262 
Journal of Agricultural Research voi. xxiii, No. 4 
small one and one of relatively low spore production, is able to develop 
dangerously large quantities of conidia each night; while a large plant 
with maximum spore formation in its nocturnal cycle liberates a startingly 
vast number of conidia. From extensive, badly infested maize fields in 
which 50, 70, or even more than 80 per cent of the plants are heavily 
conidiophore-bearing, an incredibly large number of conidia is produced. 
Moreover, it should not be forgotten that this production continues night 
after night for months, thus achieving a total output of conidia that is 
almost unbelievable. With the magnitude of this spore production in 
mind it is easy to understand the rapid spread of the downy mildew under 
Philippine conditions from even small isolated centers of infection, and 
to appreciate the menace to the whole maize-growing countryside which 
one infected field or even one single badly diseased plant may represent. 
Table III .—Calculation of approximate number of conidia liberated from the conidio¬ 
phore-bearing surfaces of representative maize plants during one night 
NATIVE YELLOW 
Conidiophore-bearing surface. 
Average 
number 
Average 
number 
Average 
number 
of conid¬ 
ia per 
conidic- 
phore. 
Succes¬ 
sive 
Plant 
No. 
Leaf sur¬ 
face 
(2Xarea). 
Sheath 
area. 
Total. 
of 
stomata 
per 
square 
centi¬ 
meter. 
of ma¬ 
turing 
conidio- 
phores 
per 
stoma at 
onetime. 
Number of 
conidia pro¬ 
duced on 
plant at one 
time. 
“crops” of 
conidio- 
phores 
produced 
during 
night 
(average). 
Total number 
of conidia 
produced on 
plant during 
night. 
s 
Sg. cm. 
703.40 
Sg. cm. 
39-77 
Sg. cm. 
743-17 
4,250 
2 
40 
252,677,800 
3 
758,033,400 
6 
1,670.00 
12. 54 
1,682. 54 
4,250 
2 
40 
572,063,600 
3 
1,716,190,800 
7 
2,29O.90 
98-93 
161.16 
2,389-83 
4,250 
2 
40 
812,542,200 
3 
2,438,126,600 
9 
2, 292. 10 
2 , 453-26 
5,829.48 
4,250 
2 
40 
834,108,400 
3 
2,502,325,200 
5,946,069,600 
IQ 
5,258.40 
S7i-o8 
4,250 
2 
40 
1,982,023,200 
3 
MEXICAN JUNE 
12 
2,098.60 
12.45 
2,111.05 
4, 250 
2 
40 
7 i 7 , 757 ,ooo 
1,038,156,000 
3 
2,153,271,000 
3,114,468,000 
14 
3,022.80 
30.60 
3 , 053-40 
4,250 
2 
40 
3 
J 5 
3,868.96 
470 . 71 
4 , 339-67 
4,250 
2 
40 
i, 475 , 487 , 8 oo 
3 
4,426,463,400 
DISPERSAL OF CONIDIA 
When the conidia are being produced their distribution, and the result¬ 
ing spread of the downy mildew, is accomplished primarily by wind, to a 
less extent by splashing or wind-blown rain, and to a still less extent by 
surface water. Obviously, distribution of effective, germinable conidia 
can take place only at night or possibly, under unusually favorable con¬ 
ditions, until a little after dawn, because the conidia can not survive 
drying. Obviously, also, any conditions restricting or favoring conidium 
production in like manner directly affect their dispersal. 
DISPERSAL by WIND 
The two Philippine Sclerosporas of maize are admirably adapted to 
distribution by Wind as their conidia are produced in vast numbers and 
are sufficiently small so that they may be readily carried in the air. The 
great number of conidia produced has already been considered. The 
size of the individual conidium, although far larger than that which 
