Studies on Fungicides — III 65 



diffusion. However, it does appear possible . that the difference in the 

 alkalinity of the two types of bordeaux may have been a contributing 

 factor. So far as is known, all other factors were the same. 



The results of this series of experiments are believed to point definitely 

 to a solvent action exercised by some excretion from the fungus spores. 



The sphere of influence of spore excretions 



Hypothesis. It is assumed that actual contact of the spore with the 

 colloidal particles is not necessary in order that the copper be rendered 

 soluble. 



In the preceding study there was definite indication that this is the case, 

 and an effort has accordingly been made to determine how great the 

 separation between the spore and the copper particle may be before a 

 lessening of toxicity is evident; in other words, to determine the sphere 

 of activity of the solvent spore excretions, and hence of the liberated 

 copper particles. It will be recalled that Lutman (1916) observed that 

 no spores germinated when within a radius of one millimeter from a drop 

 of Pickering's bordeaux. On the other hand, Barker and Gimingham 

 (1911, 1913, 1914 b) have maintained that actual contact, or at least 

 very close association, is necessary for toxic action. 



Experiments. In a preliminary experiment, bordeaux mixture 4-4-50 

 was prepared which showed no free copper by the potassium-ferrocyanide 

 test. The bordeaux was sprayed into four watch glasses and allowed to 

 dry for 24 hours. Ten cubic centimeters of distilled water was then 

 added to each glass, and the whole was allowed to stand thus exposed 

 for 60 hours. In the meantime, fresh bordeaux mixture containing no 

 free copper was placed in the form of drops on clean glass slides. Two 

 drops were placed on each slide, each drop about 1.5 centimeters 

 from the end of the slide. These drops were allowed to dry down for 

 24 hours. The end of this period coincided with the end of the 60-hours 

 period referred to above. 



A spore suspension of Sclerotinia americana conidia was prepared and 

 two drops were placed on each of a number of clean glass slides. Two 

 drops were placed also on each of the slides holding the dried drops or 

 areas of bordeaux mixture. The drops of spore suspension and of bordeaux 

 were separated by a space of from 1 to 5 millimeters. 



The spores were allowed to settle, and thus to adhere to the surface of 

 the slides. This required about half an hour. Then, to the slides with 

 the bordeaux and spore-suspension areas, distilled water was added by 

 means of a pipette, to form a film covering and connecting each pair of 

 areas — bordeaux and spores. Thus, although the bordeaux and the 

 spores were connected by a bridge of water, there was no direct contact 

 between them. 



