EXPERIMENT STATION BULLETINS. 651 



The rain may carry the spores from the old, diseased leaves on the 

 ground to the healthy leaves above, or from the diseased leaf to the 

 leaves surrounding it. It will be recalled that in staked tomatoes the 

 disease progresses upward beginning at the lower leaves. It is a signi- 

 ficant fact that the spots on the lower leaves are large, indicating infec- 

 tion from the lower surface, while the top leaves show the characteris- 

 tics of upper leaf surface inoculations. 



The relation to water was demonstrated in the greenhouse. Twelve 

 plants were grown and staked. The cotyledons of these plants were 

 diseased. These were not removed and were allowed to fall off. The 

 plants were watered from above twice every twenty-four hours. After 

 two weeks all the lower leaves developed typical leaf-spot. Two weeks 

 following, these lower leaves developed a number of secondary spots and 

 were beginning to shrivel and dry up. The leaves above these were in 

 about the same stage as the first leaves two weeks before. The disease 

 spread rapidly progressing upward on the vines. When the disease had 

 advanced about 3 feet above the soil level, 10 healthy tomato plants 

 were transplanted between these diseased plants. Watering from above 

 was discontinued, being replaced by careful irrigation. The plants set 

 between the diseased plants did not develop the disease even after twen- 

 ty-five days. Watering from above was now resumed and in six days 

 the leaves of all the young plants were covered with spots. 



This experiment determines the fact that the spores do not fall or float 

 from the pycnidium as soon as produced but adhere in a mass until 

 liberated by water. In answer to the criticism that infection did not 

 take place because of the dryness of the leaves, it may be said that the 

 greenhouse was always humid and furnished ideal conditions for spore 

 germination. The experiments already reported demonstrating the inde- 

 pendence of infection and humidity also remove any objection to this 

 interpretation of the foregoing experiment. 



The wind, as an agency in the spreading of spores, may be considered 

 as carrying the dust which contains the spores, thus inoculating other 

 plants. The spores germinate in a filtered soil infusion, secondary 

 spores being produced. Viable spores have also been found in the soil 

 about diseased plants. However, experiments performed to determine 

 the development of the organism in the soil yielded negative results; 

 considering that exuding spores are w^ashed down on the soil from the 

 diseased leaves, it is very probable that the dust must be considered as 

 an agency for dissemination of the fungus, limited however, by the length 

 of time the spores can stand drying and the disintegrating action of 

 the soil.' 



In this connection the difference in type of infection on upper and 

 lower leaves may be significant. The lesions on the lower leaves are 

 of the type to indicate infection from the lower leaf surface. It is easy 

 to conceive that the top leaves — especially when the plants are staked 

 — are out of the zone reached by splashings of rain, but especially open 

 to infection brought by falling dust particles. 



That man is an agent in spreading disease is very evident. In cul- 

 tivating the plants when they are wet, the spores are transferred to all 

 parts of the field by the machinery coming in contact with the sticky 

 spore masses or upon the clothing. Instances of dissemination in this 

 manner have been observed. 



