mycelium is beyond the wilted circle of plants. Likewise, when the growing season is over the only points where the active mycelium of Ozonium is likely to be present are on the periphery of the dead areas. Ozonium mycelium in the form of strand hyphæ may, perhaps, live through the winter on live cotton and alfalfa roots, as on native plants already described, and when conditions are again favorable new centers of infection begin at these points where this type of mycelium has withstood the winter. It is not absolutely essential that the roots remain alive, for many observations have been made of dead cotton roots covered with Ozonium mycelium. which were not only the source of conidial mats but also producing centers of infection.

Scofield (5) and Taubenhaus and Killough (8) show by diagrams that root-rot spots in cotton fields frequently do not occur in the same place from season to season, but reappear close by in other spots. Perhaps this can be explained by the following analogy: The spread of Ozonium can be compared to the advance of a grass fire. To begin with, a lighted match thrown into the grass may or may not start a fire. If conditions are favorable it may spread rapidly. When conditions are unfavorable it may do little damage and die down. So with root rot; a number of centers of infection in June may spread very rapidly if all conditions are right. On the other hand, they may make moderate progress or very little, depending on the conditions favoring or inhibiting their spread. During some years Ozonium does much more damage than in others, only a small percentage of plants dying one year and nearly 100 per cent the next. It is especially noticeable in plat experiments that a high death rate over the plat one year is often followed by a much lower death rate the next season. Further, when practically all the plants are killed in a plat one season, the plants begin dying the following year at the outer borders of the plat. A difference between the spread of a grass fire and root rot is that the fire goes out eventually, whereas root rot is able to begin its activities each season and start its destruction from the points where it left off the previous season. Thus, in cotton fields new centers of infection at the periphery of the old spots formed the preceding year tend to advance in all directions, killing all the plants in the line of advance but less completely invading the spot formed the previous year.

These observations may explain why cotton root rot may not recur in the same spots. King (4) has shown that in alfalfa fields the new centers start about 70 centimeters from the periphery of the old spot, and subsequently these new centers merge and advance outward, the extent of regression depending on the number of reestablished plants that are present in the old spot for the fungus to attack.

PURE-CULTURE ISOLATION OF OZONIUM OMNIVORUM

A summary of the pure-culture isolations of Ozonium omnivorum showed that it could be cultured only from the small fresh lesions (depressions) on the roots. Only rarely were cultures obtained from the taproots, where the mycelial weft and strands were present in any quantity. No cultures were obtained from plants which

were severely wilted, except occasionally when a young lesion was found farther down on the taproots.

The most successful method used in isolating Ozonium was as follows: Plants just showing the first signs of wilting or healthy appearing plants next to these in a row were dug up and the loose dirt washed off. Bits of tissue with the young lesions present were cut out and dipped in mercuric chloride, 1 to 1,000, for 30 to 60 seconds. Instead of washing off the mercuric chloride as is the custom, the excess of mercuric-chloride solution was taken up with filter paper and the pieces dropped on cotton or alfalfa root plugs in test tubes. As soon as the mycelium grew from the bits of tissue on to the plugs, the pieces were removed, to prevent contamination with slower-growing organisms. By the use of this method about 75 per cent of the cultures attempted yielded Ozonium.

Attempts were made to culture the mycelium making up the conidial mats, but in no instance was a pure culture obtained. Similarly, no pure cultures were obtained from the Ozonium strands when taken from diseased roots.

THE FUNGUS

No detailed account of the various stages of the fungus will be presented, as in the main the observations made by the writers are in accord with those of Shear (6), Duggar (1), Taubenhaus and Killough (8), and others.

STERILE MYCELIAL STAGE

The type of sterile mycelium of the fungus Ozonium omnivorum universally present on the healthy roots of the plants in advance of the wilting zone is the strand hyphæ, composed of several or many bands of hyphæ, the cells of which vary in shape and size. These strand hyphae vary in color with age, but are usually buff or brownish. They branch off in a more or less regular fashion, but when spreading out over filter paper from an infected cotton root they may branch to produce a fan-shaped arrangement of strands. (Pl. 5, fig. 1.)

These strands may be compared to some extent to the rhizomorphs of some of the Basidiomycetes. As has been pointed out before, the function of these strands appears to be in the nature of advance mycelia, which spread through the soil and are responsible for the direct penetration and infection of the roots.

