24 DISEASES OF CULTIVATED PLANTS 



intestinal canal uninjured, were conveyed along with the 

 manure to the land in a condition to infect future crops of 

 potatoes, perhaps he would come to the conclusion that it 

 would be a far more economical method to burn, or in some 

 manner thoroughly destroy such diseased potatoes, than to 

 run the risk of infecting previously clean land, and have 

 diseased crops in the future. 



With the idea of making the best of a bad job, it is the 

 usual thing to throw diseased roots of all kinds that can be 

 eaten, turnips, carrots, mangolds, potatoes, etc., into the 

 piggery or cattle shed. The statement made above respecting 

 the spores of the black scab fungus is true of all cases. The 

 spores to a great extent find their way back to the land in 

 manure, and consequently diseases are unconsciously on the 

 part of the farmer perpetuated. 



I am perfectly well aware that in many instances the 

 return of the spores of fungi to the land cannot be prevented, 

 nevertheless it is just as well to know the worst. In the case 

 of wheat mildew, caused by Pucdnia graminis, D.C., the 

 last, or teleutospore, stage has its spores so firmly attached to 

 the straw on which it grew, that they are not removed by the 

 operations of harvesting, threshing, etc., and may often be 

 found, when specially looked for, on fragments of straw in 

 the manure heap. Such spores are capable of germination, 

 and if the minute secondary spores they produce alight on 

 the right host-plant, the disease commences its cycle of 

 development. 



When land has produced a crop of potatoes affected- with 

 black scab, or of turnips injured by ' finger-and-toe,' it is 

 perfectly certain that some of the spores will remain in the 

 soil. Here again, prevention of infection of neighbouring 

 land is a much more difficult matter than might at first be 

 imagined. Soil containing spores from the infected land 

 may be conveyed to adjoining clean land on various imple- 

 ments, cart-wheels, boots, etc. 



As a rule parasitic fungi attack only one particular kind of 

 plant, or at most, a few closely related kinds of plants ; con- 

 sequently when a diseased crop of potatoes has occurred, 

 potatoes should not be planted in the same land again for 

 some years. By adopting this method, the spores present in 

 the soil, and capable of infecting potatoes, may perhaps perish 

 in the absence of the proper host-plant. The black scab 

 fungus, so far as known, can only attack potatoes. ' Finger- 



