Epidemic Plant Diseases. F. T. Brooks. 237 



tations of iNIalaya one of the diseases of rubber trees is an 

 affection of the bark which is brought about frequently by the 

 combined action of an insect and a fungus. The insect is a boring 

 beetle which usually attacks only dead wood, but which under 

 certain circumstances invades the bark of h\Tng rubber trees, 

 especially if this has been exposed to injury. In the dead wood 

 inhabited by these borers the fungus Ustulina zonata is often 

 found groNsing as a saproph\-te, and as the boring insects pass 

 from the dead wood to the h\"ing rubber trees they carry \\ith 

 them the fungus which, upon being thus established in the 

 tissues of the rubber trees, spreads rapidly and acts as a de- 

 structive parasite. Such instances could be multiplied many 

 times. 



Xot only do animals of a lower order play an important part 

 in the dissemination of plant diseases, but man himself must 

 plead guilty to this offence. Ci\ihsation is responsible for certain 

 e\-ils, not the least of which is the scattering broadcast over the 

 world of serious plant pests by the hand of man. This has only 

 recently come to be recognised, imfortunately too late to prevent 

 the introduction into new coimtries of some pests which cause 

 great havoc. As often happens, the stable door has been locked 

 after the horse has gone, but many coimtries have now 

 estabhshed a S3'stem of plant import inspection which \\ill pre- 

 vent the introduction of new troubles of this kind. In the United 

 States this control of plant imports is now so rigorous that 

 certain classes of plants are entirely excluded from entry into 

 the country. A few illustrations may be given of some of the 

 ways in which the agency of man may be traced in the distribu- 

 tion of fungus pests. Less than twenty years ago, wart disease 

 of potatoes was known to exist only at a few places in England 

 and Scotland, but since then it has spread to some extent prac- 

 tically all over these countries and has reached America also. 

 Some of the districts infected ^\ith this disease are reno%\Tied for 

 the quahty of their "seed" potatoes, which, in consequence, ^&t 

 distributed far and wide. During the war, when every effort 

 had to be made to grow the maxim lun quantity of potatoes, 

 "seed" tubers were distributed from one part of the country 

 to another on a much bigger scale than ever before. There is 

 no doubt that in this process wart disease was distributed at 

 the same time. No one of course could think of using "seed" 

 tubers bearing the warty outgro\\-ths characteristic of this 

 disease, but tubers apparently healthy, may bear the resting 

 spores of this fungus in particles of soil adhering to the tubers. 

 Certain varieties of potatoes are entirely immime to this disease. 

 Notwithstanding their own immunity, these varieties may be 

 the means of carrying the disease to healthy gardens, for the 



M.S. 16 



