Apr. 29. 1918 Stemphylium Leaf spot of Cucumbers 301 



At first this flufify growth of mycelium following heavy spore produc- 

 tion was rather puzzling. Careful reisolation of all strains showed that 

 it was not due to contamination. Further observations have shown 

 that it arises from germination of the spores in the cultures. 



Relation to light. — Sixteen tubes each of string-bean and potato- 

 glucose agar were inoculated with two strains of the fungus, and one-half 

 were kept in the dark, while the others were exposed to the ordinary 

 light of the laboratory. No difference could be observed in their growth. 



Relation to moisture. — Growth was usually slower on media that 

 had dried somewhat on the surface. It was also slower in a dry atmos- 

 phere than when the tubes were placed in a moist atmosphere under a 

 bell jar or in the culture chamber. In the latter cases the myceUum 

 exuded copious drops of liquid. Spore formation was usually retarded 

 in a moist atmosphere. 



Relation to temperature. — Cultures on string-bean and potato- 

 glucose agar were exposed to various temperatures. For room tempera- 

 ture, 18° to 23° C, the regular laboratory was used. For temperatures 

 above this the cultures were kept in the incubator. For temperatures 

 below room temperature two different cold rooms were employed. The 

 temperature in these cold rooms was not entirely uniform, but did not 

 vary over 3° or 4°. The optimum temperature for development and spore 

 production was found to be from 18° to 25°. Above 30° and below 15° 

 growth was very scant, and only a few spores were produced. This low 

 optimum may explain the writer's failure to find infection in the cucum- 

 ber fields around Plymouth during the summer of 1916 before the cooler 

 weather of September had arrived. During midsummer the average tem- 

 perature was much above the maximum for the growth of this fungus. 

 The writer has had no success in securing infections where the temper- 

 ature has been high continuously at the time of inoculation. 



SPORE FORMATION 



In artificial cultures spore formation may be so abundant that the 

 contents of the mycelium are almost entirely used up. The conidiophore 

 begins as an outgrowth of a mycelial cell (fig. i, A, B, C). It is at first 

 nonseptate, but a septum is soon laid down near the mycelium, and as 

 growth proceeds, additional septa are formed, in extreme cases as many 

 as eight being produced (fig. i, C-F). The end cell, or in some cases 

 the ceil next to the end, nov/ divides by a longitudinal septum (fig. i, G-I). 

 Other longitudinal divisions follow rapidly, and in a short time a globular 

 head consisting of from 5 to 20 cells or in extreme cases even more is pro- 

 duced (PI. 24, B, C; and fig. i, M, N). Meanwhile, as the longitudinal 

 divisions progress, the cells gradually assume a dark-brown color, caused 

 by a pigment in the cell wall. 



The cells of the conidiophore, which at first are compressed tightly 

 together, gradually become globose and loosely attached to each other 

 and to the spore (fig. i, M, N). 



