Epidemic Plant Diseases. F. T. Brooks. 



■^OD 



stages is present only on a few plants in the crop. Spores are 

 produced on these plants and are then distributed by \nnd so 

 that other plants in the \-icinity become infected. The same con- 

 siderations apply to mildews and rust fungi. The power of ^^ind, 

 however, to disseminate fungoid diseases over immense areas, has 

 undoubtedly been exaggerated. It used to be thought that disease 

 could be spread in this manner from one country to another over 

 \\'ide tracts of sea, but this has been sho^^^l to be untrue. Delicate 

 fungus spores which have been in the air a long time have prob- 

 ably been exposed to many \icissitudes of temperature, insola- 

 tion, and desiccation. These changes greatly impair their \-itality 

 and by the time they reach a plant susceptible to infection, most 

 of them are dead. It has been demonstrated for instance that 

 bacterial germs are rapidly killed by exposure to strong light, and 

 doubtless the same appplies to many fungus spores. Further- 

 more, spores being Ught bodies are to a great extent carried by 

 convection currents to the upper atmosphere, where conditions 

 are of a more extreme nature, and from which many of these 

 germs will not fall in a h\'ing condition. 



The limited power of \%-ind currents to disseminate fimgoid 

 diseases is weU illustrated by certain heteroecious rust parasites. 

 One of these [PiicciniaPruni-spinosae) alternates between certain 

 species of anemone and pliun trees, in the former of which the 

 fungns lives indefinitely, producing spores each spring. These 

 spores infect pliun leaves, but it is only the trees in the immediate 

 \"icinity of the infected anemones, especially in the direction of 

 the prevailing \Wnd, that first become attacked. The spores pro- 

 duced on the anemone seem only to be able to infect the alternate 

 host within the range of about a mile or so, and it is not until 

 secondary spores are formed on the plimi leaves that the fungus 

 spreads over wider areas. These secondary spores are formed 

 witliin about ten days of infection, and the fungus is propagated 

 by these during the summer in ever-widening circles. 



Nowadays emphasis is laid upon the need for contact in the 

 dissemination of many parasitic organisms. By maintaining 

 contact, is meant bringing the parasite into the immediate 

 \icinity of a healthy and susceptible host by means of some 

 li\ing or mechanical agency. This has long been recognised in 

 human diseases, for, in many of these, indi\iduals only become 

 attacked by coming into close association with others already 

 affected, notwithstanding the fact that the germs of many of 

 these diseases must be commonly present in the air. The in- 

 ability of the germs casually present in the air to cause infection 

 is due partly to the fact that their \itality is rapidly destroyed 

 bj' exposure to bright sunhght and partly to the germs being 

 present in quantity less than the minimum requisite to cause 



