Dec. x, 1923 
Habits of the Cotton Rootrot Fungus 
4 i 5 
It was frequently observed that the rate of advance in cotton fields 
was greater than in alfalfa fields. On August 21, five cotton plants, 
located at least 20 meters apart, were found dying from rootrot in a field 
where infection had never been seen. At the time when first observed 
in a wilted condition there was no evidence of any other rootrot infec¬ 
tion within 25 meters. Only two of the spots resulting from these 
centers of infection maintained the regularity of their circular margins 
throughout the remainder of the season; the other three became irregular 
because of new centers of infection appearing nearby and merging with 
them as they spread. By October 10, the two regular circles of dead 
plants were 9.1 and 9.4 meters in diameter, having progressed this dis¬ 
tance in 50 days. 
Tabl,b IV .—Rate of spread of rootrot circles during one year of activity 
Circle. 
Diameter 
September 30, 
1921. 
Diameter 
June 22, 
X922. 
Diameter 
September 30, 
1922. 
I.. 
Meters. 
7 . 0 
Meters , 
0. I 
Meters. 
14. 6 
22 2 
a . . 
8.8 
4. 6 
y* x 
13 . 3 
8.8 
2. 
* 6 ' * 
13 . 8 
RESEMBLANCE TO OTHER FUNGI WHICH SPREAD RADIALLY 
Duggar (j) has compared the radial manner of spread of the rootrot 
organism to that of “ Rhizoctonia crocorum” In many ways the aerial 
behavior can be compared to that of the fungi which cause fairy rings. 
The rate of spread is of course much more rapid than that of fairy rings, 
but the manner of spreading centiifugally from a central point of infec¬ 
tion is the same. Ritzema Bos (5) likens the advance of a fairy circle 
to the progressive advance of a flame which results from dropping a match 
in dry grass and which will spread continually outward. No better 
figure could be used in describing the field activity of rootrot. Attention 
has already been called to the resemblance between rootrot areas and 
fairy rings in the manner of the renewal and radial expansion of vegeta¬ 
tion at the center. The outer margin of the renewed growth gradually 
advances, keeping pace with the progress of the border of the original 
circle. There are many indications that the interiors of the rootrot 
circles become free from the disease at least for a year after the attack 
has passed on. Occasionally one of the recovered plants will die, and 
the infection will spread to neighboring plants, but this behavior is not 
common. In 1918 the writer had under observation several diseased 
spots of Hairy Peruvian alfalfa in which the inside of the areas had been 
replanted with Grimm alfalfa. During the entire season none of the 
plants of the Grimm variety showed any infection. On July 12, 1922, 
the spot shown in Plate 6, A, was spaded up and planted to cowpeas. 
No rootrot infection occurred during the season, and the cowpeas grew 
luxuriantly (pi. 3, A). 
A characteristic of rootrot is that the fruiting bodies occur only near 
the margin of the diseased circle. This seems to be a further indication 
that the disease is not active except near the rim of the circle (pi. 6, D). 
It is not the purpose of the writer to try by this comparison to establish 
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