4*6 
Journal of Agricultural Research 
Vol. XXVI. No. 9 
the relationship of the rootrot fungus to the hymenomycetes, but it is 
hoped that this characteristic of the organism, heretofore not described 
may prove of some significance to taxonomists who may become inter¬ 
ested in the disease. There seems to be sufficient evidence to indicate 
clearly that the organism belongs to the basidiomycetes. 
Although technical proof is still lacking that the conidial material is 
the true fruiting form of Shear’s Ozonium omnivorum , there can be little 
doubt that it plays an important part in the life of that organism. Its 
frequent occurrence and conspicuous appearance under Arizona con¬ 
ditions makes it an important and easy means of establishing the identity 
of rootrot activity when that is likely to be confused with the effect of 
alkali salts, lightning, or rodent injuries. The presence of the mats on 
fallow or virgin lands should be sufficient indication of the risk of planting 
susceptible tree crops or other plants of a permanent nature. 
POSSIBLE CONTROL OF ROOTROT IN ITS INCIPIENT STAGE 
The resemblance of the manner in which rootrot spreads to the advance 
of fungus fairy rings suggested to the writer the possibility of some 
barrier treatment whereby the infected areas might be isolated and their 
advance checked. Several methods of treating small areas with formal¬ 
dehyde were attempted in September, 1921, and June and July, 1922, 
but these were only partially successful. On August 3, 1922, three small 
infected areas about 1 y 2 meters in diameter in an alfalfa field were segre¬ 
gated from the remainder of the field by throwing up circular dikes of 
soil 45 cm. in advance of the circle of dying plants, and the soil within 
was saturated to a depth of 1.2 meters by pouring in a solution of formal¬ 
dehyde (1 part 40 per cent formalin to 100 parts water). Three control 
areas were also designated, and the limits of the disease indicated by 
small stakes or by dikes of soil. On August 19, photographs were taken 
of one treated and one control area (pi. 7) to show the difference in the 
rate of spread 16 days after treatment. The average increase in the 
diameter of the control areas was 1.1 meters, while the treated areas 
showed no dead plants outside of the treated circle. On September 30, 
the control areas had increased by an average of 3 meters, while the 
treated areas showed no dead plants outside the dikes (pi. 7). 
On August 21, eight small areas of infection, each consisting of only 
one or two dead plants, were selected in a cotton field in which the spots 
were just beginning their seasonal appearance and were yet 25 or 30 meters 
apart, and were treated as above described for the alfalfa areas except 
that the dikes were thrown up about 90 cm. outside of the infected plants. 
It was found necessary to extend the treatment to this distance in order 
to include all infected roots. On September 30 no other plants had died 
from the disease on the periphery of these spots, although three of the 
areas had to be disregarded because of new spots beginning near by and 
invading the soil contiguous to the treated areas. Five control areas 
were located, only two of which maintained their circular form; these had 
increased in diameter 9.1 and 9.4 meters respectively. 
It was observed that healthy alfalfa and cotton plants were seriously 
affected by the formaldehyde, even when diluted to 1 part formalin in 
400 of water, but after shedding their leaves the plants soon recovered 
and resumed growth. There was no evidence that the soil fertility was 
at all impaired by the formalin solution used in the extermination of the 
