26 TECHNICAL BULLETIN 17 



inoculated and uninoculated check leaves were dark green, turgid, and 

 normal in appearance. No signs of infection were apparent. Micro- 

 scopic examination showed that many spores had germinated but so 

 far as could be detected from free hand sections, the germ tubes had 

 not penetrated. After the same incubation period, at 12 C, very small 

 blue-green water-soaked areas were visible at the points of inoculation. 

 The remainder of the leaf tissue and the uninoculated check leaves 

 were still green and normal in appearance. These water-soaked areas 

 were not yet visible at the end of 48 hours' incubation. At 18 C., by 

 the end of the third day, there were green water-soaked areas on 

 which conidiophores were beginning to appear on the inoculated leaves. 

 The tissue of the leaves was still firm and the cells were turgid. While 

 the infected areas retained a dark blue-green color, the rest of the 

 leaf was yellow. The uninoculated check leaves were light green to 

 yellow in color. After 72 hours' incubation at 23, 27, and 30 C., 

 there were large dark green blotches of infected tissue covered by a 

 velvety mass of conidiophores. The leaf tissue was beginning to soften 

 and the check leaves and non-infected areas were yellow. In the in- 

 fected areas the cells were beginning to disintegrate, but the chloroplasts 

 were still green. At 34 C. small water-soaked areas, 3 or 4 mm. in 

 diameter, were apparent after 36 hours' incubation. At this time the 

 border was beginning to turn brown. By the third day, there were small, 

 brown, definite leaf spots, similar to the normal lesions produced on 

 leaves in the greenhouse and in the field. The remainder of the leaf 

 tissue and the check leaves were yellow. 



Under the conditions just described, there was always an abundance 

 of moisture, so that the difference in reaction must have been clue to the 

 influence of temperature on host and fungus. During the first 36 

 hours the results were probably more or less comparable to results 

 obtained in growing leaves attached to the plant; during the second 

 36 hours, at some temperatures at least, the relationship was probably 

 saprophytic. The most that can be claimed for results obtained in this 

 way is that they are only indicative of what may happen on growing 

 plants. 



The results obtained from these experiments would indicate that 

 at temperatures of from 18 to 30 C., penetration into the leaf will 

 take place about equally well in the presence of sufficient moisture. 

 Below 24 the spots increase in size more slowly, above 24 more 

 rapidly. At 12 a much longer incubation period is necessary for the 

 development of water-soaked areas than at higher temperatures. At 

 6 no visible infection was obtained. At a temperature as high as 34, 

 on the other hand, the development of the spots and the browning of 



