232 te. J. BiJTLER. 



Contact with a diseased leaf is sufficient to infect a healtliy one. 

 The disease spreads from leaf to leaf on the same tree chiefly by 

 contact. It is, therefore, "contagious." How it spreads from one 

 tree to another without contact, as it certainly does, is not so clear. 



The following are the chief possible methods of spread from 

 tree to tree : through the air ; by human agency ; by birds and 

 insects conveying infectious matter on their bodies. 



Most epidemic diseases which resemble the bud-rot of palms 

 are spread through the air. The parasites depend for their propaga- 

 tion chiefly on spores, which are set free and are carried into the air 

 by the wind. Alighting on healthy plants they germinate and pro- 

 duce infection. Such are the potato blight, cereal rusts and the 

 like. Pythium palmivorum, which is very similar to the cause 

 of potato blight, produces spores which would be just as capable 

 of causing rapid spread of the disease as those of the latter, provided 

 that they are formed in a position where the wind can easily reach 

 them. This is not often the case, however, since spores have not 

 been found on the surface of diseased crowns, except in the compara- 

 tively small number of cases in which the blade, or expanded por- 

 tion of the leaf, is attacked. In the large majority of cases the seat 

 of attack is the compact mass of leaf bases, the leaf-sheaths, which 

 form a tubular covering to the top of the stem. Having entered 

 the outermost of these, the fungus grows in towards the softer under- 

 lying ones and usually does not begin to produce spores until several 

 have been penetrated. Spores have never been found on the hard 

 outer sheaths, but usually occur between the softer inner ones at 

 some distance from the surface. Here they are not exposed to the 

 air and cannot serve as an effective means of propagation. In 

 some cases, however, the young leaf -shoot at the apex of the crown 

 is found to have been attacked in the portion which afterwards 

 becomes the expanded "blade'' of the leaf. This usually occurs 

 while it is still small and hidden in the tube of leaf-sheaths, being 

 the result of direct contagion from the latter. In a very few of 

 these cases the parasite has been found still alive when the young 

 leaf has pushed out into the air, and it is probable that in periods 



