10 S. E. A. McCallan 



In explanation they say, in part (page 167 of reference cited): "We 

 must, therefore, expect that progress in the discovery of new fungicides or 

 beneficial modifications of old ones will be slow until some method is 

 adopted whereby the fungicidal pro-perties of various substances can be 

 studied in the laboratory." Since that time the method has been more or 

 less continuously employed in this laboratory by various workers who 

 have introduced modifications and improvements at different periods. 

 During the past three years the author has had occasion to use the method 

 very extensively. Naturally, some new problems have presented them- 

 selves, and, as obstacles have arisen, attempts more or less successful have 

 been made to overcome them. As now developed, it is believed that this 

 method of testing approaches actual field conditions more nearly than 

 does any other described in the literature. 



TECHNIC 



The technic of the method as now in use is as follows: 



The fungicide to be tested is sprayed or dusted on standard 3x1 -inch 

 clean glass slides which are conveniently placed on a tray. Every effort 

 must be made to get an even and well-distributed layer of the fungicide. 

 (The matter of application is presented in more detail later.) With such 

 dusts as copper-lime, which should be dusted only on wet foliage, the slides 

 must first be sprayed with distilled water until they are coated with fine, 

 uniform droplets. The dust is then immediately applied. Liquid sprays 

 are left exposed to the air and allowed to become completely dry. This 

 will usually take several hours. The slides are then ready to be placed in 

 the moist chambers. 



Moist chambers are used in order to keep, without evaporation, the 

 drops of water in which the spores are placed to germinate. These cham- 

 bers have been found more suitable than petri dishes, since four slides may 

 be placed under exactly the same conditions. Also, the air relations are 

 better. In order that the chambers may be sealed with water, they are 

 inverted for use. A satisfactory size of moist chamber is 7 by 20 centi- 

 meters. A piece of clean filter paper is placed in the bottom of the cham- 

 ber. This tends to hold the moisture and to prevent the slide-holding 

 racks from displacement, and provides an excellent background for pur- 

 poses of observation. The slides must be held above the moist filter paper 

 or the water. To accomplish this, glass slide racks are used, one for each 

 chamber. These may be easily constructed of glass tubing bent to form 

 a U. The length of the rack should be sufficient to easily hold the four 

 slides parallel to one another, and yet allow room for the chamber cover 

 to be placed in position (figure 1). 



A suspension of the spores to be tested is prepared by carefully washing 

 them from the substrate into the particular kind of water desired. If 



