OZONIUM ROOT ROT 15 



has reported successful inoculation with Ozonium on well-developed 

 plants under greenhouse conditions. King (3), however, reports 

 that he has infected cotton plants under field conditions. 



During the summer of 1924 a number of inoculation experiments 

 were attempted at Sacaton, Ariz. The fungus was grown in a steri- 

 lized mixture of sand and cotton roots in 2-quart Mason jars with 

 modified caps. These jars were allowed to stand for several weeks 

 until the soil was permeated with the strands of the fungus. In 

 placing the inoculum in the field the jars were broken and the mass 

 taken out intact, so that the mycelium which held the whole mass 

 together was not disturbed. 



All inoculations were made without wounding the roots on both 

 cotton and alfalfa plants prior to irrigation. In most instances the 

 inoculum was buried in a cotton row between two plants at a depth 

 of 8 to 10 inches. As a supplementary aid, burlap sacks were spread 

 over the point where the inoculum was buried in the soil, and these 

 sacks were kept moist, so that conditions were ideal for the fungus 

 to grow out and reach the plants. 



In only one instance was the inoculum placed in direct contact 

 with the roots of a cotton plant (the Acala variety), and this inocu- 

 lation was the only successful one. The inoculation was made Au- 

 gust 21, and complete wilting of the plant occurred September 15. 



Under most of the burlap bags where the soil remained moist, 

 abortive spore mats were found seven days after the inoculum was 

 placed in the soil. The fruiting mycelium did not spread any farther 

 after the first day and in a few days disappeared entirely. The 

 spores produced in this manner when compared with spores from 

 the large mats were identical in size, shape, and color. It seems 

 strange that these abortive mats were produced from pure-culture 

 material in the soil within a week, but that none of the plants near 

 where these mats, were produced became infected. 



To the writers it seems that the failure consistently to produce 

 infection with pure cultures of Ozonium lies in the fact that we are 

 dealing with a polymorphic fungus. As has been brought out be- 

 fore, the actual penetration of the plants is no doubt accomplished 

 by the strand hyphse, which are always found in .advance of the 

 wilting plants, while the mycelial weft so common on wilting plants 

 serves to break up the host tissues in order to build up a food reserve 

 to supply the active strand hyphse in the advance zone. Under pure- 

 culture conditions the tendency of the fungus is to produce mycelium 

 similar in nature to the hyphse which make up the mycelial weft 

 found on the roots of wilted plants rather than the strand hyphse. 



DATA COLLECTED IN PREVIOUS YEARS 



A thorough digest was made of the root-rot data collected during 

 the period from 1916 to 1924 in the rotation and tillage experiments 

 at the field station near San Antonio, Tex., but no general correla- 

 tions could be found, although some points of interest were brought 

 out by this study. In all rotation and tillage experiments root 

 rot was present to some degree every time a susceptible crop was 

 grown. Although on the average the percentage of loss was lower 

 than that in the continuous cotton plats, even on these plats the 

 loss might vary from 2 to 96 per cent or more from one season to 



