue is the destructive action of bacteria most prominent ? 

 What evidence is there that bacteria are transferred by pruning 

 instruments or by insects ? 



8. Cut some vigorous twigs from the tree and carry them to 

 the laboratory. Cut the ends under water and stand them in a 

 small jar of water. Prick through the bark of the new growth 

 with a steel needle previously sterilized and dipped into a bead 

 on a diseased twig (or a pure culture of the organisms). Watch 

 subsequently for infection. Collect some immature fruits, inocu- 

 late as above, and keep under a bell jar. 



4. Study the growth of the bacillus on agar slope, gelatin 

 stab, bouillon, milk, and sterilized pear twigs. Make stains with 

 carbol-fuchsin and Gram's stain. 



