298 



Journal of Agricultural Research 



Vol. XIII, No. s 



Table I. — Infection showing on September ly, IQ16, on cucumbers inoculated on Septem- 

 ber 13 



Manner of inoculation. 



Leaves sprayed on upper surface 



Leaves sprayed on lower surface 



Leaves with drops on upper sur- 

 face. 



Leaves with drops on lower sur- 

 face. 



Controls 



Infection on plants kept in — 



Culture chamber for 24 hours. 



Fine spotting (0.2 to 0.75 mm. 

 in diameter). 



do 



Fine spotting in groups 



....do 



No infection 



Greenhouse from begin- 

 ning. 



No infection 



Do. 

 Do. 



Do. 

 Do. 



Infection occurred equally well with all the strains used and regard- 

 less of whether the spores were applied in distilled water, tap water, or 

 beef bouillon. 



An inspection of Table I shows the following points: The infection 

 appeared on both surfaces regardless of which surface had been inoculated. 

 Infection appeared only on those plants kept in a moist atmosphere for 

 some hours after inoculation. As might be expected, those leaves that 

 were sprayed with the atomizer showed a fine spotting over more or less 

 of the entire surface, while on those inoculated with large drops of liquid 

 the spots occurred in groups. In this series of inoculations the spots did 

 not continue to enlarge and spores were produced on only a few of the 

 lesions. The fungus was reisolated both from the spots that produced 

 spores and from those that did not. 



The question then arose as to the cause of the failure to produce the 

 large mottled spots with normal spore formation so common in the field. 

 The plants had been kept in the greenhouse at a rather high temperature 

 and low humidity. Accordingly another series of inoculations was started 

 in which these factors were varied. It was found that under conditions 

 of fairly high humidity and of moderate temperature the characteristic 

 mottled spots were produced with abundant spore formation on the 

 lower surface of the leaves. Under conditions of excessive humidity 

 and partial shade, spore formation occurred on both surfaces of the leaf, 

 a phenomenon very rare in nature. 



Other inoculations, in which the writer used mycelium in comparison 

 with suspensions of spores, have shown that infection occurs equally 

 well in either case. 



On September 15, 1916, a series of inoculations was made on cucumber, 

 gourd, and squash (Cv^urbita spp.) with a suspension of spores in water. 

 The plants were placed in the culture chamber for 24 hours and then 

 kept in the greenhouse. Infection appeared on the cucumbers, but not 

 on the gourds or squashes. Later, these experiments were repeated, the 



