258 
Journal of Agricultural Research voi. xxiii, No. 4 
after the plants either when young or when quite passe were cut back to 
the ground surface. Thus conidium formation by the parasite on 
Saccharum spontaneum can be as persistently perennial as the grass itself. 
QUANTITY OF CONIDIUM PRODUCTION 
The number of conidia produced from a single maize plant infected 
with downy mildew even during one night is exceedingly great, while 
the total conidium production from such a plant throughout the entire 
season is appallingly vast. It is necessary to consider this matter further 
in order to appreciate the infectious menace to a maize-growing district 
which is embodied in even a single diseased plant. The amount of coni¬ 
dium production from any one plant during any one night depends on 
three factors: First, the extent on the host of the discolored area which 
is most heavily invaded with mycelium and especially active in conidio- 
phore production; second, the number of stomata in both surfaces of 
this area; and third, the proper environmental conditions to induce 
conidium formation. 
The number of conidia borne by a plant is directly proportional to the 
extent of the yellowed, whitened, or otherwise symptomatically dis¬ 
colored part of its surface. Although the hyphae of the parasite are to 
some extent found in and may give rise to conidiophores on the appar¬ 
ently unaffected parts of the host, it is from the discolored areas which 
have become etiolated through the destructive invasion of abundant 
mycelium of the fungus that the great mass of conidiophores emerges. 
The development and progressive increase in extent of this abundantly 
conidiophore-bearing surface has been studied both in inoculated plants 
grown under controlled conditions and in naturally infected plants in the 
field. The procedure is as follows: After the young plant is infected, 
there ensues a latent period of from a few days to more than two weeks, 
its length being directly proportional to the age of the plant when in* 
oculated. The mycelium spreads throughout the plant tissues and con¬ 
centrates especially in those areas of the leaves and leaf sheaths which 
begin to show symptomatic etiolation. The production of conidiophores 
on the whitened areas then commences. At first the conidiophores appear 
scatteringly (PI. i, A; 4, A, B), but production gradually increases during 
subsequent nights until the affected areas are covered with a dense down 
of conidiophores (PI. 4, C, D). In proportion as new leaves unroll from 
the bud, and as the leaves, which were already partly developed when 
conidium production began, continue to elongate by intercalary growth, 
the conidiophore-bearing surface is constantly augmented (PI. 1, C). 
Finally, when all the leaves are fully developed, the production of conidio¬ 
phores reaches its maximum. Even then, in badly infected cases, this 
production may be still further increased by the formation of conidia 
from the tassel, from the husks, which are often abnormally developed 
(PI. 5, B), and from the peculiar bractlike structures which frequently 
form in abnormal tassels (PI. 5, A) or around abortive ears. Thus the 
conidiophore-bearing surface, which may be established when the infected 
plant is only about a week old, and which at first may be only a few 
square centimeters in extent (PI. 2, A; 3, A), gradually increases in area 
(PI. 2, B; 3, B) until it attains its maximum of some two or three thou¬ 
sand square centimeters; and then, decreasing in extent as successive 
