82 Journal of Agricultural Research voi. v, no. » 



morning, however, the fungus, which, on microscopic examination 

 proved to be P. infestans, 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 

 Uttlc 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 

 foUage 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, o\\'ing 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 \*igorous one with 13 shoots, all normal except 3. 

 The smallest of these 3 was 6 inches tall, while the others were fully 

 t^^^ce this height. The plantlet was well shaded by the others and was 

 detected only on careful examination of the hill (PI. VII, fig. i). 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 sHde 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 



