DISEASES 155 



known, are centres of infection from which myce- 

 lium spreads in the soil in every direction. The 

 most effective means, therefore, of checking the 

 spread of mycelium would be to remove all stumps. 

 This, however, for various reasons cannot be done, 

 at least not at once. As the strands of mycelium 

 extend through the soil, growth takes place only 

 at the tip, the back portion dying away. The tip 

 or growing point consequently requires food at 

 every stage of its progress. If such food is with- 

 held it dies at once. This knowledge affords a 

 means of checking the spread of mycelium in the 

 soil by means of open trenches. Such trenches 

 need not be more than a foot wide, and about a 

 foot in depth. If an open trench of this descrip- 

 tion was made round every stump, the spread of 

 mycelium from such infected centres would be 

 practically arrested. Similar open trenches break- 

 ing up the entire portion planted into small areas 

 would also go far towards checking the under- 

 ground spread of fungi, which cause far more in- 

 jury to cultivated plants in the virgin forest than 

 is usually realised. Of course, as in the case of 

 spraying, the difficulty of keeping open trenches 

 in the tropics would be serious in many localities, 



