PROBLEMS IN PREVENTING PLANT-DISEASE EPIDEMICS 75 



is estimated that spraying to control this disease and certain insects consti- 

 tutes as much as 50 per cent of the operating costs in banana production in 

 some areas. But bananas could not be grown successfully without the spraying. 



The late blight of potatoes also is reasonably well controlled by spraying. 

 But it is necessary to spray the plants from four to twenty times in order to 

 control the disease. The cost in certain potato-growing areas is at least $35 an 

 acre; consequently attempts have been made to develop resistant varieties 

 that do not require spraying. But there are many physiologic races of the 

 late-blight fungus, and most varieties have therefore been only temporarily 

 resistant. The problem here, as with stem rust of wheat, is to produce a variety 

 that resists all physiologic races under all conditions. Many crosses have 

 been made between wild potatoes that are resistant but otherwise of no value 

 and cultivated potatoes that yield well in the absence of blight. Whether it 

 will be possible to control blight permanently by means of resistant varieties 

 alone remains to be seen. Every increment of resistance that can be incor- 

 porated into potatoes, however, will help to reduce the cost of spraying. 



Theoretically, plant-disease epidemics could annihilate certain species of 

 plants entirely unless man intervened. The danger is particularly great when 

 new disease organisms are introduced into an area where they have not 

 previously existed, because neither nature nor man has selected for resistance 

 against them. The chestnut blight fungus, which is a native of the Orient, was 

 unwittingly introduced into the United States on ornamental chestnuts about 

 1904. It rapidly spread from New York, where it was first found, through- 

 out the range of the very valuable American chestnut. Desper-^te attempts 

 were made to check its spread by cutting barrier zones in which all chestnuts 

 were cut down, but it was a losing fight, because birds and other agents of 

 spore dissemination ignored the barriers. The chestnut forests of the United 

 States have been annihilated. Most of the species of Oriental chestnuts, on 

 which the blight has long existed, have at least sufficient resistance to enable 

 them to grow successfully; but the American chestnut was so susceptible 

 that it quickly succumbed, and there seems to be no chance that the forests 

 will be regenerated because there appears to be no resistance within the 

 species. 



Even if the permanent epidemic development of introduced pathogens 

 can be checked, there is always the problem of cost. The bacterium that 

 causes citrus canker was introduced into Florida several decades ago and 

 threatened the existence of the industry. The disease has been brought 

 under control, but only by literally burning the disease out by actually 

 putting a flaming torch to all infected trees and reducing them to charcoal. 

 It was a long and expensive campaign, but it was successful. The white pine 

 blister rust and the Dutch elm disease also are importations from abroad and 

 have become epidemic in certain areas, have gradually extended their range, 

 and are a menace to a group of our best forest trees and one of our best 



