26 PLANT DISEASES 



In many instances where the soil has become infested 

 with resting-spores or sclerotia, it is very unwise to continue 

 growing the same kind of crop, and under the circumstances 

 rotation of crops ; selecting those that the fungus present in 

 the soil does not attack is undoubtedly the best thing to 

 do under the circumstances. Deep ploughing is some- 

 times resorted to for the purpose of burying the spores or 

 sclerotia, but for other reasons this cannot always be done. 

 Where rotation of crops cannot be followed out, very 

 frequently what is termed a trap-crop is found to be useful 

 in clearing the soil of disease germs. 



As an illustration of this method, we may take the 

 case of the disease known as ' finger-and-toe ' of cabbages, 

 turnips, radishes, and other cruciferous plants. The 

 myxogaster causing ' finger-and-toe ' is essentially an under- 

 ground organism, living on organic matter in the soil, 

 or as a parasite in the roots of plants. It has been 

 proved that in the case of badly infected soil, if a crop of 

 any quick-growing cruciferous plant is grown early in the 

 season, the plants become diseased. After the trap-crop has 

 thus caught up the disease, it should be removed at once, 

 and immediately afterwards the same ground may be 

 planted wdth cabbages or turnips, which wdll remain free 

 from the disease. This method would undoubtedly prove 

 of great service in many forms of disease, where the inocu- 

 lating bodies are present in the soil. The points to re- 

 member are : the trap-crop should consist of some plant 

 readily susceptible to the disease it is intended to catch, 

 and that after becoming diseased, it should be removed 

 before spores or sclerotia are produced. 



Rofatio7i of crops a?id the €mployme7it of trap-crops are to 