The acicular type of hyphæ is generally associated with the strands found on the roots. In culture this type has been found on cotton and alfalfa root plugs and sterilized soil, but not on any media containing agar.

The mycelial weft so prominent on the roots of diseased plants consists principally of large Rhizoctonialike hypha, together with some acicular hyphæ and a few strands. The large-celled hyphe also form the major portion of the pseudosclerotia, although the outer surface may be covered by strands with acicular hyphæ.

The large-celled hyphæ break up and collapse very readily. Only the young growing tips of these hyphæ appear normal. The

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 another 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 omnivorum (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 barrelshaped cells of hyphæ could be observed arising from these strands. This mycelium is more or less branched in a fan-shaped or tufted manner. On the smaller ends and sides of these tufted hyphæ the spores were borne. These abortive spore mats were very small (1 to 2 centimeters in diameter) and only a few layers of hyphæ

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 4 centimeters in thickness. Usually the first indication of their development is the production of a loose mass of cobweblike 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.) (Pl. 6, fig. 1.) Where the mat continues to grow for more than one day each day's growth is clearly defined. (Pl. 6, fig. 2.) On the second day the first day'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 become 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 favorable for their development.

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All attempts to germinate the spores of the conidial stage failed, in spite of the fact that spores of all ages were used under many varied conditions, with a large number of soil and plant decoctions, stimuli, and other methods used in spore germination. In a few instances the spores obtained from newly formed spore mats placed in distilled water and held in an ice box became slightly swollen, and some semblance of a germ tube was produced; but otherwise no reaction of any sort occurred.

The conidial stage of Ozonium seems to be very similar to the Corticium stage of Rhizoctonia solani Kühn. In both instances the spore stage does not appear to be necessary in the life cycle of the fungus, as both organisms can maintain themselves from year to year by means of the sterile mycelium. The Corticium spores are very difficult to germinate; in fact, the only successful method developed has been the placing of spore-bearing material over an agar plate, and only a few of the mature spores which fall on the agar germinate. Although the germinating Corticium spores develop on agar to produce the sterile mycelium of Rhizoctonia, which in turn can cause infection, no one has yet succeeded in infecting susceptible plants with the spores. As in the case of Corticium vagum B. and C., it is doubtful whether the conidia of Phymatotrichum omnivorum will be found of any importance in the propagation, development, and spread of Ozonium.

PERFECT STAGE

Shear (7) has recently described a perfect stage of this fungus which he has named Hydnum omnivorum, based on the connection between typical Ozonium hyphæ and the formation of a hymenium of the above-described Hydnum.

In this connection it might be stated that during the summer, and especially after every irrigation at Sacaton, Ariz., a number of small (2 centimeters) fan-shaped mats were found growing on the soil in well-shaded spots under cotton plants. Most of these mats were of an orange-yellow color. A number of these were dug out and found to arise from a rhizomorphic mycelium very similar to Ozonium strands on the old cotton roots invariably found under these mats. Specimens were sent to E. A. Burt, of the Missouri Botanical Gardens, who identified them as resupinate Hydnums.

Perhaps these specimens found so abundantly at Sacaton are the same as those described by Shear. Of course, final proof of this connection, as Shear states, must be obtained by germinating the spores of the Hydnum to produce the typical mycelium of Ozonium. It is interesting to note in this connection that the soils of the Southwest seem to be particularly adapted to the growth of many rhizomorphic fungi.

INOCULATION EXPERIMENTS

Of the many attempts to produce infection with Ozonium by those who have studied this disease, very few have been successful. Taubenhaus and Killough (8) reported the results of successful inoculations on 10 young plants of cotton. While no doubt the organism caused a wilting of the plants and could be recovered in culture, the plants were grown under very abnormal conditions. So far no one

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 sterilized 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 inoculation was the only successful one. The inoculation was made August 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 before, the actual penetration of the plants is no doubt accomplished by the strand hyphæ, 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 hyphæ in the advance zone. Under pureculture conditions the tendency of the fungus is to produce mycelium similar in nature to the hyphæ which make up the mycelial weft found on the roots of wilted plants rather than the strand hyphæ.

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 correlations 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