Jan. 27,1923 
Production of Conidia in Philippine Sclerosporas 243 
The production of conidia from the diseased plant takes place almost 
without exception at night. Of the conditions which obtain during the 
night in the Philippines, the most essential to conidiophore production 
is apparently the presence of a layer of moisture on the surface of the 
infected plants. In this regard dew plays the foremost part under custom¬ 
ary field conditions, while gentle rains are of somewhat less importance. 
The film of moisture must not be temporary but must persist for at least 
four or five hours to permit the conidiophore buds to protrude from the 
stomata, develop into conidiophores and mature conidia. The maxi¬ 
mum production of conidia is attained when the whole plant surface is 
drenched and dripping with a heavy deposition of dew, as happens when 
still, clear, cool nights follow hot, rainy days. A gentle, drizzling rain 
persisting through the night and keeping the surface of the com plant 
covered with a layer of moisture also results in conidium formation, 
though to a less extent. During violent, nocturnal storms, however, 
when the plants are beaten with driving torrents of rain, comparatively 
little conidium production occurs, and then only on the more protected 
parts of the plant where the delicate conidiophores can develop uninjured. 
No conidium formation takes place on nights when there is no deposition 
of dew on the infected plants, or when the temporary surface layer of 
moisture, whether of dew or rain, is dried in a short time by inopportune 
winds. 
That the nocturnal production of conidia by the downy mildew depends 
on the presence of a layer of moisture over the host leaves was found to 
hold true not only under natural conditions in the field but also under the 
controlled conditions of various experiments. A number of maize seed¬ 
lings, after being inoculated with the downy mildew, were divided into 
two lots, one of which was exposed to the usual field conditions, while the 
other was kept indoors protected from dew, rain, or other surface mois¬ 
ture. After the normal incubation period both lots developed the dis¬ 
colored markings characteristic of the disease; but although on successive 
moist nights the exposed plants produced abundant conidiophores, the 
plants indoors, on the contrary, consistently failed to produce any coni¬ 
diophores whatever. After this had gone on for 10 days a few plants 
from the exposed lot were brought indoors, and a like number of the 
indoor lot were placed in the field. On the first favorably moist night 
thereafter conidiophore production took place on the latter newly ex¬ 
posed plants, although previously, while indoors, this had never occurred. 
On the contrary, the plants which had been supporting abundant coni¬ 
diophore production in the field now, when brought indoors, ceased to 
bear any conidiophores whatever. Those other plants of the original 
lots which had been left indoors or had remained in the field continued 
to behave as heretofore. By varying the procedure in a series of experi¬ 
ments such as this, the dependence of conidiophore production on noc¬ 
turnal moisture on the host was clearly demonstrated. When once this 
dependence was realized, it was comparatively simple to manipulate 
conditions so that the formation of conidiophores was induced or stopped 
at will. 
It should be noted, however, that conidiophore production on an 
infected plant can not thus be induced month after month indefinitely, 
but if continued at intervals in experiments will gradually cease, 
being limited in duration as it is in plants growing undisturbed in the 
field. For example, if under field conditions the production of conidia 
