LEAF DISEASE OF PARA RUBBER 13 
the tree itself has changed or that the plantation trees are descended 
from unusually susceptible types also exists. 
CLIMATIC FACTORS 
The careful infection studies made by Stahel (23) with the 
fungus of the South American leaf disease indicate that its spread 
and severity are largely dependent upon certain climatic factors. 
The requisite conditions are not essentially different, however, from 
those which obtain through a large part of the year in all moist 
equatorial regions. They are, specifically, enough rain and wind 
to disseminate the spores and the presence of moisture on the plant 
continuously for a period of at least 10 to 12 hours, which are re- 
quired for germination and infection. Abundant moisture is fur- 
nished in some localities by prolonged mists and fogs, but usually 
by heavy dews. It is only on clear quiet nights that the dew re- 
mains long enough to allow infection, and the disease has always 
been found to be more severe during the dry season. In British 
Guiana, on the contrary, Bodkin (Harrison 9, p. 16) states that 
it is particularly virulent during protracted periods of heavy rain- 
fall and assumes a milder form during relatively dry periods, the 
leaves remaining green though riddled with small holes. However, 
Stahel observed that during the average rainy period the actual 
surfaces of the plant were not continuously wet for a sufficient 
length of time to insure infection. 
Any circumstance which hinders or delays dew formation re- 
duces the amount of injury from the leaf disease. Wind is doubt- 
less the most important single factor, but if it is too strong it has 
a deleterious effect on the growth of the tree (10, p. 6). The local 
alternating land-sea breezes which set up as a result of unequal 
heating of land and water operate ideally on unobstructed coastal 
areas in shortening the period of dew formation. The mature 
plantings of Hevea examined by the writer on the east coast of 
Trinidad probably in large part owe their relative freedom from 
the leaf disease to the prevalent night breezes, although the com- 
paratively short wintering period is doubtless a contributing factor. 
The strong trades striking the Guiana coast, however, necessitated 
the leaving of protective belts of jungle, which have, as earlier 
indicated, favored the disease. On exposed sites in the same region 
the injury was always less. 
Differences in soil apparently have little or no influence on the 
prevalence of the leaf disease. 
POSSIBILITIES OF CONTROL 
CONTROL IN INFECTED REGIONS 
Numerous suggestions are offered in the literature for controlling 
the leaf disease. Although a good deal of money is said to have been 
expended by a few of the larger plantations in carrying out some of 
these suggestions, very little carefully controlled experimental work 
appears to have been done. The only measure which seems to have 
been finally adopted was to cut down the trees and replace them with 
more profitable crops. It was not learned to what extent this has 
been done ; but it is known that many estates; have been merely aban- 
