698 FUNGI 



crop do not belong to the Oomycetes it has been found 

 convenient and useful to mention them in this chapter. 



(i) ' The Potato Disease,' ' Late Blight,' ' Pbytophthora Blight.' 



Symptoms. — This disease, which is met with wherever the 

 the potato is grown, usually makes its appearance in the British 

 Isles about the end of July, and is first observed upon the 

 leaves of the plant. The latter lose their green colour, and 

 become spotted with yellowish patches which soon die and turn 

 dark brown or almost black. In dry weather the dead patches 

 increase very little, but in damp, foggy weather they spread 

 over the leaflets on which they occur at an exceedingly rapid 

 rate. After destroying the leaves, the disease attacks the stem 

 and in severe cases the whole of the plant above ground is 

 reduced in a few hours to a damp, shrivelled, and blackish mass 

 of plant-debris, with a peculiar and characteristic foul odour. 



Around the margin of each dead spot on the under surface of 

 a diseased leaf there is a more or less distinct border of white 

 or greyish ' mildew,' sometimes resembling fine flour, and most 

 readily seen when the lower side of the leaf is viewed in a 

 slanting direction. The presence of a whitish rim round each 

 dead patch is highly characteristic of this disease, and enables 

 us to distinguish it from others which kill the leaves, and from 

 the natural withering and death of these organs at the end 

 of the growing season. 



As the growth of the tubers is dependent on the supply of 

 materials manufactured by the leaves of the plant, an early 

 destruction of the latter results in a diminished crop of potatoes. 

 Moreover, when the leaves and stems above ground are 

 severely attacked, the substance of the tubers, especially just 

 beneath the skin, is frequently affected with brown dead patches. 

 The amount of tissue thus injured varies considerably; in some 

 cases only small spots are met with, while in others the whole 

 tuber rots. 



