

1917] STEVENS— PLANT PATHOLOGY 301 







genus a good example is Septoria with 1200 species, or Phyllosticta 

 with 1 1 50 species. The former has nearly 700 species between 

 20 and 50 ijl in spore length. The latter has 128 species with spores 



* 



measuring 5-6^ long. Septoria has 115 species on Compositae, 

 and 77 species on the Gramineae (26 of these are within the limits 

 of 20-40 /j, in spore length) . 



Our present knowledge of such genera, as given by Saccardo, is 

 essentially that of a preliminary cataloguing of these forms by 

 their hosts, the necessary first step. And we may add, as examples, 

 the species of such genera as Photna, Rhabdospora, Cercospora, 

 Nectria, Sclerotinia, Guignardia, Physalospora, and Phyllachora. 

 The bearing of this condition upon practice is evident, since 

 numerous forms described as separate species upon the same 

 economic host plant in reality may be identical or may be co- 

 specific with forms described as distinct species or as belonging to 

 other genera, families, or orders on the same or other hosts. The 

 next step, well exemplified by such work as the monographs of 

 Theissen, will consist in morphological comparison and readjust- 

 ment of the species. This raises the question of life histories, of 

 course, and shows the need of much such work as that of Shear 

 and Wood on Glomerella, Higgins on Cylindrosporium (Coc- 

 comyces), Clinton on Venturia, Wolf on rose black spot (Diplo- 

 carpon), etc. 



In connection with these problems arises the question of host 

 relation and of biological specialization, as best exemplified, per- 

 haps, in the rusts and the powdery mildews. What is the status 

 of such specialization in the Fungi Imperfecti, in Phyllosticta, 

 Septoria, Cercospora, etc., in the Ascomycetes, Nectria, Sclerotinia, 

 Phyllachora, and many other genera? This forms a large and 

 enticing field, in which much good work has been done, but a vast 

 amount remains still to be done. 



Coupled with these problems, come of necessity physiological, 

 morphological, and cytological studies. The Oospora-Actinomyces- 

 Streptothrix problem will require, apparently, all the possible side- 

 lights before solution. This illustrates admirably the dependence 

 of practice upon science, since fundamental questions of practice 

 must rest their answer upon the degree of biological specialization 





