98 DISEASES OF CULTIVATED PLANTS 



forms subglobose, glassy-looking pustules up to i mm. 

 broad, on the leaves and petioles of Trifolium montanum, 

 T. pratense, and T. repens in Italy and Germany. Spores 

 subglobose, smooth, wall double, outer thin, brown, endospore 

 thick, 40-58 fJL diam. 



SYNCHYTE1UM (DE BARY and WOR.) 



Zoospores penetrate the tissue of the host and form a 

 plasmodium which becomes surrounded by a thick membrane 

 and forms a resting-spore, which on germination liberates its 

 contents as zoospores, or as a single mass which divides 

 directly into zoospores, or into a group of thin-walled 

 sporangia containing zoospores. 



No mycelium present at any stage. Parasitic in the tissues 

 of plants, more especially in the epidermal cells. 



Black scab of potatoes. This disease is caused by Synchy- 

 trium solani (Massee.) It is also known locally as 'Wart 

 disease,' ' Cauliflower disease,' and ' Canker fungus.' 



In a typical example of black scab the tuber bears one or 

 generally several prominent rugged outgrowths or warts, 

 which vary in size from a marble to that of a walnut. These 

 warts are coloured like the potato at first, but eventually 

 become almost black, hence the name black scab. Less 

 frequently the lower leaves are also infected and converted 

 into fleshy, shapeless masses. The tuber itself is never 

 infected, but only the sprouts, which are attacked when 

 quite young. The act of infection by the parasite causes 

 hypertrophy or much increased local growth of the infected 

 sprout. Continued infection from outside stimulates the 

 sprout, which gradually spreads to a greater or less extent 

 over the surface of the tuber, until a tumour-like mass is 

 formed. When all the sprouts are attacked, the entire 

 surface of the tuber is often covered with excrescences. 

 Infection is always effected by the zoospores of the fungus 

 present in the soil, and only very young superficial cells not 

 protected by periderm can be entered. When a cell is once 

 infected or entered by the parasite it is incapable of further 

 division, but those superficial cells that escape infection 

 rapidly divide and form a mass of tissue that soon overlaps 

 the original surface cells. This process is frequently repeated, 



