16 BULLETIN 12S6, U. S. DEPABTMEXT OF AGBICULTUBE 
to differences in climate. Although in general it has doubtless been 
a discouraging factor, in the eastern section at least there have been 
no important losses to producing estates. 
The rubber plantations of British Guiana and Dutch Guiana 
(Surinam) have been either largely abandoned or the trees cut 
down and replaced by more profitable crops. The leaf disease has 
been considered largely responsible, although the relatively high 
cost of labor was doubtless a contributing factor. 
The most striking symptom of the disease occurs when the very 
young leaves blacken, wither, and fall off. Leaves less severely in- 
jured continue to grow, the larger spots becoming papery and torn. 
This, combined with stunting, gives the mature foliage a thin 
ragged appearance. The lower surface of the spots is at first 
velvety with conidiospores, and later, on the upper surface, black 
pycniclia and perithecia are often characteristically arranged in 
minute ringlets. On young twigs, fruit stalks, and other young 
organs black hypertrophied lesions are produced. 
The causal fungus (D othidella uLei P. Henn.) is known to pos- 
sess conidial, pycnidial, and perithecial fruiting stages, although the 
first is almost entirely responsible for the rapid spread and defolia- 
tion of the trees. Only the very young leaves up to 7 days old are 
susceptible, and within a week after infection a new crop of conidia 
may be formed. According to Stahel, none of the spore forms are 
very resistant to drying; but further viability studies are needed, 
especially to determine the longevity of the mycelium in twig lesions. 
The fungus, which is apparently an obligate parasite, is prob- 
ably never carried by the seed or by large cut -back seedlings, or 
" stumps," since neither of these is susceptible to infection. 
The optimum conditions for the epidemic spread of the leaf dis- 
ease are (1) the continuous presence of young foliage and (2) 
abundance of moisture on the plant for a period of 10 to 12 hours, 
furnished sometimes by prolonged mist or fog but generally by 
heavy dews. Since the duration of the latter is greater on quiet, 
clear nights, the disease is generally more severe during the dry 
season of the year. 
Of the numerous suggestions made in the past for controlling the 
leaf disease, none appears to have been practicable. The situation 
is not considered hopeless, however, because a sufficiently detailed 
knowledge of the disease as a necessary basis for control measures 
is lacking. 
Attention is called to the promising possibilities existing in the 
deserted Guiana plantings for the selection of disease-resistant 
strains of the rubber tree. Until such strains are obtained or until 
further investigation has demonstrated a practicable method of pre- 
vention or control of the leaf disease, renewed planting in the in- 
fected regions, at least in the Guianas, is unlikely. Plantations may 
be established in other parts of the American Tropics, however, in 
regions where the disease does not exist and other conditions may 
be equally suitable for the growth of the tree. In this case effec- 
tive restrictions on the importation of propagating material to pre- 
vent the introduction of the disease should be adopted. 
