672 Wart Disease (Black Scab) of Potatoes, [dec, 



tuber, and later several warts by growing together form a brown 

 spongy scab, which finally becomes black. 



Black scab has no connection with ordinary potato scab and 

 does not resemble it in appearance, but as both are frequently 

 Jound on the same plant they are sometimes confused. 



Although the disease has been known to potato growers in 

 the Liverpool district for some fifteen years it was not reported 

 to the Board until 1901. Since then a number of cases have 

 occurred in Lancashire, Cheshire, Shropshire, Staffordshire, 

 and Warwickshire, while a few outbreaks have been recorded 

 in other English counties, in Wales and in Scotland. 



The disease has caused most damage in gardens or allotments 

 where potatoes are grown every year, but in a few cases there 

 has also been serious loss in potatoes grown in a four-course 

 rotation. Land may be unfit for potato-growing for as long as 

 six years after infection. 



The occurrence of the wart disease in the affected districts 

 is similar to that of the Finger-and-toe disease (Leaflet No. 77) 

 which, on certain soils and in some seasons, has caused serious 

 damage to turnips. Although in the first instance only a few 

 plants may show the wart disease, it is almost sure to spread 

 more or less rapidly and the disease must be treated as a 

 dangerous enemy, which, if neglected, may entirely prevent the 

 growth of potatoes. 



Cause of Infection. — The diseased tubers or haulms contain 

 numberless " spores " (the " seeds " of the disease) which are 

 not only capable of infecting healthy potatoes the following 

 season, but may also lie dormant in the soil for more than four 

 years. 



The diseased tuber is the usual source of infection and the 

 spread of the disease from one holding to another is mainly to be 

 attributed to the planting of infected sets. 



When disease appears it may, if neglected, spread over a 

 farm and render the soil useless for potato growing in the 

 course of a few years. It is spread by manure, by the decay of 

 affected haulms, tubers, and runners in the soil, and it may even 

 be carried from one field to another on implements and boots. 



Infection is very readily conveyed by manure if raw diseased 

 tubers are fed to stock, or if diseased haulms are thrown on the 

 manure heap. Dung which has been contaminated in this way 



