38 James G. Horsfall 



hyphae can frequently be seen in one or two cells beyond this point and occasional threads 

 of hyphae can be seen in the mesophyll cells of this region and may extend to within two or 

 three cells of normal tissue. The cells in this zone have their contents more or less dis- 

 organized and an occasional palisade cell is filled with some dense brownish substance, but 

 there is little if any collapsing of cell walls. There is no evidence of cicatrice formation. 



The minute apothecium in the center of older spots is another infallible 

 evidence of the disease. After a rain or in moist weather these little fruit 

 bodies push through the epidermis and glisten as minute amber-colored, 

 jelly-like globules, but in drying they shrink and darken so that they 

 are difficult to detect. It is these which make determination simple on 

 the alsike and white clovers. 



Etiology 



Name, history, and classification of the pathogene. Pseudopeziza trifolii 

 (Fr.) Fckl. is the cause of this leaf spot. The organism was described 

 first in 1816 by Bivona-Bernardi as Ascobolus trifolii and was cited six 

 years later by both Persoon (1822:342) and Fries (1822:165) under that 

 name. In 1834 Madame Libert called it Peziza trifoliorum (1834:324). 

 Boudier changed the name again in 1869 to Phacidium trifolii when 

 he revised the Ascobolaceae (1869), but in the same year Fuckel (1869: 

 290) erected the genus Pseudopeziza, making P. trifolii (Biv.-Bernh.) 

 Fckl. the type species. It has remained P. trifolii to this day, but the 

 specific name is ascribed to various persons. Frequently, (Biv.-Bernh.) 

 Fckl. is cited. The h in the second name is an error apparently first made 

 by Fries (1822:165) and copied by subsequent writers. Persoon (1822: 

 342-343) omits the h. This authority is correct according to the so-called 

 American code which accepts the very first specific name applied. If, 

 however, the International code be followed (Briquet, 1912), the authorities 

 are (Fr.) and Fckl. Oudemans (1921 : 875) lists the following as synonyms: 

 Ascobolus trifolii Biv.-Bern., Mollisia trifolii Phill., Peziza trifoliorum 

 Lib., Phacidium trifolii Lib., Phacidium trifolii Boud., and Trochila 

 trifolii DeNot. 



Much confusion existed for a time through the supposed identity of the 

 forms on clover and alfalfa, but Jones (1919) has shown rather clearly that 

 they are different. In this paper, therefore, the organisms on species of 

 Trifolium are considered to be P. trifolii and those on species of Medicago 

 to be P. Medicaginis. 



Pathogenicity. Jones (1919) was the first person to grow P. trifolii in 

 pure culture. The writer has isolated the fungus from red clover by 

 planting pieces of leaves bearing very young lesions in separate tubes of 

 oat agar. Apothecia are formed within a month if a bit of sporulating 

 culture is placed at the top of an oat-agar slant and is allowed to discharge 

 spores over it as pointed out by Jones (1919). Since the fungus grows 

 very slowly at first, it is isolated only with difficulty. Subcultures grow 

 somewhat more rapidly, however; but unless transfers arc made frequently, 



