FUNGICIDES 369 



tory; under actual field conditions other agencies such as rain may also be 

 a factor. Although considerable research had been done on the action of 

 weathering, possible changes which may occur in the copper-lime-sulfate 

 ratio of Bordeaux under the influence of rain had not been studied. 



It was found that when glass plates are sprayed with an 8-8-100 Bor- 

 deaux mixture and exposed outdoors, the sprayed film undergoes a change 

 in composition under the leaching influence of rain and dew, leading to a 

 mixture relatively richer in copper. ^^ This change in composition is 

 accompanied by an increase in soluble copper. The highest amount ob- 

 served was 0.45 mg per plate (225 sq cm), when the plate was agitated 

 with 50 cc of water. Carbonation of the excess lime was complete in a 

 few hours, as judged by pH measurements, but the increases in soluble 

 copper did not occur until much later. The results could be duplicated in 

 the laboratory using artificial rain, but only if a sprayed, dried film of Bor- 

 deaux was used. Washing the Bordeaux precipitate in bulk by centrifuging, 

 or on a Biichner funnel, did not lead to substantial increases in soluble 

 copper. When Bordeaux mbctures low in lime were subjected to the leach- 

 ing action of rain, soluble copper appeared sooner than with an 8-8-100 

 mixture. Treatment of the sprayed films "\\ith CO2, either wet or dry, did 

 not lead to much increase in soluble copper. It is considered that the in- 

 creases in soluble copper observed can be best explained by assuming that 

 the weathered Bordeaux precipitate is an adsorption complex, or a solid 

 solution containing copper, lime, and sulfate, the copper of which is soluble 

 in water to an extent which varies with its composition. The appearance 

 of small amounts of soluble copper must be considered as a factor in con- 

 nection w^th foliage injury, as well as in fungicidal action where it may 

 supplement the solvent action of spore excretions. 



The Laboratory Slide-Germination Method of Evaluating 



Fungicides 



Although the ultimate evaluation of fungicides must depend on their effi- 

 ciency under actual field conditions, such tests are costly and time-consum- 

 ing. It is most desirable to be able to test fungicides under the rapid and 

 simple conditions of the laboratory and greenhouse and considerable effort 

 has been devoted to developing such methods and to a clarification of the 

 important factors mvolved.^- ^°' "■ i^' ^^- ^^- ^^- ^°' '^' 2"- 25. 26. 29. 30, 31, 32. 39 



Theoretical principles. In the beginning of laboratory testing "^ little 

 or no attention was paid to such factors as concentration or deposition and 

 variability of results, while statistical interpretation was unheard of. 

 Pioneering studies were undertaken in these matters. The relative preci- 

 sion of spore germination tests was demonstrated,-" and it was pointed out 

 that the errors arise from two sources, namely, faulty technique and random 

 sampling; the former may be reduced but the latter cannot be avoided. 



Toxicity surface. When tests are performed with three variables of con- 

 centration, time, and response, i.e., per cent germination, it is possible to 



