omnivora. 



190 



Some varieties of potatoes have been found to resist 

 the disease better than others. Tubers, for instance, with 

 exceptionally thick skins offer more resistance to the entrance 

 of the hyphse than do others with thin skins. In order to 

 keep up the power of resistance of the potato plant to the 

 disease, care must be taken to always select the most resistant 

 tubers for continued cultivation, and to raise new and stronger 

 varieties from the seed obtained by crossing good, selected, 



/ c? o 



varieties. Diseased stalks, leaves and tubers should be burnt 

 and, to prevent risk of infecting sound tubers from living 

 conidia lying on the ground, the tubers should not be raised 

 for three weeks after the complete dying down of the plants. 



As moisture favours the development and spread of the 

 fungus, wet localities should be avoided as far as possible. 

 Irrigation of the crop should be carried out with caution, and 

 the tubers should be stored in a dry well-ventilated place. 

 Phytoph- 167. Very closely allied to the 



above fungus is another named Phytophthora omnivora De 

 Bary, which in Europe has proved very destructive to the 

 seedlings of forest trees. It is probable that the " damping 

 off " of seedlings in many of our forest nurseries in India 

 will be found to be caused by it or by some other nearly 

 related fungus with an almost identical life-history and 

 mode of attack- The first signs of attack are usually dark 

 spots on the stern, cotyledons, or first leaves of the seedling, 

 and if the weather is warm and damp the young plant is 

 soon destroyed. Frequently only the " collar ' is attacked 

 and the seedlings then fall over. If only the upper leaves are 

 attacked the seedlings may recover. The damage gradually 

 spreads from one seedling to those around it, and the 

 circle of diseased plants gradually spreads outwards from 

 the centre. The growth and injurious action of the my- 

 celium of this fungus in the tissues of the attacked plant 

 are almost precisely similar to those of P. infestans, except 

 that the hyphse more commonly absorb their nourish- 

 ment by means of haustoria. The most important point of 

 difference between them consists in the fact that this species 

 produces sexual resting spores (oospores) in the tissues of the 

 attacked plant. These spores have the power of remaining 

 dormant for several weeks or months before germinating and 

 hence their name of resting spores. In the formation of these 

 spores the end of a hypha swells up into a rounded knob, and 

 the protoplasm inside it forms into a ball thus producing what 

 is called the oosphere or egg-cell. The end of another hypha 

 then coming in contact with the swelling, sends out a small 



