DISEASES AND PESTS OP PLANTS. 



33 



be kept for a time under these conditions, the contents of the spore divide up into 

 a number of smaller units. These soon come out and are capable of swimming about 

 actively in the water on their own account. Hence they are termed zoospores or 

 " animal " spores. Very soon each zoospore comes to rest, germinates, and produces 

 an infection. In this way the number of infections may be very greatly increased, 

 since one spore of the first kind may give rise to as many as thirty of the second, 

 the whole procedure from the first spore to the new infection only occupying a few 

 hours. However, under certain conditions the first type of spore may germinate 

 directly and produce an infection, without any formation of active spores. 



Although the destruction of the tops may in itself be serious by preventing the 

 proper maturation of the tubers, it is not the worst form of the disease, since the 

 tubers are themselves attacked. On the mature tuber the affected areas appear 

 of a dull leaden colour and somewhat sunken. On cutting, brown diseased tissue 

 is seen to underlie them. This may be confined to a thin superficial layer or may 



Fig. 0. Tuber infected with the fungus of 

 late blight. Note the discoloured areas under 

 the skin, corresponding with the depressed 

 discoloured areas visible on the surface. 



involve most or all of the tuber. This form of the disease is commonly spokeii of 

 as a " dry rot." Although the dry rot produced by the fungus may work much 

 injury, it is usually more important indirectly, since it permits the entrance of other 

 organisms, especially bacteria, which rapidly destroy the tuber, producing a "wet 

 rot." This may take place in the soil if this is wet and heavy, or In storage. The 

 tubers apparently become affected, in the first instance, by spores washed against 

 them by the rain. Exposed tubers or those near the surface are, therefore, especi- 

 ally likely to show the disease. The manner in which the first infections of the 

 growing plant come about has been the subject of much discussion. Apparently the 

 fungus remains alive in many of the infected tubers, and, if such be planted, becomes 

 active, producing conidia either on the seed-tubers themselves or on the sprouts 

 from them. These conidia, being subsequently carried to the leaves, give rise to 

 the early infections from which an epidemic may later develop. 



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