62 PLANT DISEASES 



THE POTATO DISEASE 



(Phytophthora infestans, De Bary.) 



This terrible scourge was first observed near Boston, 

 U.S., also in Denmark and Norway, between 1840 and 

 1842, and by 1845 it had spread over Europe, doing 

 immense damage. Although the injury done at the 

 present day is not so severe as during the first decade 

 after its invasion, it is still with us, and during damp, warm 

 seasons still does a considerable amount of injury. In 

 addition to the cultivated potato (Solatium tuberosum), the 

 parasite also attacks several cultivated exotic species of 

 Solatium, the common British wild Solanum dulcamara^ 

 and more especially the tomato. 



The earliest indications of the disease are the appearance 

 of small brownish blotches on the leaves ; these gradually 

 increase in size, followed by a curling of the leaves, and in 

 an exceptionally severe attack the leaves and stems become 

 blackened and decayed within a few days, emitting a dis- 

 agreeable smell. If the brown spots on the under side of 

 a leaf are examined with a pocket-lens, numbers of delicate 

 white threads will be seen, especially towards the circum- 

 ference of the diseased patch. Higher magnification 

 reveals that these delicate threads are simple or branched 

 conidiophores, which originate from the mycelium of the 

 fungus, ramifying the tissues of the leaf, and emerge singly 

 or most frequently in small clusters through the stomata 

 of the leaf, for the purpose of producing conidia on the 

 surface of the leaf, whence they are readily dispersed by 

 wind, rain, passing animals, etc. The conidia are egg- 

 shaped and colourless, and are produced at the tips of the 

 conidiophores, but when a conidium is once formed, the 



