6 BULLETIN 934, U. S. DEPARTMENT OF AGRICULTURE. 



Inoculation experiments are therefore probably even more neces- 

 sary in damping-off investigations than in studies of most other dis- 

 eases in order to demonstrate etiological relationships. Unfortu- 

 nately, most of the inoculation work with damping-off organisms 

 prior to 1900 was either crudely done by placing diseased seedlings 

 against healthy ones or consisted of experiments in which purity 

 of cultures and validity of controls did not receive sufficient con- 

 sideration. Recent investigations not primarily directed toward 

 damping-off, but which have decidedly increased our knowledge of 

 the relation between Corticium and the disease, are those of Peltier 

 (98) and Fred (43). The latter established a strong presumption 

 that the difficulty in securing stands of various field crops having 

 oily seeds in soil where green manures had been recently turned 

 under is due to the killing of the sprouting seed by damping-off 

 organisms. 



In tobacco, sugar beet, and pine, whose damping-off has received 

 considerable attention, it has been found that the damping-off proper 

 is commonly preceded by the killing of many of the sprouting seeds 

 in the soil (38; 68, p. 522; 81, p. 5) and followed, after the plants 

 become too large to be killed by the damping-off organisms, by root 

 sickness and the death of small roots (38, p. 161 ; 64 ; 100) . This latter 

 has been reported also as a serious matter in the case of Corticium, 

 vagum for potato (51), a host on which damping-off is not important 

 because of the lack of commercial propagation from seed. Pythium 

 debarycmwni further has been reported as continuing to work in the 

 cortical tissues and leaves of tobacco plants which have been in- 

 fected too late to result in death (81). 



The fact that a number of the damping-off fungi are able to attack 

 young or soft tissues of so great a variety of plants and are much 

 less able to kill older plants suggests that resistance to damping-off 

 may be in part based on purely mechanical factors. Hawkins and 

 Harvey (71) recently have extended to Pythium debaryanum the 

 idea, developed by Blackman and Welsford (12) and Brown (16) for 

 Botinjtis cinerea, of the importance of mechanical penetration in the 

 fungous invasion of plant tissues. While for B. cinerea, mechanical 

 pressure was found to be the main factor only in cuticle penetration, 

 with P. debaryanum the penetration of the cell walls of all parts of 

 the potato tuber was apparently largely dependent on mechanical 

 puncturing by the hyphse, only tubers with mechanically weak cell 

 walls being susceptible to decay by the fungus. The extreme sus- 

 ceptibility to P. debaryinuin and Corticium vagum of soft, thin- 

 walled tissues and the resistance of older stems and root parts would 

 fit in well with such a theory as to the method of wall penetration, 

 as in the older tissues the thicker cell walls would obviously be a 

 serious bar to the extension of a fungus dependent partly or en- 



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