252 
Journal of Agricultural Research voi. xxm, no. 4 
conidiophores was apparent. Microscopic examination showed that the 
first “crop” of conidia was maturing but that the conidiophores were 
dwarfed and scantily branched. A little after 4 a. m. conidiophore 
production was terminated when the dew dried as the breeze increased. 
In this case formation and liberation of conidia lasted for about an 
hour only. 
The entire process of conidium production from the emergence of the 
internal hyphae through the stomata to the final ejection of the mature 
conidia takes place with remarkable regularity. Whether on infected 
maize, teosinte, sorghum, sugar cane, Saccharum spontaneum, or Mis- 
canthus japonicus, whether on young seedlings or senescent plants, 
whether on leaves, tassels, bracts, or other infected parts, the cycle of 
conidium production under favorable climatic conditions follows a 
relatively constant schedule, which may be outlined as follows: 
7 to 8 p. m. 
10 to 11 p. m. 
11 to 12 p. m. 
12 p. m. to 1 a. m. 
1 to 2 a. m. 
2 to 3 a. m. 
3 to 4 a. m. 
4 to 5 a. m. 
5 to 6 a. m. 
Clear, still evening; dew deposition begins. 
Plant surface wet with dew; groups of knoblike conidiophore 
initials are forming over the stomatal pores on the leaf surface. 
From the knoblike bases the clavate main axes are developing 
and beginning to form primary branches of the conidiophores. 
The branch system of the conidiophore is now completely, devel¬ 
oped, and conidia are beginning to bud out, while the basal 
cell is being cut off by a cross wall. 
Discharge of the conidia and their dispersal by air currents begins 
to take place. 
Discharge and dispersal of conidia is at its maximum. 
Discharge of conidia lessens. Some of the first conidia dis¬ 
charged and scattered have reached the water held in the 
unrolling leaves or in the leaf axils of young susceptible plants, 
and there have germinated and started infection. 
Few conidia are discharged or further conidiophores developed 
but germination and infection by conidia already spread 
continues. 
Sun rises and soon dries the plant surfaces, killing the few coni¬ 
diophores and conidia still developing from infected plants, 
and also the conidia which have not yet achieved the infection 
of the new hosts they have reached. Those conidial germ tubes, 
however, which have already penetrated in to the host tissue 
are not killed when the plant surface is dried but continue to 
invade the host within whose tissues they are protected. 
The procedure which has just been outlined is beautifully adapted 
to the limitations of the delicate conidia which when dried are killed 
immediately. Borne on conidiophores which are submerged in dew, 
carried in countless numbers through the cool darkness on currents of 
damp air, dropped into the moisture contained by such susceptible parts 
as unrolling leaves, and there sending germ tubes into the succulent 
interior tissue of the host, the conidia, despite their delicacy, are able to 
accomplish infection in a remarkably efficient manner. 
This procedure is also exceedingly successful in accomplishing the 
rapid spread of infection and distribution of the disease. Plants thus 
diseased will in turn begin producing conidiophores after about 10 days 
or, if they are young, even in 3 to 5. As a result, a small center of 
inoculation can under favorable conditions cause the infection of large 
areas in a relatively short time. 
DURATION OF CONIDIOPHORE PRODUCTION 
The development of conidiophores and conidia from the downy 
mildew mycelium ensconced within the diseased maize plant is not limited 
to one night, nor does it cease after it has occurred several nights, but 
