82 
Journal of Agricultural Research 
Vol. V, No. a 
morning, however, the fungus, which, on microscopic examination 
proved to be P. infestans y had fruited, a 500 c. c. beaker having been 
inverted over the plant in the evening. For three successive mornings 
after this date evidence of a new crop of spores of this fungus on the 
little plantlet was found (PI. VII, figs. 2 and 3). Later the plantlet 
fell over, owing to destruction of tissue by the fungus and soft-rot organ¬ 
isms which followed. The stem was found to be discolored all the way 
down to the parent tuber, a distance of 6 inches. The plant was allowed 
to remain in the field in order to ascertain whether it might infect the 
foliage of surrounding plants, but no infection developed and the plantlet 
soon died and dried up. Conditions were probably unfavorable in this 
case for the development of secondary infections, owing to a poor stand 
in the row where this infected plant happened to be. This condition 
makes it necessary for the spores to be carried a greater distance than 
might have been the case had a higher percentage of the seed planted in 
this row grown. The stand in the row in question and also the infected 
hill are shown in Plate VII, figure 3, This case is of special interest in 
showing that no further development of the fungus occurred, although it 
did grow up the stem from the diseased parent tuber to the surface of 
the soil and sporulate. 
It was not until July 25 that another case of infection by P. infestans 
was discovered on any of the six rows under experimentation. This 
case developed in one of the hills growing from a whole diseased tuber. 
The hill was a vigorous one with 13 shoots, all normal except 3. 
The smallest of these 3 was 6 inches tall, while the others were fully 
twice this height. The plantlet was well shaded by the others and was 
detected only on careful examination of the hill (PI. VII, fig. 1). When 
first found on July 25, fully an inch of the stem above the surface of the 
soil was discolored and a hand-lens examination showed that a fungous 
growth was present. Some of this growth, collected on a slide and 
examined microscopically, proved to be spores of P. infestans . The 
weather for five or six days previous to July 25 had been rainy, cool at 
night, and quite warm in the day time, conditions highly favorable for 
the rapid growth and spread of the fungus, as demonstrated in the 
laboratory studies. 
The infection spread up the stem into the petioles of the lower 
leaves and produced spores in abundance. On the 29th, or four days 
after the infection was first noted, two leaflets in the hill showed 
infection, and discolored areas appeared on the stems of three of the 
adjoining shoots about 2 inches above the surface of the soil. The next 
morning five new leaflets in the hill showed early stages of infection. 
These infections occurred on leaves in the lower third of the hill, and each 
day the number of infections increased on the foliage. On July 31 one 
leaflet was found infected near the top of a plant in one of the adjoining 
