Jan. 27,1923 
Production of Conidia in Philippine Sclerosporas 253 
rather may continue to take place every favorable night for a month, six 
weeks, or even so great a proportion of the plant’s existence as two or 
more months. Because this has not been realized hitherto, the danger 
of even a single conidium-producing plant to the surrounding fields has 
never been sufficiently appreciated or emphasized. 
The duration of nightly conidium production was studied in a large 
number of infected plants, including maize, teosinte, sorghum, sugar 
cane, Saccharum spontaneum , and Miscanthus japonicus. At first the 
plants were examined each night, but later the following more convenient 
method was used. On the badly infected leaves of each plant an area, 
on which formation of abundant conidiophores occurred nightly, was 
outlined on both the upper and lower surfaces with waterproof ink. 
Each afternoon these areas were carefully scraped clean from all remains 
of previous conidiophore production; and early the following morning 
the presence or absence of conidiophores and conidia showed whether 
the fungus had been active during the night. The lengths of the periods 
of nightly conidiophore production in several typical cases of infected 
maize are presented in Table I. 
A series of such observations extending through the wet, dry, and 
transitional seasons brought out the following points. The length of 
time during which nightly production of conidia may take place on maize 
depends primarily on the time normally required by the host to attain 
maturity, on the vigor of the individual attacked, and on its age when 
infected. In the case of maize varieties which mature most rapidly, such 
as the “dwarf early” sweet and pop corns (60 to 70 days), or the Chinese 
or Mindanao waxy endosperm maize (70 days), the time over which 
nightly conidium formation may extend is relatively short, usually not 
over a month. In maize varieties, which mature more slowly, such as 
the several native flint types which are grown throughout the Philippines 
and other tropical places, the time is somewhat longer; while those large 
hardy, dent varieties and flint-dent crosses which require the longest 
time (3 to 4 months) to mature support the formation of conidia by the 
parasite for the longest period, for even more than 2 months. Apparently 
while the host is actively growing and developing, its internal condition 
is such as to favor both the growth of the parasite and the abundant pro¬ 
duction of the conidia; but when the host plant has matured, and its 
vitality begins to decline, its tissues apparently undergo chemical and 
physical changes which render them unfavorable for the further develop¬ 
ment and activity of the mildew. 
The vigor of the individual plant infected also affects the duration of 
conidium production. Weakling plants, which result from planting im¬ 
ported seed of foreign varieties unsuited and unacclimatized to Philippine 
conditions, succumb to the downy mildew in a comparatively short time; 
and conidium production by the parasite, although exceedlingly abundant 
for a while, is soon brought to an end by the untimely death of the host. 
On the other hand, if strong, vigorous hardy plants are attacked they 
do not succumb but continue to support the persistent parasite even for 
as long as the lifetime of a healthy plant (PI. 5, B). In some plants, after 
many weeks of abundant nocturnal spore production, conidia were still 
found developing on the middle and still green part of the last or next to 
last leaf, although all the other leaves and even the tips of these sporulat- 
ing leaves themselves were drying and brown. 
