36 James G. Horsfall 



field in New York. In Europe, it occurs in Italy (Bivona-Bernardi, 1816), 

 Wales (Stapledon and others, 1922:16), Russia (Jaczewski, 1916), Belgium 

 (Lambotte, 1880:264), England (Massee, 1914), and Germany (Voges, 

 1909). 



Economic importance 



Pseudopeziza leaf spot undoubtedly is the most serious leaf disease of red 

 clover in New York, as a reference to the survey data will show. From 

 these data it appears that statements to the effect that no especial damage 

 results (Haskell, 1924:307) are misleading. If New York be considered 

 typical of the northern clover-growing States, then the disease is of great 

 importance. Miss Massee (1914:65) states that the disease is far more 

 prevalent and injurious in England than is generally suspected. 



Symptomatology 



This disease is easy to recognize in the field. The diagnostic symptom 

 is the fimbriately margined, dark reddish, purple, dark olive, or even 

 almost black spots which show on both leaf surfaces. The fimbriate 

 margin does not occur invariably, but when it does, it is an unmistakable 

 symptom of the disease. Such spots are shown in figure 7, A. Sometimes, 

 especially late in summer, the fimbriate condition is almost wholly lacking. 

 This seems to be due to the dry conditions under which the spots develop. 

 Such spots are illustrated in figure 7, B. The fimbriate spot is most 

 pronounced on T. pratense and T. medium, but it is usually apparent on 

 T. hybrid u in. This character is much less prominent on T. repens. 

 Furthermore, the general color tone of the spots on T. hybridum and T. 

 repens is lighter than on T. pratense or T. medium. 



In the greenhouse very young infections are much lighter in color than 

 are the older spots observed in the field, matching Prout's brown or mummy 

 brown of Ridgway's color standards (1912). It is practically impossible 

 to diagnose this disease in its early stages in the field because the color 

 of the lesions is not markedly different from that of the other leaf spots. 

 Young spots resulting from artificial inoculation are illustrated in figure 7. 

 The spots may vary from mere specks four or five days after inoculation 

 up to 2 or 3 millimeters in diameter under the wet conditions that often 

 prevail in thick stands of clover in the spring. The disease also appears 

 as small elongated dark streaks on leaf petioles. Pure cultures of the 

 pathogene have been obtained from such lesions. 



Cunningham (1928:742) says that the histologic symptoms of this 

 disease on red clover are the same as for those on alfalfa attacked by the 

 congeneric parasite P. medicaginis. 



In the holonecrotic area [that is, the dead area in the center of the spot] the palisade cells 

 are almost entirely replaced by the stroma of the fruit body while the cells of the spongy 

 parenchyma are either collapsed or filled with the hyphae. In the plesionecrotic zone [that 

 is, the diseased tissue surrounding the dead area] the epidermal cells may be collapsed. The 



