10 DISEASES OF CROP-PLANTS 



increased local susceptibility, which results in the death of the 

 cells at the point of infection and prevents the mycelium from 

 establishing the necessary connection with living tissues. 



The production of resistant strains of cultivated plants has 

 usually been accompHshed by the propagation of chance-found 

 individuals showing the required powers. A frequent defect of 

 such varieties is that resistance may not happen to be associated 

 with a high standard in other desirable qualities. Some more 

 or less successful attempts have been made to secure the desired 

 combinations by crossing, a seductive but uncertain line of work 

 which has possibly had more advertisement than the stability 

 of the results yet warrants. 



The most striking example of the use of resistant varieties in 

 West Indian agriculture is the replacement of the Bourbon cane 

 by seedlings resistant to red rot and less susceptible to root 

 disease, a measure which has preserved the sugar industry of 

 these islands. 



Susceptibility to Exotic Diseases. 



It seems most probable in the light of the available evidence 

 that the failure of the Bourbon cane in the West Indies arose 

 from the introduction of a fungus to which it had not previously 

 been exposed. In this manner have originated many of the 

 great epidemics of plant disease. Where host and parasite are 

 both native or have been long associated there is a natural 

 selection of resistant species or strains and the parasite is 

 frequently inconspicuous in amount and in its effects. Introduced 

 into a new country the parasite may find appropriate hosts quite 

 unable to withstand it. The chestnut bark disease, introduced 

 a few years ago into the United States on resistant species from 

 China, has made a clean sweep of the extensive forests of the 

 American chestnut in the Eastern States, and shows no real 

 abatement in its progress. Similar occurrences have been the 

 introduction of the mildews of the grape-vine and the gooseberry 

 from America to Europe, of the white pine blister rust in the 

 reverse direction, of citrus canker from Japan to Florida and 

 South Africa, and, according to the most probable theory, of the 

 introduction from Africa of the coffee leaf disease which nearly 

 exterminated the main industry of Ceylon. 



A similar result may come from bringing exotic plants within 

 the range of native parasites, especially those occurring on related 

 species. The leaf blister mite of cotton in the West Indies, which 

 appears to be indigenous and is known to attack native Mal- 

 vaceous plants, caused first an epidemic and would now result 

 in an annual infestation of the Sea Island crop if a close season 

 were not enforced. The witch-broom disease of cacao in Surinam 

 comes from a wild Theobroma in the forests of that country. 

 The fungus causing pink disease of various cultivated trees and 

 shrubs, one of the serious troubles of the Malayan rubber planter. 



