68 PROTECTION OF PLANTS, 1921-22 



(c) Common scab {Actinomyces scabies.) 



(d) Skin spot {Oospora pustulans). 



(e) Silver scurf {S-pondylocladium atrovirens). 



(f) Dry rot of tubei's (Fusarium spp) 



(g) Fusarium net-necrosis {F.oxysporum). 

 (h) Black clot disease (Vermicularia varians). 



8. Diseases caused by conditions which adversely affect the normal phy- 

 siological processes: 



(a) Black heart. 



(b) Frost necrosis. 



(c) Net necrosis. 



(d) Internal brown spot. 



(e) Spindling sprout. 



(f) Hollow heart. 



(g) Arsenical injury. 



9. General considerations in control. 



It will thus be seen that there are some twenty or more common potat 

 diseases with which the growers may have to contend at different seasons. 



GROUP 1. 



In this group are placed three (or four) diseases in which insects play an 

 important role. 



(a) Hopperhurn, or "tipburn" as it used to be called, occurs periodically 

 whenever dry conditions prevail in potato growing areas. It was noted first in 

 Iowa in 1876 by Osborn, but wilting of early varieties was the most pronounced 

 characteristic. Osborn recorded the presence on such diseased plants of Empoasca 

 Mali (Le Baron) known as the leafhopper. In 1908 its work was observed in 

 New York, and again in 1909. Fraser reported it from Quebec in 1913, it was 

 again serious in Iowa in 1915, and in Vermont in 1917. Ball in Wisconsin, in 

 1918, studied the disease which that year extended from Montana and Kansas to 

 New York and New Jersey. This year the leafhopper was very prcA^alent in 

 Quebec and hopperburn is certain to have caused considerable loss (1921). 



Symptoms of the Disease 



The first signs of the hopperburn usually appear during the latter part of 

 July or early in August with most severe effects from the middle of August to 

 the middle of September. On some of the lower leaves a slight yellowing begins 

 at the tip or edge of terminal leaflets. As the injury progresses the yellow areas 

 become brown, curl up and dry out. Thus the leaflet finally possesses a dark- 

 brown, dried, uprolled margin with a band of green in the middle bordering the 

 midrid (see plate). The remaining leaflets gradually become affected so that 

 the whole leaf is involved (plate). When the season is hot and dry such leaves 

 shrivel completely, the petiole gradually dries towards the stem and hangs limp 

 so that a touch will cause it to drop. Whole fields may succumb to this injury 



