8 BULLETIN 1417, U. S. DEPARTMENT OF AGRICULTURE 



apparently healthy to the 115-centimeter level. Down to 35 centimeters the 

 bark was more or less disorganized, including some of the wood elements. 

 The mycelial weft extended from the crown to about 20 to 25 centimeters 

 down the roots, then strand hyphse of varying numbers and colors extended 

 to about 45 centimeters farther down the surface of the roots. Below this 

 point for about 10 centimeters there were numerous depressions at points where 

 the fibrous roots entered the taproot. The roots on the outer edge of this zone 

 were not as badly rotted as the others. 



Zone 3 : Tops healthy ; roots infected. The plants nearest zone 2 would 

 wilt soon, while those of the outer edge of zone 3 would not have wilted for 

 two or three days. This was the advance zone, where the strand hyphae 

 spread out and caused infection. In this zone also the soil was permeated 

 with strand hyphse. (See pi. 5 fig. 1.) Strand hyphse were numerous on 

 the taproots for about 25 centimeters. The mycelial weft was just begin- 

 ning to organize on the upper 15 to 20 centimeters of the root, and many 

 depressions were found around the point where the fibrous roots entered 

 the taproot. The lesions were still small and only in a few cases had they 

 penetrated through the bark. On the last plant in this zone only one or two 

 strand hyphae were found. No depressions had developed as yet. 



Zone 4: Tops and roots healthy. No mycelium of any sort was found on 

 the roots. At the rate the plants died in the wilting zone the strand hyphae 

 must travel several' centimeters or more a day through the soil. 



SUMMARY OF CONDITIONS OBSERVED IX EXCAVATIONS 



The notes made from an examination of a number of excavated 

 plants can be summarized as follows : On the underground parts the 

 first evidence of Ozonium is the presence of a few strand Iwphse on 

 a healthy root. The mycelial accumulations generally occur at the 

 base of the small rootlets on the taproot at any point in the first foot 

 of soil. The formation of the depression and the invasion of the 

 cambium and stele are similar to those described for cotton. The 

 amount of mycelium present on the alfalfa roots is more plentiful 

 than on cotton. Kotting of the roots proceeds faster down the tap- 

 root than up. The line of demarcation between healthy and dis- 

 eased tissue at the crown is not nearly so sharply delimited as in the 

 case of cotton nor is there any constriction between these tissues. 



Under favorable conditions the time interval between the presence 

 of the strand hypha? on healthy roots and the wilting of the plant is 

 approximately a week. Of all the diseased alfalfa and cotton roots 

 examined, no instances were observed in which rotting proceeded 

 from the ends of a side root to the taproot, and no observations were 

 made that showed that the fungus progressed from one plant to 

 another by the contact of a diseased root with a healthy one. There- 

 fore, it appears evident that the fungus progresses through the upper 

 foot of soil irrespective of the root contacts, and only those plants 

 are attacked at the collar which are in the line of the advancing 

 mycelium in the upper foot of soil. 



FRUIT TREES 



Owing to the manner in which fruit trees are set and grown in 

 an orchard, the point of infection and the progress of the disease can 

 not be determined as easily as in the case of alfalfa or cotton. The 

 time required by Ozonium to kill a fruit tree depends principally on 

 the size of the tree. To judge from the fruit and ornamental trees 

 that were examined, the principal point of attack seems to be at the 

 crown and on the surface roots, although centripetal infection may 





