ANTHRACNOSE OF CUCURBITS. 



29 



petioles the intracellular mycelium about 'the advancing edges of the 

 lesion is easily seen without staining. 



As to the effect on host cells besides the shrinkage and the staining 

 reaction above noted, there appears to be an enlargement of the host 

 nucleus, often followed by its division (figs. 6 and 7) and in some 

 cases by cell division (fig. 11). In cucumber-stem lesions there seems 

 to be a tendency toward callus formation. 



To sum up, it may be said that the mycelium is intracellular and 

 that shrinkage and collapse of the invaded host cells occur. There 

 are indications of a stimulus to cell division. 



LIFE HISTORY OF THE CAUSAL ORGANISM IN RELATION TO THE DISEASE. 



SEASONAL DEVELOPMENT. 



Previous observations indicate that anthracnose is a disease 

 which becomes serious only rather late in the growing season of its 

 host. Observations made 

 during the course of this 

 work lead to the same con- 

 clusion. This appears to be 

 due to the mode of origin 

 of the disease in rather iso- 

 lated and restricted cen- 

 ters in each field, from 

 which subsequent spread, 

 dependent upon heavy 

 rains, is rather slow until 

 a considerable reservoir of 

 infection has developed. 

 The strict relation of epi- 

 demics of this disease to 

 wet weather is well recog- 

 nized in the literature (41, 

 43, 44), and the importance of climatic conditions can not be over- 

 estimated. 



DEVELOPMENT IN 1915. 



Observations upon the disease as it occurs among cucumbers were 

 made during the summers of 1915, 1916, and 1917. The summer 

 of 1915 was cool with frequent and abundant rainfall. At Prince- 

 ton, Wis., the disease was first noted in a 2-acre experimental field 

 on July 14. This infection was confined to lesions on the first and 

 second leaves of two adjacent plants planted seven weeks previously. 

 The next day the disease was found in a neighboring private field. 

 By July 20 several centers of anthracnose were noted in the latter 

 field. On July 21 anthracnose was found in another private field 

 in town. 



Fig. 11. — Cross section of a leaf lesion 121 hours after inocula- 

 tion. Appressorium shown. Mycelium intracellular. The 

 division of invaded palisade cells and collapse of epidermal 

 and palisade cells are visible. (Camera-lucida drawing; 

 magnified about 570 times. ) 



