CIRCULAR No. 263 JANUARY, 1933 
UNITED STATES DEPARTMENT OF AGRICULTURE 
WASHINGTON, D.C. 
Phd 
~ 
DOWNY MILDEW OF TOBACCO 
By E. E. Crayton, Pathologist, and JoHN G. GAINES, Assistant Pathologist, 
Division of Tobacco and Plant Nutrition, Bureau of Plant Industry 
CONTENTS 
Page Page 
introduction =.42 se es es iL Control—Continued. 
Description of the disease_______-___ 2 Transplanting in relation: to dis- 
Life history of the mildew fungus____ 2 ease devyelopment——_ =) _ = 6 
COTO ee a ee es ee eS a) AS) SUELO See oS = eI oe era 6 
Number and location of beds____ ay 
INTRODUCTION 
Downy mildew, or, as it is frequently called, blue mold, caused 
by the fungus Peronospora hyoscyami DBy., is a disease compara- 
tively new to tobacco growers of this country. It has long been a 
problem, however, in Australia, where its destructive nature is only 
too well understood. In Australia, as early as 1891, this disease 
was said to be threatening the tobacco industry with extinction ; 
and, as late as 1931, a responsible authority there again referred 
to it as the industry’s most serious menace. 
In the United States, the disease was first observed on cultivated 
tobacco in 1921, but neither in that year nor in 1931, when it was 
again seen, did ‘it. do any serious damage. Consequently, while its 
possibilities were recognized, prior to 1982 there was no way of 
knowing with certainty how serious a problem it might become in 
this country. However, this question has now been answered very 
definitely, for beginning i in Georgia in January, 1932, and continu- 
ing until early summer, it was active over a constantly enlarging 
area that finally reached from Florida to Pennsylvania. The worst 
damage was done in North Carolina, South Carolina, and parts of 
Georgia, where plants were destroyed in enormous numbers. This 
disease was an important factor in a 350,000-acre decrease in the 
1932 crop of fiue-cured tobacco. In addition, much of the crop set 
in the affected areas was late, because of the shortage of plants, 
and the stands secured were poor and irregular in growth. Com- 
petent observers report that the mildew disease, as it developed in 
~ 19382, showed all the destructive characteristics that have long made 
it the Australian tobacco growers’ most serious problem. 
Since 1932 is the first year that the mildew has been important in 
the United States there are many questions about it yet to be 
answered. The first is what it may be expected to do in the imme- 
diate future. Judging from experience with this disease in other 
countries and from the history of similar diseases in this country, 
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