4 
Journal of Agricultural Research v 0 i. xxin, no. i 
entering the root from one side, spreads and involves the whole plant. 
The remaining stalks are doomed to quick collapse, for the fungus has 
advanced from the other side of the plant, which has already been killed, 
and has penetrated the upper portion of the roots feeding this side. 
Within a few hours these stalks would present the appearance shown 
in Plate 4, B. 
This disease should not be confused with one very commonly found 
in rhubarb fields during the summer months, which consists of a dry 
necrosis of the base of the petiole and its consequent partition from the 
plant. The latter condition is pictured in Plate 3, C. It is in no way 
related to the Phytophthora disease, even though stalks so parted from 
the plant may appear the same (except for more marked yellowing of the 
leaves), and may go through identical processes of further decomposi¬ 
tion and decay. 
Rhubarb shows a remarkable power of withstanding and checking 
further advance of the disease if conditions are not entirely favorable to 
the causal organism. A change in the weather may prevent the spread 
of the fungus from a point of primary infection to the rest of the plant. 
In artificially inoculated plants in which the fungus was known to have 
entered the roots, however, the writer has seen the disease continue in 
its destruction even during a period of dry weather. 
INFLUENCE OF WEATHER CONDITIONS ON PROGRESS OF THE 
DISEASE 
Marked variation in the prevalence of this disease from year to year 
was noted. This appears to be due to differences in weather conditions, 
especially in the rainfall during the period of greatest danger. In 1917 
reports of the disease were numerous, and it was found by personal 
surveys to be rather general. During the next summer very few reports 
of rhubarb diseases of any kind were received. Field trips into localities 
that had been badly affected in 1917 did not bring to light any of the 
disease in its active condition. During the summer of 1919 the writer 
had no opportunity to investigate its prevalence, but the appearance of 
the experimental field the next spring indicated rather heavy loss of 
plants. In 1920 it appeared worse than ever, as was evidenced by 
numerous oral and written reports and by personal observations. A 
field at Arlington Experimental Farm, Va., just across the Potomac 
River from Washington, D. C., was attacked and plant after plant killed 
by the disease. . . . 
A study of the climatological data for Washington, D. C., and vicinity 
over the summer months of 1917 to 1920, inclusive (, 26 ), disclosed some 
interesting facts with regard to rainfall. The total precipitation for 
June, July, and August, 1917, was 16.43 inches; for the same period in 
1918, when the disease was not to be found at all, it was 7.73 inches, or 
less than half that of the previous year; for 1919, 13.65 inches; and for 
1920, 15.21 inches. It is a prolonged period of continuously wet weather, 
however, rather than a high total precipitation, that is favorable to the 
development of the downy mildews in general. In this respect, particu¬ 
larly, the weather records show an interesting correlation with the preva- 
lenceof the rhubarb disease. In July, 1917, two prolonged periodsof daily 
rainfall and cloudy weather occurred. During the first, from July 8 to 
July 11, an average of 0.36 inch fell every day, followed by several cloudy 
days. Again, on July 22, 1917, 0.59 inch of rain fell, followed by two 
cloudy days (49 and 46 per cent of possible sunshine), and on the next 
