374 



JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



The warty condition is not so evident after the tubers have been 

 removed from the soil for a time. The warts become flattened and 

 discolom'ed, so that raised, more or less chocolate-coloured scars 

 remain in their stead. This is clearly shown in fig. 99 e, which should 

 be compared with fig. 99 a. 



The warts frequently develop in a group at the end of the tuber 

 away from the point of attachment to its underground stalk. This 

 particular form of the disease is known in some parts of Britain as 

 " corky end." This grouping is illustrated by a series of figures (fig. 



100 a-d) showing all intermediate conditions between an aggregation at 

 the end and a scattered arrangement of the warts. 



The tissue forming the wart contains the youngest stage in 

 the life-history of Spongospora. The parasite works -its way into 

 the tuber from the skin and is able to penetrate and exist within the 

 living cells of its host. Spongospora appears within the cells in the 

 form of minute protoplasmic lumps (myxamcebse), each containing 

 a single nucleus. It is impossible to say at present how the original 

 entry into the tissue is effected. The cells of the potato tissue con-j 

 taining the organism, e.g. those immediately beneath the skin, are| 

 now stimulated to divide, and at the same time the minute parasitic 

 bodies (myxamcebse) also divide, with the result that the parasite 

 becomes distributed in the newly-formed cells. The abnormal local 

 increase in the number of cells brings about tissue swelhngs which 

 take the form of warts (fig. 99 a) or raised patches (figs. 101 c (b) and 



101 D (b) ) at the point or area of original infection. 



During the further development of the wart, the fungus present ir 

 the tissue passes through a peculiar life-history. The separate bodies 

 present in each infected cell become approximated to form, not a truej 

 Plasmodium, but a colony, and the parasite exists for a time in thid 

 form, making use of the nourishment which it is able to obtain ir 

 the cells of its host. Very soon, however, the parasite enters upon 

 new phase of its life-history, and this brings the warty stage of thJ 

 disease to a close. The colony goes through a series of comple:| 

 changes, and finally becomes converted into a spongy ball made u]! 

 of a number of spores, which do not fall apart but remain in clos^ 

 contact with one another. Each of these spores is capable of bring 

 ing about the infection of some other potato tuber. During th. 

 development of the spore-ball the contents of the host-cell are uset 

 , up and destroyed, so that the tissue of the wart or warty are 

 dries up and shrivels, leaving a mass of spore-balls in the pit c 

 depression left by the decay of the tissue. This stage is shown i; 

 fig. 101 A. Here the crater-like hollows with torn skin at the margij 

 are easily distinguished. The hollows are filled with an exceedinglj 

 fine yellowish powder made up of spore-balls and the remains of tbi 

 tissue in which the spore-balls were formed. The appearances pr^ 

 sented by tubers at this stage suggested, in all probability, the nam 

 " powdery scab " given to this disease by Johnson in 1909. 



If the tubers are only slightly attacked by Spongospora and tl 



