6 BULLETIN 1417, U. S. DEPARTMENT OF AGRICULTURE 



SUMMARY OF UNDERGROUND STUDIES 



The detailed notes from a number of root excavations of cotton 

 plants can be summarized as follows: The strand hyphse are first 

 observed on the taproots. At certain intervals they branch to form 

 a small ball of hyphse. Directly below these accumulations of 

 strands a small depression in the bark is found, and in this depres- 

 sion a weft of large-celled hyphse is present. Many such depressions 

 are found on one root. After the depressions have penetrated 

 the bark they apparently do not increase in diameter. When the^ 

 bark is peeled .off, the woody tissues present a dark-brown spot just* 

 beneath the depression. During the period in which the depressions 

 are increasing in diameter and depth the mycelium becomes more 

 plentiful both in and around the depressions and on the root. 



The next stage in the disease seems to be a penetration of the 

 cambium and the stele by the mycelium, which appears to progress 

 up and down the main taproot and out into the laterals. In most 

 instances the mycelium is found very abundantly on the bark in 

 advance of the rot. 



To judge from the rate at which the plants die in a cotton row, 

 the time from the formation of these depressions to the actual 

 entrance of the fungus is a matter of not more than one or two 

 days. Meanwhile the collar of the plant is inclosed in a fungous 

 mass composed of strand hyphse and mycelium. The wilting of 

 the plants after this stage is reached is a question of only a day 

 or so. 



During this stage (wilting), while the strand hyphse are still 

 very much in evidence, the number of large-celled hyphse increases 

 rapidly to form a weft. The amount of this type of mycelium 

 formed depends on the percentage of soil moisture and the extent 

 of the breaking down of the root tissues. High soil moisture favors 

 the development of pseudosclerotia in the crevices of the bark or 

 at the lenticels. 



Rotting is much more rapid from the point of entrance down into 

 the taproot and out into the laterals than upward. Usually there 

 is a marked constriction between the healthy and diseased tissues 

 at the collar. The healthy tissue is also very sharply delimited from 

 the diseased portion. The fact that the fungus has been observed 

 only in the first foot of soil when not in contact with the roots seems 

 to preclude the possibility of an attack below this level. 



Two types of infection were repeatedly observed, depending on 

 the point of attack. In the first type the strand hyphaa first accu- 

 mulate at a point on the root just below the constriction between 

 the root and the stem, and the depressions are formed in this zone. 

 Wilted plants attacked at this point have healthy laterals except 

 at the point of attachment with the taproot. Plants infected in 

 this manner always show a pronounced constriction of the taproot 

 just below the collar. This point of infection is the one usually 

 found on cotton plants in root-rot spots. 



The point of attack for the second type of infection usually is 

 where the main side* roots originate. Here infection occurs through 

 depressions located where the small rootlets formed in the cotton 

 seedling stage die off after the permanent side roots are formed. 

 These small rootlets are not connected to the stele, as is the case 





