xiv PLANT DISEASES 



to infected land, and afterwards, for at least five years, sow 

 cereals or a crop not attacked, which means avoid growing 

 root crops. 



The only safe method of dealing with diseased tubers is 

 burning. 



The spread of disease by means of hybernating mycelium. 



It has been proved in the case of potatoes attacked by 

 Phytophthora infestans, causing the well-known potato 

 disease, that the spawn or mycelium of the fungus enters 

 the young tubers, and when such tubers are used as ' sets,' 

 the mycelium passes from the tuber and grows up along 

 with the stems, enters the leaves, and if climatic conditions 

 are favourable, produces the disease on the leaves. The 

 mycelium also passes from the parent plant into the new 

 tubers, which in turn produce diseased plants. By this 

 method it follows that the offspring of a diseased tuber 

 is infected for all time. When the foliage of a potato 

 plant infected by the method explained above, produces 

 spores, such spores, dispersed by wind or animals, infest 

 other plants, and the disease is thus distributed. It is 

 owing to the production of spores on the foliage of potatoes 

 originally infected by mycelium spreading from the tuber, 

 that spraying is of some service. Spraying protects the 

 crop to the extent that it would suffer from the dispersion 

 of such spores, but it does not prevent the infection of the 

 new crop of potatoes from infection by the spawn travelling 

 from the stem of the plant into the new tubers. The fact 

 that a crop of potatoes remains apparently perfectly 

 healthy and free from disease, by no means proves that 

 there is no mycelium present in the tubers or foliage. It 

 has been definitely proved by experiments conducted at 



