268 
Journal of Agricultural Research volxxiii, no. 4 
Wind, in so far as the writer was able to determine, is the most im¬ 
portant factor in scattering the conidia of the Philippine maize mildews. 
In the other cereal downy mildews of the Orient various methods of 
conidium dispersal have been emphasized by previous investigators. 
Unfortunately, Butler’s (5) discussion of the downy mildew of maize in 
India does not consider this point. With regard to the Sclerospora of 
com and spgar cane in Formosa, Miyake (15) states that he obtained no 
direct experimental data on the methods of spread of the conidia in the 
field because he could not collect spores with germinating power. From 
field observations, however, he concludes that the spores are distributed 
by wind, by rain, and by irrigation water; but he regards the distribution 
of diseased cuttings of cane as the most dangerous and effective method 
of spread. In the case of the Javan maize mildew, Rutgers ( 20 ) disre¬ 
gards the matter of conidium dispersal and considers it doubtful if 
infection by conidia occurs normally in the field in spite of the fact that 
Raciborski ( 19 ) had secured infection with conidia and had concluded 
that they undoubtedly were spread by the wind. Palm (16), however, 
rightly infers from field observations that the disease spreads by the 
wind dispersal of conidia, and he fully appreciates the importance of 
this method. It should be noted, however, that his experimental evidence 
(the finding of conidia in Petri dishes exposed in the path of the wind 10 
to 50 meters from a 60 per cent infected field) is valueless. As these plates 
were exposed during the day and “Het weer was zonnig en droog,” the 
conidia caught were undoubtedly the dried remainder from a previous 
night’s crop, quite dead, and quite incapable of germination or infection. 
By his experiments “the presence of the conidia in the air was shown” 
as he says (in translation), but as these conidia were incapable of germi¬ 
nation their presence in the air gave no proof of their ability, wind-borne 
through they were, to spread infection. It would be analogous to reason 
that certain species of grasses are distributed by ocean currents because 
the seed, although killed by immersion in salt water, are carried long 
distances in the sea. 
SPLASH DISPERSAL 
Dispersal of the conidia is also accomplished by splashing. On dewy 
nights this is constantly taking place as the large drops of accumulating 
dew drip from the upper leaves, strike the conidiophore-bearing surfaces 
of the leaves below and splash off, carrying innumerable conidia to 
neighboring plants. Droplets thus splashed off, when caught on sheets 
of glass and examined with the microscope, were found to contain large 
numbers of conidia which germinated vigorously in dew or on agar. 
Although the spreading of infection undoubtedly occurs in this direct 
way, it is of comparatively limited extent and involves only plants in 
the immediate vicinity. During rains, however, provided conidium pro¬ 
duction is taking place, the conidia are splashed far greater distances; 
and, when the splashed-off droplets are carried by the wind, whether 
by the gentle air currents of still dewy nights or by the gales of violent 
tropical storms, the distance of dispersal is greatly and correspond¬ 
ingly increased. 
The distance over which such droplets are spread either by simple 
splashing after falling from various heights and upon different kinds of 
surfaces or by spattering into and being carried away by air currents 
of various strengths has been studied very thoroughly by Faulwetter (10). 
