258 STATE BOARD OF AGRICULTURE. 



SPRAYING FOR TOTATO BLIGHT IN 1905. 



BY C. A. M^CUE. 



Bulletin No. 236. 



For several seasons the potato growers of Michigan have suffered 

 serious losses in their potato crops through the devastation wrought by 

 late blight (PJiytophtJiora infcstnns). The disease was particularly bad 

 during the last season and the loss to growers of the State ran up into 

 thousands of dollars. 



The disease is prevalent in nearly all, if not all, parts of the State 

 where late potatoes are grown. The history of the disease in other states 

 seems to indicate that the trouble is here to stay and that we may look 

 for it in every favorable season. It has been known for nearly two 

 score years that spraying with Bordeaux mixture would prevent the 

 blight to a greater or less extent, but it has been only within the last 

 few years that the knowledge of this fact has been applied to com- 

 mercial potato growing; and then only in a few localities. The prac- 

 tice is by no means common. In Michigan it is hard to find a grower 

 who systematically sprays his potatoes for blight every year. The rea- 

 son for this is not that the grower is careless and neglects the crop, 

 but rather that he is ignorant of the cause of the trouble and is at a 

 loss to know what to do to stop it. In the correspondence carried on 

 by the author with a large number (100) of potato growers in the 

 State it was astonishing to find the large number of growers who did 

 not even known what Bordeaux mixture was; and the majorit}' of them 

 believed the blight to be incurable. The reasons why potato spraying 

 has not been more generally practiced in the Siate might be stated as 

 follows: 



1. The grower is ignorant of the cause of the disease. 



2. He is not aware that there is anything that will prevent it. 



3. He does not believe that the results obtained would pay for the 

 time and trouble of spraying. 



The growers should be set right on these questions as far as possible : 

 hence the mission of this bulletin. No new facts will be found in these 

 pages, but an endeavor will be made to present the cause and the pre- 

 vention of potato blight to potato growers of the state for their con- 

 sideration. 



I. THE CAUSE OF THE BLIGHT. 



Late blight is caused by a fungous disease that works upon the tops 

 of tlie potato plant and which also causes rotting of the tubers. 



The disease is very energetic and spreads with great rapidity, some- 

 times ruining a crop within a few days. It usually makes its pres- 

 ence known soon after the 15th of August in this state and thrives 



