Jan. 6,1923 
A Phytophthora Footrot of Rhubarb 
5 
day 3.27 inches, followed by two cloudy days. During either period 
conditions undoubtedly favored the development of a downy mildew. 
During the summer of 1918 no such period of prolonged wet weather 
occurred. In 1920, when the next observations on the disease were 
made, several wet periods occurred, the longest of which was from July 
15 to July 23, every day being listed as cloudy. In fact, only one day, 
the 19th, showed as high as 40 per cent possible sunshine. During these 
nine days 5.6 inches of rain fell. It was during the latter part of the 
period that the rhubarb disease appeared at its height. 
The extensive occurrence of rhubarb footrot only during such weather 
conditions agrees with the observations of other workers on other diseases 
of the same type. Clinton wrote in 1905 (6): 
The prevalence of most parasitic fungi is largely influenced by the character of 
the weather, particularly in regard to moisture. This is especially true of the downy 
mildews. An abundance of cloudy or rainy weather at certain periods of the year 
determines whether or not these troubles will be injurious. 
CAUSAL ORGANISM 
ISOLATION 
When investigations on the disease were started, several organisms 
had to be isolated and studied before the true cause was discovered. 
Prominent among these was Colletotrichum erumpens Sacc., which has 
already been mentioned as being almost universally found on decumbent 
rhubarb stems. This fungus was reported by Stevens (24) as the 
cause of a rhubarb disease in Illinois in 1919. One feature that cast 
suspicion on it in particular was that it was found fruiting in distinct 
lesions, such as are shown in Plate 3, B, on freshly fallen rhubarb stems. 
Furthermore, it appeared to be constantly associated with the footrot; 
the abundance of infective material on old and dried as well as on freshly 
fallen leaves would seem to be conducive to such damage as was noted. 
Single spore isolations were made after considerable difficulty to free 
them from bacteria, and inoculations were made upon living plants in 
the greenhouse, under most favorable conditions as regards moisture and 
temperature. Results were consistently negative. Stems cut and 
inoculated in a moist chamber became infected, but the symptoms 
differed essentially from those of the footrot disease. Several bacteria 
were isolated and used for inoculations without positive results. The 
finding of a coenocytic fungus in the tissues of fresh material at this time 
led to a concentration of effort on that and abandonment of further 
systematic study of the other organisms. 
This Phycomycete, after its presence was first noted in newly collected 
material, was constantly found associated with the typical disease. Re¬ 
peated attempts were made to isolate it by the tissue fragment method 
on poured plates of corn meal and other agars. While it grew very 
readily in most of the media used, it was always accompanied by con¬ 
taminating bacteria, often even out to the very tips of the growing 
hyphae. Finally in September, 1917, pure cultures were obtained. On 
corn meal agar plates the organism, contaminated as it was with bacteria, 
sometimes made a rather profuse aerial growth. By cutting sheets of 
sterile agar from a poured plate and laying them one on top of another, 
little inverted pryamids were built on the under surface of the cover of 
a Petri dish. These were turned with the cover so as to be directly over 
the colonies with good aerial growth. In two cases out of several attempts 
