96 PAKASITES BELONGING TO THE GENUS GLOMERELLA. 



DIFFERENCES IN VIRILITY OF RACES FROM THE SAME HOST. 



From the experiments of the writers it appears clear that there is 

 considerable variation in the virility of different races or strains 

 of Glomerella just as there is in their morphological characters. As 

 mentioned in the records of inoculation experiments, conidia from 

 cultures from different apples when inserted in the fruit showed 

 considerable difference in the production of rot. Some of these races 

 produced rot more slowly than those derived from other hosts. This 

 fact increases the difficulty of developing a resistant variety of any 

 host as a means of counteracting disease, as has been attempted with 

 cotton and bean. Barrus (9) at first reported certain varieties of 

 beans as entirely resistant to the anthracnose fungus with which he was 

 working. Later he (10) found that when other strains of the 

 fungus were used these varieties became infected with the disease. 

 The observations of Scott (73) that the Ben Davis apple is very 

 severely attacked by bitter-rot in Arkansas, while it is seldom seri- 

 ously injured by the disease in Virginia, although other varieties in the 

 same locality are destroyed, is also perhaps to be accounted for by the 

 presence of strains of different virility in the two regions. A variety 

 or race of a host may be very resistant to a certain strain of a fungus 

 but may succumb to a more virile strain of the parasite at any time. 

 This also accords with the results of animal pathologists as reported 

 by Slack (82) and others in the case of pathogenic bacteria. We 

 must not fail to recognize that the parasite is apparently capable of as 

 great variability in eveiy direction as the host. 



METHODS OF PREVENTION AND CONTROL, 



In the cases in which these diseases are known to be transmitted 

 through the seed, as in cotton and beans, the trouble may be largely 

 prevented by the careful selection of fungus-free seed, as shown by 

 Barre (7) for cotton and Whetzel (95) for beans. The work of 

 Duggar (27) and Barre (7) with cotton, Barrus (9, 10) with beans, 

 and Bain and Essary (4) with clover seems to indicate that more 

 or less resistant varieties may be obtained. In this connection it 

 should be constantly borne in mind, as already stated, that the fun- 

 gous parasite is an organism evidently subject to the same laws and 

 possesses the same or possibly greater capacity for variation, not 

 only in form but in virility, as its host plants. This has been demon- 

 strated by the writers' work, as indicated elsewhere in this paper, and 

 is confirmed by the experience of Barrus (10) and others. Sanitary 

 measures and eradication methods will prove helpful. Cutting out 

 and destruction of cankers where they occur, the destruction or 

 burying through cultivation of mummied fruits and infested leaves, 

 should be practiced wherever practicable. 



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