192 Melanose and Stem-End Eot [390 



planation of these geographical limitations may be found. 

 Such problems as this are worthy of serious attention. Some 

 of the logical possibilities of this particular case may be 

 mentioned, by way of preparing for further observations and 

 for constructive experimentation. 



One possibility that always presents itself in connection 

 with a limited geographical distribution of any parasite is 

 that sufficient opportunity or time may not yet have been 

 afforded for the parasite to become distributed throughout 

 the area occupied by the host. But this possibility seems not 

 to apply in the present case. As has been mentioned, after 

 melanose had become common in central peninsular Florida 

 there took place a free interchange of many kinds of citrus 

 nursery stock between Florida, on the one hand, and Cali- 

 fornia and Cuba, on the other. Many carloads of young 

 citrus trees were shipped from nurseries located in the 

 Florida area where this Phomopsis was most virulent, no 

 effective quarantine regulations were in operation at that 

 time, and it is impossible that the fungus has not long since 

 been thoroughly distributed. All or nearly all of the citrus 

 varieties grown in Florida have been planted, at one time 

 or another, in California, and the recent and extensive 

 Cuban plantings have been made with nursery stock from 

 Florida. 



Of course, climatic conditions may furnish an explanation 

 of the facts here dealt with, but the climatic relations of a 

 fungus like Phomopsis citri are probably even more complex 

 than are those of higher plants. For the growth of such a 

 parasite it is not only necessary that the climatic conditions be 

 suitable for this organism, but it is also essential that the com- 

 plex of these conditions be naturally so arranged or balanced 

 that the host-plant may be in just the proper state to favor 

 the virulent development of the parasite. The time factor 

 is especially important in the process of infection; it must 

 happen that the host is in a condition to be readily infected 

 just at the time when the fungus spores reach it. 



