FUNGICIDES 377 



the chemical under controlled conditions by means of a paint spray gun, as 

 shown in Fig. 148. After drying they are inoculated with a known concen- 

 tration of fungus spores produced under standard conditions and placed m 

 high humidity infection chambers at controlled temperatures (see Fig. 149) 

 for 24 hours, then removed to the greenhouse. Necrotic lesions develop in 



Figure 149. Insulated, temperature-controlled humidity infection chambers. Atomiz- 

 ing jet nozzles with water and air pressure line for maintaining high humidity enter at 

 upper sides of each chamber, two may be seen above the chamber letters B and C. 

 Refrigeration machinery for B at lower left. The chambers are fitted with a movable 

 shelf in center, plate glass false ceiling below nozzles to prevent direct water spray on 

 plants, and fluorescent Hghts on top. 



several days, are counted, and expressed as per cent of the number of 

 lesions on the control plants. Prior to inoculation, the plants may be sub- 

 jected to several cycles of growth, high humidity, and laboratory "rain" 

 in order to determine the tenacity of the chemical being tested. ^^ ^Yhe 

 details of this method are well worked out and for a greenhouse procedure 

 are considered precise. Special attention is required in order to ensure an 

 adequate supply of pathogenic Alternaria solani spores; this is accomplished 

 by scraping the Petri dish cultures and irradiating for 20 seconds under 

 ultraviolet lamps transmitting to about 250 mju.^^ 



When the number of lesions is expressed as a per cent of the check, the 

 dosage results may be plotted on logarithmic probability paper and, like 

 the spore germination curves, they tend to give straight lines; however, 

 the former are much flatter. Perhaps this may indicate that when fungi- 



