Pethybridgu — Experiments tvith Phytophthora. 23 



dormant mycelium of Phytophthora, and that of some of the smut fungi ; 

 but sucli comparisons are entirely out of place until it has been definitely 

 established that the former fungus does possess dormant mycelium, which is 

 at present far from being tlie case. The general habits and behaviour of tlie 

 smuts are so far removed from those of Phytophthora and its allies that the 

 existence of any such similarity as is assumed is most improbable. Tiie 

 dormant mycelium tlieory is in reality but a modification of one brouglit 

 forward in the first instance by de Bary, but discarded by him some five-and- 

 thirty years ago as not being in accordance with known facts. 



If this theory is correct, it seems almost impossible to explain tlie 

 undeniably beneficial results accruing from spraying the crop with Bordeaux 

 or Burgundy mixtures. Every grower of potatoes knows how essential it is 

 to carry out this process of spraying before the blight has attacked the plants, 

 for, if done later, the efficacy of the treatment is very seriously diminished. 

 Spraying must be looked upon as a preventive method against the attacks 

 of the blight from without and not from within. If the fungus once gains 

 an entry into the tissues, external spraying does not prevent its spread in 

 them. In some experiments which I carried out in 1909, I found that 

 carefully painting over areas affected with blight by hand witli Burgundy 

 mixture, and even dipping afi:ected foliage into the mixture, did not arrest 

 the progress of disease, provided that the conditions of moisture and warmth 

 were suitable. The mycelium of the fungus was even seen to emerge and 

 form " spores " through places on leaves thickly coated and blue with the 

 mixture. 



Not only is this theory of dormant mycelium advocated to account for 

 the infection of the over-ground parts of the potato-plant during the summer, 

 but it is also employed to account for the attack of the new crop of tubers. 

 It is stated that it has not been proved that the tubers become diseased as a 

 result of the falling of the " spores" from the foliage on to the soil, and the 

 ultimate arrival of them, or the products of their germination, on the surfaces 

 of the tubers. It is strange that such a statement should be made at this 

 time of day, in view of the many experiments which have been carried 

 out to settle this point, and which force one to the conclusion that this in 

 reality is at least the principal, if not the only, way in which the tubers do 

 become infecteJ. 



If the new tubers are infected directly from the old ones by invading 

 mycelium, why is it that those tubers nearest the surface of the soil, and 

 therefore generally farthest away from the diseased set, become first and 

 worse diseased than the deeper-lying ones ? Why is it that the diseased 

 areas of the new tubers are nearly always, if not always, superficial, and 



