582 STATE BOARD OP AGRICULTURE. 



Relation of Parasite to Host. 



METHOD OP infection. 



The factors involved in the physiology of parasitism have been baffling 

 problems which investigators have attacked with varying degrees of success 

 every since the study of plant diseases attracted scientific research. Much 

 of the work done has hinged around the method by which the parasite en- 

 ters the host and around relations of the pathogen to the internal tissues of 

 the host. It is to be hoped that a thorough knowledge of these phases will 

 aid in the discovery of at least some of the fundamental factors which con- 

 stitute the difference between parasites and saprophytes. Aside from the 

 scientific value connected with this kind of investigation, there is often con- 

 siderable practical significance attached to the method of host penetration 

 and invasion. 



Parasites employ a number of different methods in entering host tissue. 

 Germ tubes may enter through stomata, as Gregory* has shown for Plas- 

 mopora viticola, and as Pool and McKay f have shown for Cercospora beticola. 

 Other natural openings are often very important. Wounds break down the 

 plant's defensive equipment and, with many parasites, are the path of in- 

 vasion. Still other organisms enter by boring directly through the epider- 

 mal cell walls. In the case of the Botrytis of lily, Ward| very early believed 

 that the fungus gained entrance by dissolving the cell walls by means of a 

 "ferment substance", an observation which has been confirmed repeatedly 

 and which has been found to be true for a great number of other plant path- 

 ogens. 



Whether or not all organisms which penetrate cell walls employ some kind 

 of dissolving substance as they come in contact with the host, is not definite- 

 ly kno^vn. It is possible that the substances are not necessary for all para- 

 sites, Hawkins and Harvey § think that in Pythium de Baryanum they 

 have an organism capable of exerting enough mechanical pressure to enable 

 the threads to push their way through the cell wall. 



With a view to determining the relation of the Phoma Root Rot organism 

 to its host in the early stages of infection, a number of inoculations and mi- 

 croscopic studies have been conducted. Virulent strains of this parasite 

 uniformly give infection regardless of whether spores or mycelium are used 

 as the inoculum, but on account of the greater uniformity of time required 

 for penetration spores have been used for studying methods of infection. 



Leaf and petiole inoculations have been made to avoid the difficulty of 

 making observations on crowns and roots. The spores were first germinated 

 in water, then bits of filter paper were soaked in this spore suspension and 

 placed on the plant under flecks of wet cotton. The spots inoculated were 

 examined at different times by stripping off pieces of epidermis, or by placing 

 bits of leaves under the microscope. In this way, it was possible to study 

 relation of host and parasite under high power without resorting to stains. 



Forty-eight hours after inoculation the germ tubes were found to have 

 entered the host. The method of entrance was a direct penetration of the 

 epidermal cell walls by the fungus hyphae. The threads pass into the cell 



* Gregory, C. T. Spore germination and infection with Plasmopora viticola. Phytopath. 2:235- 

 249. 1912. 



t Pool. Venus W. and M. B. McKay. Relation of stomatal movement to infection by Cercospora 

 beticola. Jour. Agr. Res. 5:1011-1038. 1916. 



t Ward. Marshall, H. A lily disease. Ann. Bot. 2:319-382. 1888. 



§ Hawkins, Lon H. and R. B. Harvey. Physiological Study of the Parasitism of Pythium de 

 Baryanum Hesse on the potato tuber. Jour. Agr. Res. 18:275-298. 1919. 



