BACTEKIAL DISEASES OF PLANTS 87 



Exercise 127. Study of Oidium (Oospord) lactis 



This fungus is frequently found in milk or milk products and 

 is believed to be of some importance in "cheese manufacture. 



It forms chains of barrel-shaped cells which readily break up. 



It may usually be obtained from old flasks of sour milk, 

 where it forms a superficial pellicle. From these places pure 

 cultures may be obtained. 



1. Put raw skim milk to a depth of 3 cm. into each of two 

 1-liter flasks. Plug them with cotton. Heat one of the flasks to 

 75 C. for ten to fifteen minutes. 



2. Keep both flasks at room temperature for three or four 

 weeks, noting the changes which occur in the milk. Note the 

 changes in acid content. Note the sequence of organisms which 

 grow in the flasks. 



3. Obtain 0. lactis from the velvety pellicle on the surface of 

 the raw milk. 



4. Make cultures on gelatin stab, agar slope, potato, milk, 

 and litmus milk. 



5. Make stains with dilute methylene blue and gentian violet. 



SECTION XIV 

 BACTERIAL DISEASES OF PLANTS 



A relatively small number of bacteria are known to produce 

 diseases in planis. As further investigations are made the number 

 will no doubt be increased. Some plant diseases due to bacteria 

 are important because of their destructiveness and the great 

 difficulty experienced in attempting to control them. 



Several distinct types of disease are produced by organisms 

 belonging to 'this group : The blights are represented by the 

 destructive pear blight, caused by B. amylovorus ; galls and 

 tumors by the crown gall of nursery trees, caused by Bad. tumc- 

 faciens ; leaf spots by the bacterial leaf spot of stone fruits, 

 caused by Bart, pruni ; wilts by the wilt of sweet corn, caused 

 by Pseudomonm stewartii ; and rots by the black rot of cabbage, 

 caused by B. campestris. 



