240 



Late Blight 

 Caused by Phytophthora infestans (Mont.) De Bary 



In most of the states where potato growing is impwrtant, the most 

 dreaded disease is late blight. In Illinois it was first reported in 1882 

 by A. B. Seymour, who collected specimens August 7 at Camp Point, 

 Adams county, and at Freeport, Stephenson county, September 13. A 

 collection by Burrill, without date or place, is reported by G. ^^^ Wilson.' 



These are the only records of the occurrence of late blight in Illinois. 

 It is noteworthy that in all the examinations of potato fields made by our 

 Survey during the past three seasons no instance of late blight has been 

 foimd. It has never been known to cause serious loss in Illinois. 



Scab 



Caused by Actinomyces scabies (Thax.) Giiss. 



Scab is a very common tuber disease of the potato in Illinois. It is 

 characterized by roughening and pitting of the tuber. It first appears 

 on the surface of the tuber as a tiny reddish or brownish spot, which 

 increases in size, deepens in color, and eventually develops the rough, 

 corky incrustation so commonly seen. 



It has not been widely reported in Illinois. It was reported present 

 in Champaign and Coles counties in 1911, and in Champaign county in 

 1912 by Burrill. It is now known to occur in 16 counties, the distribu- 

 tion of which is shown on Map 98. This indicates a wide range over the 

 state, and it is probable that further search will prove the disease to be 

 much more general than it now appears to be. Observations made in 

 192.3 indicated an injury equivalent to a crop reduction of approximately 

 1 per cent. In terms of the 1923 yield, this is equivalent to 96,000 bushels 

 valued at $84,400. 



Control of scab is difficult, since the disease infests soil in which it 

 has once occurred. Rotation of crops on infected soil, treatment of in- 

 fected seed-potatoes with formalin or mercuric chloride, and withholding 

 fertilizers which favor scab development serve to keep the disease in 

 check. 



Black Leg 

 Caused by Bacillus astroscpticiis van H. 



This is a bacterial disease which causes a stunting of the plants and 

 a rotting of the stem below the ground, later resulting in the death of the 

 plant. It was present, but not seriously abundant, in 1923 in three coun- 

 ties, Logan, Lawrence, and Monroe. It is seed borne, and prevention 

 demands careful selection and treatment of seed tubers. 



' Bui, Torr. Bot. Club. 34 : 392. 1907 



