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LEAF DISEASE OF PARA RUBBER 11 
individually is transformed into the " secondary Scolecotrichum 
stage," where only long many-celled conidiophores push up in com- 
pact clusters from a sort of subcuticular pseudoparenchyma. 
When the infected leaf is barely full grown the second or pycnidial 
stage of the fungus appears. This belongs to the type genus Apo- 
sphaeria. The pycnidia occur only on the upper side as minute black 
points scattered individually over the leaf or arranged in character- 
istic fashion around the borders of tiny hypertrophied areas and dead 
spots. (PL VI.) The pycnidia may develop either from the pseudo- 
parenchyma of the secondary conidial stage or arise independently. 
They are formed regularly on diseased internodes and fruits, but 
according to Stahel the pycnospores have been found incapable of 
infecting even the youngest leaf. Hence this stage of the fungus 
evidently plays no part in spreading the disease. 
. On approximately 2-months-old or fully grown moderately infected 
leaves there appears scattered among the pycnidia the third or peri- 
thecial stage (Dothidella), which arises in the same way and is 
indistinguishable from the pycnidial fructification without micro- 
scopic examination. On old leaves the perithecia predominate, but 
Stahel (23) does not consider them significant in spreading the dis- 
ease. Fresh ascospores were unable to withstand drying in a desic- 
cator for more than four to six hours. Although they germinated 
weakly in water in about two hours, they required 16 hours of con- 
tinuous moisture to form appressoria and effect partial penetration 
of very young leaves sown with the spores and kept in a moist 
chamber. Sufficient penetration and invasion of the tissues to be 
termed infection were never obtained, but there were also appar- 
ently no tests made in which fresh ascospores were sown on leaves 
remaining attached to the plant. The disease has therefore never 
been reproduced experimentally from inoculations with ascospores, 
and Stahel has had to depend upon microscopic investigation to 
prove the relationship of the three spore forms. In summarizing 
these studies he states that the connection between the Scolecotri- 
chum, the Aposphaeria, and the Melanopsammopsis (Dothidella) 
forms has been established beyond all doubt along anatomical lines. 
Since earlier workers had also observed the constant association of 
the last two fruiting forms and Stahel was able to find occasional 
traces of the first one on the original material described by Hen- 
nings, there is little ground for questioning his conclusion, in spite 
of the fact that it is based entirely on morphological evidence. 
PERPETUATION AND SPREAD OF THE DISEASE 
From the above account the importance of the conidial stage and 
dependence upon it for perpetuation of the disease, once it has ob- 
tained a foothold in a planting, is realized. Although this stage 
develops as a rule only on very young leaves and other immature 
organs, it sometimes continues to appear on older leaves where nor- 
mally only pycnidial and perithecial stages are found. Such pro- 
longed production of viable conidia doubtless enables the fungus 
to bridge over short periods when susceptible tissues are absent. In 
the Guianas this is usually unnecessary, because on account of the 
wide variation between different trees and even between different 
branches on the same tree in the time of their annual leaf change, 
