OZONIUM ROOT ROT 13 



principal function of this type of mycelium appears to be to complete 

 the destruction of the root tissues after penetration is accomplished 

 and incidentally to serve as a food reserve for the advancing strands 

 beyond the wilting zone. 



There are intermediate types of mycelium between those described, 

 which vary in color, size, and shape, but they merge one into an- 

 other so frequently that they do not stand out as sufficiently distinct 

 to be described as different from those already mentioned. The 

 greatest differences in the mycelium occur in the soil. 



CONIDIAL STAGE 



The conidial stage of Ozonium (Phymatotrichum omrdvot v am 

 (Shear) Duggar) was observed at the United States field stations 

 at San Antonio, Tex., and Sacaton. Ariz. The simplest form of 

 the conidial stage may be called " abortive spore mats.'' These 

 usually appear in cotton fields where root rot has been present in 

 past seasons or is active, usually after every irrigation. In most 

 instances these small abortive mats were found directly over old 

 dead cotton roots, covered with Ozonium strands. These strands 

 could be traced from these old cotton roots directly through the 

 soil to the surface, where mycelium consisting of large barrel- 

 shaped cells of hyphse could be observed arising from these strands. 

 This mycelium is more or less brauehed in a fan-shaped or tufted 

 manner. On the smaller ends and sides of these tufted hypha? the 

 spores were borne. These abortive spore mats were very small 

 (1 to 2 centimeters in diameter) and only a few layers of hyphse 

 in thickness. 



The large well-organized mats vary greatly in size. Specimens 

 were examined that ranged from 2 to 25 centimeters in diameter and 

 from 0.5 to -i centimeters in thickness. Usually the first indication 

 of their development is the production of a loose mass of cobweb- 

 like mycelium, composed of large barrel-shaped cells. This mycelium 

 usually collects a little beyond the outer edge of the wilting zone 

 in a spot of root rot in an alfalfa field or on the periphery of a spot 

 of cotton root rot, generally where it is well protected and shady. 



This is followed by an accumulation of a creamy, fluffy mycelium, 

 the extent and thickness of the mass being dependent on prevailing 

 weather conditions. (PL 6, fig. 1.) Where the mat continues to 

 grow for more than one clay each day's growth is clearly defined. 

 (PL 6, fig. 2.) On the second day the first clay's growth begins to turn 

 dark. Spores are rapidly formed, so that by the end of the day 

 the mass of mycelium has been replaced by spores, which in mass are 

 of a buff color. Just as soon as the mats cease to develop they be- 

 come a buff-colored pulverulent mass of spores, similar to some of 

 the puffballs. 



The spore mats may appear at any time during the active growth 

 of Ozonium, but only when conditions for their formation are 

 favorable — cloudy weather with rain, together with a high soil 

 moisture and high relative humidity. King (4) has given an 

 account of the numbers which may be produced in Arizona, their 

 relative position about a spot of root rot and the conditions favor- 

 able for their development. 

 92792—26 3 



