io DISEASES OF CULTIVATED PLANTS 



soluble glucose, which is conveyed to those parts of the plant 

 where growth is proceeding. This soluble glucose or its 

 modifications, sugar, etc., along with other cell-contents, 

 constitute the food of parasitic fungi ; during fine weather 

 when a plant is actively growing, the daily supply of glucose 

 and other substances is. monopolised by the plant itself, and 

 the fungus present is literally starved, or obtains so small a 

 share of food that it is but little in evidence. On the other 

 hand, during a spell of weather unfavourable to the plant, 

 growth is more or less suspended, the amount of starch after 

 conversion into glucose is not quickly attracted to growing 

 points, but in common with other substances formed under 

 conditions unfavourable for growth more or less saturates 

 the tissues, and consequently supplies the fungus with a 

 copious supply of food. Infection of a plant by fungus spores 

 is also most readily effected under the conditions indicated, 

 hence an epidemic follows. It is common knowledge that 

 only the young and actively growing parts of plants are 

 attacked by the majority of kinds of parasitic fungi. This is 

 because at such points there is the greatest concentration of 

 soluble glucose and other constructive substances required by 

 the fungus. For this reason the mycelium of Phytophthora 

 present in the tuber of a potato follows the growing stems ; it 

 also explains why the ' bunt ' fungus, that attacks seedling oats 

 in the ground, follows the upward growth of the plant, being 

 always most in evidence at the tip or growing-point, where its 

 food is in greatest abundance. 



The above remarks apply more especially to the first or 

 conidial form of parasitic fungi, which is most markedly 

 parasitic in habit ; the later stages of the same fungus usually 

 develop on the fading or even dead host-plant, and are 

 consequently more saprophytic than parasitic in their nature. 



HOW PLANTS ARE INFECTED BY FUNGUS SPORES 



It is a well-known fact that the spores of a given parasitic 

 fungus cannot infect indiscriminately every kind of plant 

 that the spores happen to alight upon. On the other hand, 

 the majority of the most destructive parasites known can only 

 infect and set up a disease on one particular kind of plant, 

 or at most, a few closely related plants. 



The fungus causing potato disease, since its introduction to 



