WART DISEASE OF POTATOS. 



309 



name now given is 5. endobioticum Perc. Later, Behla (10) worked 

 out the life-history, and more recently Miss Curtis (22) has recorded 

 that sporangia continue development after decay of the host and 

 give rise to numerous uniciliate zoospores, which, after a period of 

 motion, enter the epidermal cells of any actively dividing region. Pre- 

 sumably there is a resting period in the soil before these zoospores are 

 liberated. In view of the fact that these sporangia are known to lie 

 in the soil for at least 10 years, the question arises as to their state 

 during this time and the conditions which cause their rupture. At 

 present we have no evidence, but it seems probable that some secretion 

 of the potato plant is necessary for germination, or during the years 

 in which no potatos are grown germination would take place and 

 zoospores would die of starvation. 



Miss Curtis's paper reveals the fact that sori are formed from which 

 facultative gametes are liberated. These gametes may infect the host 

 directly, giving rise to other sori (Johnson's summer sporangia), 

 and this process may go on several times during the growing season, 

 as it only takes about three weeks. On the other hand, fusion of 

 gametes may take place and a zygote be formed. Infection by a 

 zygote which becomes uninucleate will give rise to a resting 

 sporangium, but this process takes months. Orton and Kern (55) 

 also describe the entrance of the parasite into the host cell, and infer 

 there is a special attraction of the parasite to the host nucleus. 



Spread of the Disease. 



It is obvious, from the early history of the disease which has been 

 dealt with previously, that the disease can be carried by infected 

 seed. It is known that tubers from an infected plant which are 

 apparently quite healthy may bear the sporangia, and these release their 

 zoospores into the soil. Any growing plant within the range of the 

 zoospores therefore becomes infected and sori are formed, the process 

 being continued throughout the growing season if soil conditions 

 are favourable. Fresh districts become infected by " seed," but 

 the disease is spread locally by want of care in the use of tools and 

 implements, and chiefly by feeding diseased material to animals, 

 especially cows and pigs. In the early days of Wart Disease case after 

 case of whole fields being spoilt by infected manure came to the notice 

 of the authorities, as the resting sporangia passed through the animals 

 unchanged. Although not wind-borne, doubtless cases have occurred 

 where the spread of the disease was caused by pheasants and crows, 

 which are fond of pecking at the warty mass between the haulms, 

 dropping sporangia with their excreta. 



The slow spread of the disease in its early days is a matter of 

 surprise to some persons. It must be understood, however, that the 

 idea of change of " seed " had only recently become generally accepted 

 and the disease spread very slowly and locally in its early stages, 

 as it was then outside the usual " seed "-growing districts. When it 



