524 DISEASES OF CULTIVATED PLANTS 



The plasmodium or vegetative condition remains buried 

 in the matrix from which it obtains food, until ready to form 

 spores, when, for the object of effecting the wide dispersion 

 of the spores, it creeps to the surface and forms its sporangia 

 in a position where the spores will be readily dispersed by 

 wind, etc. 



The group is cosmopolitan in its distribution, although 

 numerically small under five hundred species. The majority 

 are minute, and live as saprophytes amongst humus, dead 

 wood, etc. ; on the other hand, a few species are amongst the 

 most destructive of known parasites. 



Some of the larger species that are not parasitic, creep 

 over living plants and suffocate seedlings. The dense masses 

 of spores sometimes prove injurious to animals. 



Finger-and-toe, also known in different districts as ' Club- 

 root,' ' Anbury,' ' Grub,' etc., is caused by a Myxogaster called 

 Plasmodiophora brassicae (Woronin). 



Nearly all kinds of cruciferous plants, both wild and 

 cultivated, are attacked. The cultivated plants that suffer 

 most in this country are turnips and the various kinds of 

 cabbage. The root is the part attacked, which becomes 

 much distorted and more or less covered with large swellings 

 or finger-like outgrowths. Finally the entire root is resolved 

 into a loathsome, rotten, foetid mass. 



Infection is effected by swarm-spores liberated 'from spores 

 present in the soil. These swarm-spores enter through the 

 delicate root-hairs, and pass into the root, where they 

 stimulate a rapid increase in the number of cells. The 

 infested cells also increase enormously in size and become 

 filled with the plasmodium or vegetative condition of the 

 organism, which at a later stage become transformed into a 

 mass of spherical spores about 3 ft in diameter. 



When the root decays, these spores are liberated in the 

 soil, and in due course infect future crops. 



This disease has undoubtedly increased very much in 

 severity in this country during the past fifty years. This 

 period agrees roughly with the cessation of the previous 

 general use of lime in favour of artificial manures, crushed 

 bones, etc., many of which contain crude acid. Now it 

 has been proved that the presence of an acid greatly favours 

 the development of Plasmodiophora, in fact it may be said 

 to be indispensable. On the other hand, the presence of an 



