De CIRCULAR 263, U. S. DEPARTMENT OF AGRICULTURE 
it is likely that the mildew problem is here to stay, and that destruc- 
tive outbreaks will depend on the occurrence of weather conditions 
favorable for the disease. It is possible that the mildew may even 
extend to new districts as yet untouched, such as the Burley areas 
of Kentucky and Tennessee. It is to be regretted that the informa- 
tion available regarding this disease is very incomplete, and that. 
despite years of work, the Australians have not as yet developed 
satisfactory methods of control. Federal and State agencies are 
cooperating in research work designed to develop control measures, 
which is being pushed as rapidly as possible. This circular is 
intended to summarize the information now available. 7 
DESCRIPTION OF THE DISEASE 
Downy mildew is primarily a seed-bed disease, though occasion- 
ally it attacks plants in the field. The first indication of the disease 
in a bed is the appearance on a few leaves of indefinite yellow 
blotches. On the under surface of these spots is a cottony fungus 
growth (fig. 1) that is either white or pale violet in color. On the 
upper surface of the spots numerous brown flecklike lesions soon 
appear, and in a few days these enlarge and run together, forming 
irregular dead areas (fig. 2) that give the leaves the appearance of 
having been spattered with boiling water. At the same time that 
the attacked leaf tissues die and dry out, the fungus growth disap- 
pears from the lower surface of the spots. Later the dead leaf tis- 
sues drop out, and the leaves may become twisted as portions still 
alive continue to grow. There are many things which may cause 
spots on tobacco leaves, but finding the cottony growth on the lower 
leaf surface is an almost sure proof that the trouble is downy mil- 
dew. Final verification can only be had by observing the charac- 
teristic conidiophores and spores (fig. 3) under a microscope. 
Small plants are likely to be killed outright by the first attack of 
the disease. Older plants may lose most of their leaves and still 
have strength to make new fohage if aided by a favorable change of 
weather. Transplanting from diseased beds before recovery has be- 
gun often results in the death of large numbers of the plants set. 
In the field, affected plants show dead brown spots of irregular 
shape, but this leaf injury usually is not serious. A more serious 
type of field injury has been the stunted uneven growth of plants in 
portions of fields set from diseased beds. This condition has been 
very common in some localities. 
LIFE HISTORY OF THE MILDEW FUNGUS 
The mildew fungus, Peronospora hyoscyami, viewed through a 
microscope, has definite structures, and the cottony growth of the 
under leaf surface is made up of mycelium, conidiophores, and 
spores. These correspond to the roots, trunks and branches, and 
fruit of a tree. The spores are very light and are produced on the 
tips of the conidiophores (fig. 3) in tremendous numbers. They are 
probably carried long distances by the wind. When a spore reaches 
a tobacco leaf, it germinates, and the mycelium grows into the leaf, 
killing the cells and producing a spot. About a week after germi- 
nation and leaf penetration have occurred, the surface fungus growth 
