90 



Journal of Agricultural Research 



ing of August 17, 191 1, after a spell of wet weather, two potato plants were 

 sprayed with a suspension of spores of P. infestans, the spores having been 

 taken from infected plants in the greenhouse. The inoculation of the 

 two plants was made in the usual way and typical spots became visible 

 in the course of five days. The amount of infection was not extensive. 

 The ground was very moist, owing to the fact that several rains had 

 fallen the previous week, and the weather was continuously cloudy from 



August 22 , the date in- 

 fection first appeared, 

 until August 27. 



On August 30 infec- 

 tions were found on 

 two plants adjoining 

 those artificially in- 

 fected, and the next 

 day four more plants 

 immediately adjoin- 

 ing showed infection 

 on several leaves. 

 Careful examination 

 showed no infection on 

 any of the plants far- 

 ther away. The new 

 infections that had oc- 

 curred on August 31 

 were on the six plants 

 immediately sur- 

 rounding the two arti- 

 ficially infected. The 

 fungus had made no 

 further spread in the 

 half-acre potato plot. 

 After August 30 new 

 infections were daily found farther and farther from the two plants first 

 infected, and on September 7 infected leaves could be found everywhere 

 throughout the plot, though none of the vines were conspicuously 

 bhghted. By this time all the plants within a radius of 8 feet of the two 

 plants initially infected were killed. Farther away the infection was 

 much less in extent, though present in abundance. By September 12 the 

 plot was very badly blighted; not a single plant anywhere was free from 

 infection, and many were wholly dead. No further records except as to 

 the time of harvesting and the amount of loss were kept when the tubers 

 were harvested on October 10. Less than 50 per cent of the crop was fit 

 to put in storage, and less than 10 per cent kept until spring, although 

 held in good storage. 



Fig. a. — A portion of the same section of a potato plant shown in 

 figure I, showing the mycelium in the pith region of the stem. 



