Cockayne. — Spread of Phytophthora infestans. 319 



whence it has originated, I am inclined to combat ; although I 

 must admit that in the majority of the cases I have examined 

 secondary infection by bacteria and other fungi, notably Fusa- 

 rium oxysporum, plays an extremely important part in the rotting 

 that occurs in tubers which have been primarily attacked by 

 P. infestans. It is a well-known fact that tubers bearing the 

 characteristic marks associated with Phytophthora attack often 

 remain during the whole winter without any trace of rot setting 

 in. When these are examined under a magnification of about 

 100 diameters, large amounts of dormant mycelium will be seen 

 in those portions of the tissues of the tubers that abut on the 

 darkened and discoloured areas, which are said to be caused 

 by Phytophthora ; and in such cases no other fungi or bacteria 

 will be found associated with the Phytophthora mycelium. This 

 observation gains considerably in significance when it is stated 

 that in those tubers on which an active rot is present resting 

 mycelium can hardly ever be discovered, although there will be 

 an abundance of bacteria and other fungi, both parasitic and 

 saprophytic. 



If slices of potatoes showing resting mycelium of Phytoph- 

 thora infestans are placed in petri dishes, and kept moist in a 

 temperature of about 60° to 70° Fahr., the mycelium will rapidly 

 become active, and in a few days an abundant crop of spores 

 will be developed. This shows that the mycelium is not in a 

 very dormant condition, but is really on the border-line between 

 active and hybernating mycelium. This is a very important 

 point, inasmuch as it shows the liability, under ce tain condi- 

 tions, of the mycelium to become active even when no growth 

 on the part of the host takes place, and in this particular dis- 

 tinguishes it sharply from the more specialised resting stages 

 in the life-history of other fungi, such as many of the Ustilayinece. 



When tubers with the resting mycelium of P. infestans are 

 p/anted, the fungus develops rapidly through the tissues of the 

 developing plant, and if the weather is humid and warm the 

 mycelium becomes markedly negatively geotropic, and if the 

 conditions remain favourable for its development it quickly 

 makes its way into the leaves, on the under surfaces of which 

 it soon produces an abundant supply of spores, which, blown 

 by the wind, can soon spread infection far and wide. If, how- 

 ever, the weather-conditions remain unfavourable for its de- 

 velopment, no spores at all may be produced, and to all outward 

 appearance the potato-plants remain quite healthy. In such a 

 case it is not known whether the fungus can again form resting 

 mycelium without the intervention of a spore- producing stage, 

 as is done in the case of Lolium temulentum, but I am inclined 

 to consider that such can and often does occur. 



