274 Journal of Agricultural Research voi. xxiii. n 0 . 4 
have been given above, one is able to understand and adequately to 
explain the most important phases of the problem presented by these 
maize mildews in the Philippines. The gradual local spread of the 
disease in some regions and its sudden scattering over extensive areas 
in others, the apparent immunity of maize in certain localities and the 
destruction of the whole crop in others, the sudden appearance of the 
disease in areas hitherto uninfected and the equally rapid disappearance 
of long persistent infection elsewhere are all no longer baffling. 
It should be noted, however, that the dispersal of conidia does not 
necessarily imply distribution of the downy mildews. Vast quantities 
of germinable conidia may be scattered by the breezes; but if they fail 
to reach susceptible parts of favorable hosts before being killed by 
drying, no distribution of the Sclerosporas will result. The following 
points must be borne in mind. To secure the production of large numbers 
of conidia there must be a heavy and lasting deposition of dew on the 
infected plants; to effect the dispersal of the conidia so produced there 
are required night breezes of favorable strength and duration; while to 
accomplish infection from the quantities of conidia thus scattered there 
must be available and accessible in the field, within carrying distance, 
young susceptible maize plants into whose dew-bathed buds and leaf 
axils the breeze-borne conidia may fall. These conditions are the limiting 
factors affecting distribution of the downy mildew through the agency of 
conidia. In the Philippines, climatic conditions and agricultural prac¬ 
tices are such that these requirements are very generally fulfilled. As a 
result dissemination by means of conidia goes on, year in and year out. 
unhindered. 
In achieving discontinuous, step-by-step distribution of the maize 
mildews, the conidia are preeminently important. At the beginning of 
each rainy season the downy mildews gradually spread in the newly 
planted maize from centers where the conidial stage has lived through 
the dry season on maize planted out of season or maintained under 
irrigation for forage. Along the edge of the broad Batangas plain, for 
example, small areas are kept in maize during the dry season at Sto. 
Tomas, where the proximity of Mount Maquiling insures unseasonable rains, 
and at Lipa where Mount Malepunyo serves similarly. At both places 
downy mildew persists on these plantings; and when the rainy season 
begins the new maize crop, exposed each night to large numbers of 
wind-borne condidia, becomes rapidly and extensively infected. Also, 
in the foothills of Mount Banahao, at San Pablo, Lilio, Nagcarlan, and 
Majayjay numerous densely and successively planted maize plots are 
maintained through the dry season to supply forage for the tough little 
ponies that are the chief means of transportation over the steep roads. 
In these plots the downy mildew is carried along to spread with disastrous 
effect to the more extensive plantings of the main maize-growing season. 
Not only can the progress of the disease be followed in limited areas 
during early stages in the development of a single maize crop, but in 
the course of the growing.season, which in some regions accommodates 
two or even three successive crops and occupies two-thirds of the year or 
more, spread over extensive areas can easily be traced. For example, 
in the Batangas plain and along the border of the Lake of Bay, where the 
progress of the maize mildew was followed closely during two years, it 
was observed to spread from the persistent foci of infection which have 
been mentioned and extend with ever-increasing severity and expansion 
