Dec. i, 1923 
Habits 0} the Cotton Rootrot Fungus 
4i7 
rootrot mycelium. In a few weeks following the treatment weed growth 
became profuse within several of the treated areas. 
While it is not considered likely that such a method as is here described 
will be commonly practiced, because of the labor and expense involved, 
it seems to hold out promise of further success in the effort to master this 
destructive parasite. Even without modifications, this method might 
have possibilities for use on especially valuable fand such as city property, 
experiment farms, and bearing orchards, where the limits of the diseased 
spots could be defined by planting some kind of leguminous cover crop 
SUMMARY 
(1) Rootrot, a disease caused by Ozonium omnivorum , is becoming 
more and more serious in the Salt and Gila River Valleys of Arizona, 
where, owing to the extension of the cotton industry in recent years, it 
is found to be more widespread than was formerly thought when alfalfa 
was the principal crop. 
(2) In alfalfa fields it is the habit of the disease to spread radially 
and to form almost perfect circles, the more recent activity being defined 
by the ring of recently wilted plants on the circumference. 
(3) Conidial mats such as have been described by Duggar have been 
observed closely associated with the disease for six seasons in the vicinity 
of Phoenix and Sacaton, Ariz. 
(4) With but few exceptions, a characteristic behavior not heretofore 
described is that the conidial mats appear only in close proximity to the 
plants that have most recently succumbed to the disease. 
(5) The chief requisites for fructification of the fungus appear to be a 
heavy type of soil with a dense cover crop and humid weather with inter¬ 
mittent rainfall. 
(6) In the majority of fields examined the amount of humus and 
organic carbon was less per surface foot of soil inside of rootrot areas 
than in adjacent areas not infected. 
(7) Diseased areas in alfalfa fields become occupied by various kinds 
of weeds and alfalfa plants, originally thrifty, which have recovered by 
sending out adventitious roots from fragments of old taproots that 
have remained alive just beneath the crown. 
(8) In some seasons many of the larger spots show no renewed infec¬ 
tion, but in the case of most of the smaller spots the diseased area of one 
year seems to be merely an extension of the diseased area of the previous 
season. 
(9) The rate of enlargement of regular circles in an alfalfa field was 
about 8 meters increase of diameter per year. In cotton fields regular 
circles increased in diameter about 9 meters in 50 days. 
(10) The behavior of rootrot in alfalfa fields resembles that of fungus 
fairy rings in their manner of spreading radially, in areas becoming free 
from the disease after the active mycelium has passed on, and in the 
formation of fruiting bodies about the ring of most recent activity. 
(11) In Arizona the fruiting form provides an easy means of estab¬ 
lishing the identity of rootrot activity. 
(12) A method of controlling the disease by promptly providing 
barriers for segregating new centers of infection, and saturating soil 
around them with formaldehyde solution was tried and found effective, 
and points to the possibility of some more practicable means of control. 
