14 DISSEMINATION OF PARASITIC FUNGI 



Another large giou]) of cases could be selected based on the total absence of 

 specific diseases from certain areas, though long known elsewhere. Particular 

 stress is not laid on these cases, since we do not always know the factois which 

 govern acclimatisation to a new locality, and have some reason to believe that 

 they are often more restrictive in their effects where parasitic fungi are con- 

 cerned than in respect to the host plants. It seems better, therefore, to icly 

 more on cases like those quoted above, where subsequent experience has 

 proved that the more or less prolonged absence of a disease from a cei-tain 

 locality was not due to any want of ability on the part of the parasite to live 

 in that region when introduced. 



One of the most remarkable of this group of cases is the absence of 

 the common yellow rust of wheat {Puccinia glumarum) from 

 North America, Australia, and South Africa. The other two 

 wheat fusls (Puccinia graminis and Puccinia triticina) occur 

 practically wherever wheat is cultivated, though in India, 

 P. glumarum has actually a wider range than P. triticina. Why 

 yellow rust, which stands the extremes of cl'mate in all the 

 wheat-growing parts of India well, should fail to attack wheat 

 in the United States, Australia or South Africa is a mystery, 

 unless indeed we accept the simple explanation that its spoies 

 have not yet reached these areas. ^ 



The lilac is attacked by two diseases of restricted distribution. Iii 

 Germany and Holland a blight caused by Phytofhthora 

 Syringce has been prevalent during the past few years ; in the 

 United States a mildew (a foim of MicrosfJicera Alni) is very 

 abundant on it ; neither of these have been recorded elsewhere, 

 at least up to 1914 for the former ai;d 1908 for the latter. 



There are two serious diseases of tobacco in the Dutch Ir.dies. Ore, 

 the bacterial disease caused by Bacillus solanacearum, attacks 

 several other plants, including the potato. It is of very wide 

 distribution in most tobacco-growing countries of the w'oild 

 and can leadily be conveyed in potato tubers and on other 

 living plants. The other, caused by PhytojjhtJwra NicGtiancE. 

 is not known on anything but tobacco and has not extended 

 beyond the Dutch Indies. It is probable that living tobacco 



' Since the above was written yellow rust of wheat has been reported in the United States 

 (Phytopathology, VI, 191 C, p. 96). The area affected is as yet limited, and it is probably £^ 

 fecent introduction. 



