POTATO DISEASES 



1719 



The plants do not die quickly, as they 

 do when attacked by Fusarium wilt, but 

 may live nearly as long as healthy ones. 

 The growth is checked and the formation 

 of tubers prevented. Often no potatoes 

 are set, or only small ones clustered 

 around the base of the stem, while num- 

 erous rudimentary tubers are formed on 

 the stolons. 



The browning of the woody part of the 

 potato stem and the presence of a brown 

 discolored ring at the stem end of the 

 tubers is not so much a character of the 

 leaf roll, but is rather to be taken as an 

 indication of the presence of another dis- 

 ease, the Fusarium wilt. 



The formation of aerial tubers is some- 

 times a feature of leaf roll, but in other 

 cases is the result of stem cankers caused 

 by the fungus RMzoctoma. 



Leaf roll is considered to be hereditary 

 through the seed potatoes; that is, if po- 

 tatoes borne on plants affected by leaf 

 roll are planted the resulting crop will 

 be diseased and usually much worse than 

 the first crop. 



The cause of leaf roll remains unknown, 

 though it has been prevalent in Europe 

 since 1905, and has been given much study 

 there. It is now believed to be a physio- 

 logical disorder rather than one caused by 

 a parasite. Many consider it due to some 

 unfavorable soil or climatic condition, but 

 no one has been able to show what condi- 

 tions produce it or how it may be con- 

 trolled by any cultural practices. 



No fully satisfactory remedy for leaf 

 roll has been discovered. We have, how- 

 ever, the benefit of seven years of Ger- 

 man experience with the same trouble. 

 The anxiety caused by its appearance in 

 Germany has been somewhat allayed with 

 the passage of time, and the best author- 

 ity on potato diseases there even states 

 that through the awakening of interest in 

 better culture and in improvement of seed 

 the leaf roll will prove in the end a bene- 

 fit to German agriculturists, and their po- 

 tato production will be permanently in- 

 creased. 



Leaf Soobch. See Tip Burn, this sec- 

 tion. 



Lime- Sulphur Dwarfs Potato Plants 

 Bordeaux Best for Potatoes 



Lime-sulphur solution cannot replace 

 Bordeaux mixture as a preventive of po- 

 tato diseases. Orchardists who also grow 

 potatoes hoped that they might use the 

 lime-sulphur spray in the field as well as 

 in the orchard and dispense with the Bor- 

 deaux altogether, as it would be conven- 

 ient to prepare only one fungicide; but a 

 careful test made at the Geneva (New 

 York) Station in 1911 proves the lime-sul- 

 phur harmful to potatoes. The plants in 

 rows sprayed with lime-sulphur were 

 dwarfed by the fungicide, died early and 

 yielded about 40 bushels less to the acre 

 than plants in check rows; while the Bor- 

 deaux-sprayed rows produced 100 bushels 

 to the acre more than the checks. 



Geneva (N. Y.) Bulletin 347. 

 Little Potato. See RhizoctoniUj this 

 section. 



Potato Scab 



Oospora scaUes 

 F. D. Bailey 



Potato scab is a fungous disease that 

 is prevalent in Europe and the United 

 States and probably in all countries 

 where potatoes are raised. The chief loss 

 is in the depreciation in value due to the 

 appearance of affected potatoes, although 

 it is claimed that the yield is also re- 

 duced. 



In the Northwest scab is prevalent 

 only in fields that have been heavily 

 manured, where wood ashes have been 

 applied, or in alkaline soils. An abun- 

 dance of moisture also favors its spread 

 and development. 



Symptoms 



This disease is confined to the tubers 

 and is readily recognized by the charac- 

 teristic rough corky patches on the sur- 

 face (Pig. 1). The affected spots may be 

 quite deep, even forming cracks if the 

 attack is made when the potatoes are 

 small- The lenticles or openings in the 

 surface are the points more often af- 

 fected. The first indication is a small 

 reddish-brown surface spot; this grows 

 both outward and downward and soon a 

 brown corky growth begins to form over 



