EXPERIMENT STATION BULLETINS. 349 



The investigations conducted upon this disease have shown that the trouble 

 in the Upper Peninsula is caused by a bacterial organism which is carried 

 from year to year in diseased tubers. The trouble is spread for the most 

 part in storage bins where diseased tubers rot and infect the neighboring 

 tubers. When such tubers are planted high percentages of Blackleg in the 

 field is the result. It has also been found that the disease will wdnter over 

 in diseased tubers which are left in the ground. Potatoes should not, there- 

 fore, follow potatoes in rotation and at least four years should intervene 

 between crops. 



Blackleg of potatoes can very easily be controlled by treating the seed 

 tubers with corrosive sublimate, as described for Rhizoctonia, and by rogue- 

 ing out diseased hills during the growing season. It is important that the 

 rogueing be severe and that the entire hill, showing the disease, be dug up. 

 Quite often hills will be found in which only one or two stems show the char- 

 acteristic blackening while the others are apparently healthy. Should these 

 be left in the field they too will later go down with the disease and produce 

 tubers which will spread the disease during the storage season. 



HOPPERBURN (tIPBURN). 



Tipburn of potato leaves has been known for a long time and until recent- 

 ly was thought to be due to weather conditions. It was noted that during 

 clry seasons the trouble was always severe and it was thought that the burn- 

 ing of the tips and margins of the leaves was due to sun effects. In 1918 

 Dr. E. D. Ball of Wisconsin, demonstrated that a leaf hopper (Empoasca 

 mali LeB.) was the cause of the trouble. Dr. Ball's discovery, however, 

 was so revolutionary that plant pathologists and entomologists alike were 

 rather hesitant in accepting it as a fact. It was to confirm or contradict Dr. 

 Ball's work that the Hopperburn investigations were undertaken. By 

 means of cage experiments in 1919 it was conclusively demonstrated that 

 leaf hoppers were the cause of the burning of the tips and margins of potato 

 leaves. Results of these experiments were reported in Ph;y-topathology 

 10:61-62,1920. 



Fig. No. 20. Cages used in leaf hopper experiments. Hopperburn was produced in cages into which 

 leaf hoppers were introduced. No burning resulted in the cages which were kept free from aU 

 insects nor in those cages into which plant lice were placed. 



