276 Journal of Agricultural Research voi. xviii, no. s 



Brown (6) in a recent study of parasitism in Botrytis has shown that 

 this fungus secretes an enzym which breaks down the middle lamellae of 

 tissues which it invades. He demonstrated that the enzym secretion was 

 more powerful from freshly germinated spores than from old cultures. 

 A toxin, which is apparently closely associated with the enzym, is also 

 secreted. This toxin is not oxalic acid or an oxalate. Blackman and 

 Welsford (5), in the second of this series of papers, have shown that this 

 fungus apparently penetrates the cuticle of the broad bean leaf by push- 

 ing its way through mechanically. These writers are in agreement with 

 Brown that an enzym secreted by the fungus breaks down the tissues 

 in the interior of the leaves. Brown (7) later showed that the infecting 

 germ tubes were unable to affect chemically the cuticle of the host plant 

 and that the toxic substance could not pass through the cuticle. He 

 concludes that penetration of the cuticle must take place in a purely 

 mechanical way. In the fourth paper of this series (8) he contrasts thick 

 and thin sowings of spores and finds that thick sowings 2 to 4 days old 

 yield the most active preparations of enzym. He considers that the 

 cytase is much more active at the tip of the hypha. 



From this review, which of course covers only part of the literature 

 on this subject, it is apparent that there is good evidence that some para- 

 sitic plants make their way into their host plants by breaking through 

 the tissues mechanically ; but there is no doubt that some fungi secrete 

 enzyms which break down the cell walls of certain plants and are thus 

 able to make their way through the tissues of their hosts. 



The parasitism of Pythium debaryanum Hesse on some of its numerous 

 hosts has been investigated, but how it gains entrance into the host 

 plant seems not to have received any considerable attention. This 

 fungus has recently been shown to be the cause (75) of a tuber-rot of 

 potatoes which is of considerable commercial importance in the San 

 Joaquin Valley of California. A method of controlling this disease under 

 commercial conditions has been worked out and described (17). In 

 the present study the effect of the fungus on the sugars, pentosans, and 

 starch of the potato tuber was determined, and the rate of growth of 

 the fungus in three different varieties of potatoes was measured. An 

 attempt was made to correlate certain physical and chemical character- 

 istics of the potatoes with their susceptibility or resistance to this dis- 

 ease, and the growth of the fungus in the potato tissue was observed and 

 studied. Some information on the mode of entrance of this fungus into 

 the cells of the potato was obtained, and a possible explanation was 

 found as to why some varieties of potatoes are much more susceptible 

 to this disease than others. 



