CHAPTER XVI 



Plant Diseases 

 By Donald Reddick 



Some knowledge of the habits of the organism causing a disease is 

 asually necessary in order successfully to combat it and prevent its 

 ravages. Those diseases caused by powdery mildew fungi (which are 

 surface infestations) can be cured. Practically all others must be 

 prevented. 



Fungi attacking parts of plants above ground are usually dissemi- 

 nated by means of spores. Water is often necessary to liberate the 

 spores from the fungus proper, and is nearly always necessary to permit 

 spore germination and infection of other plants. Heavy dew sometimes 

 furnishes sufficient moisture, but prolonged drizzling rains are more 

 favorable. For this reason a fungicide, in order to be effective against 

 such parasites, must be applied before the rain. If it is going to rain 

 to-morrow, spray to-day. But how know whether it is going to rain? 

 This can best be told from a study of the United States weather maps, 

 which are printed and distributed from the many weather stations, or 

 else appear in the daily papers. Storm periods, indicated by a " low " 

 barometer, travel quite regularly from west to east, and are usually 

 accompanied or followed by rain. This can be determined by noting 

 the amount of precipitation, if any, in the wake of the storm. Local 

 conditions are often a factor to be considered. A few minutes' study of 

 the weather map each day will soon make one reasonably efficient in 

 predicting the weather. See Chap. I. 



It is unfortunate that a definite system of naming plant diseases has 

 not been formulated. Diseases of plants of a similar nature should 

 bear the same common name. The term "blight" is commonly used 

 for many kinds or forms of diseases. It might well be restricted to 

 bacterial diseases like fire-blight of pear or bean blight. When some 

 definite system of naming diseases is adopted, it is likely that a tabula- 



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