I'O MASS. EXPERIMENT STATION BULLETIN 350 



Table 5. Influence of Turgidity of Foliage on Infection 



Turgid Plants 



1 



2 



3 

 Flaccid Plants 



1 



2 



3 



closing of the stomata. The wilting plants were subsequently maintained under 

 soil moisture conditions making for turgidity, and at the end of 48 hours con- 

 siderable spore germination was noted. Next to light, the moisture supply of 

 the leaves is the factor which most universally affects the opening or closing of 

 the stomata. Turgidity, therefore, contributes to the conditions necessary for 

 infection. Open stomata and respiration are definitely correlated with turgidity 

 and give rise to a greater local humidity. The greater amount of tissue disorganiza- 

 tion and fungus mycelium in the spongy parenchyma and about the tracheary 

 tissue than in the palisade parenchyma in newly infected leaves suggests that 

 conditions in the spongy parenchyma play a part in the preponderance of infection 

 on the lower leaf surfaces. 



Both Dyke (7) and Weiler (38) have noted that tomatoes are attacked by 

 leaf mold in new houses more readily than in old houses. Dyke declared that 

 tomatoes do not take the disease as readily under shaded as under clear glass, 

 on the assumption that glass which allows more heat and light to pass is more 

 favorable to the progress of the disease. According to the writer's observations, 

 new greenhouses in locations outside of vegetable trucking areas are usually free 

 of leaf mold for the first one or two years of growing, because of the absence of 

 nearby sources of the fungus. In established trucking distiicts epidemics of leaf 

 mold of equal severity have been noted on the first cropping of tomatoes in new 

 and old greenhouses. 



Bright sunlight is regarded as a lethal or unfavorable factor. It is reported 

 that the disease develops earliest and best in parts of the house made hottest by 

 the sun, corresponding to the south and west sides, and some reports appear to 

 attribute this to light. Around Cleveland, Ohio, houses built north and south 

 are reported to be freer of the disease than east and west houses (2) because in 

 the former more sunlight is allowed to penetrate between the rows and tempera- 

 ture fluctuations from sunlight are less rapid and severe. Makemson (13) noted 

 that dark, cloudy days furnished ideal conditions for the disease. Volk (36) 

 found that tomatoes were more seriously diseased in darkness than in light. The 

 toxic influence of light on fungous pathogenes of plants in greenhouse culture 

 was recognized by Stone (34). 



Since infection is obtained only under conditions which favor the opening 

 of the stomata, light is an important factor for infection; and since the stomata 

 close in darkness infection does not occur at night. Infection on the upper leaf 



