PHYCOMYCETES.— (a) OOMYCETES 



419 



like those of Pythium, they are motile for a time by means of two 

 cilia (Fig. 355). Coming then to rest, the cilia are dropped : each 

 zoospore rounds itself off, and, investing itself with a wall, puts out a 

 hyphal tube. If this takes place on the surface of a potato leaf, as it 

 well might do under conditions of rain or heavy dew, all is ready 

 for the infection. This may either be by entry through the pore of 

 a stoma or by direct perforation 

 through the epidermal wall (Fig. 

 356). By either route the parasite 

 may reach the intercellular spaces 

 and establish a new infection. 



It is not only the leaves but also 

 the stems and tubers of the Potato- 

 plant that may be traversed. The 

 mycelium spreads through the 

 tissues down the haulms tO' the 

 tubers. A perforation of the j^oung 

 tubers by a new infection from 

 conidia is even possible, so long as 

 their skin is thin. In bad cases 

 tubers thus infected may decay at 

 once. More commonly the hyphae 

 enter a dormant state, and in this 

 condition the tubers are harvested. 

 But such tubers often decay during 

 the winter. If, however, they are 

 not heavily infected, and thus 

 escape decay, and they are used as 

 " seed " potatoes for a new crop, 

 the young plants start infected from 

 the first, and a recurrence of the 

 disease is inevitable. The measures to be taken are to destroy 

 by fire all infected haulms and leaves, to avoid carefully the use 

 of tainted "seed" tubers ; and, as a preventive, to spray the young 

 growing crop with suitable disinfectants, especially if the season 

 is wet in the middle summer. But a more hopeful line of prevention 

 is by the use of " immune varieties," which are able to resist the 

 attack of the parasite. 



No mention has been made of sexual reproduction in the Potato 

 Fungus. As a matter of fact sexual organs have not been proved to 

 e.xist in Phytophthora infestans under normal conditions of life. Like 



Fig. 354. 



Piece of the tissue of the stem of a Potato- 

 plant, showing the hyphae ol PhytapMhora 

 penetrating the middie lamella of the cell-walls. 

 a=nucleus of acell. Highly magnified. (After 

 Marshall Ward.) 



