368 GROWTH OF PLANTS 



in an insoluble form, and various theories have been advanced to explain 

 its fungicidal action.^ These may be divided into three groups: (a) that 

 atmospheric agents such as CO2 and rain liberate soluble copper; (6) that 

 the fungus itself secretes materials which free the copper; and (c) that excre- 

 tions from the host plant act on the insoluble copper. 



Studies were made of the action of fungus spores on Bordeaux ^^ and of 

 the weathering of Bordeaux mixture. ^^ The amount of copper that goes 

 into solution in distilled water above a dried deposit of Bordeaux was 

 determined to be less than 0.3 ppm, an amount insufficient to affect mate- 

 rially the germination of most fungus spores. 



Action of spores. Fungus spores were obtained by a vacuum technique 

 which prevented their contamination by the nutrient medium.-^ These 

 were suspended in water, allowed to stand for several hours, and filtered. 

 The filtrates were then placed over dried Bordeaux mixture and agitated 

 overnight. The amount of copper rendered soluble varied with the seven 

 different species tested and was directly proportional to the number of 

 spores. For example, the water extracts from 100,000,000 spores of the 

 most active fungi, Uromyces caryophyllinus and Sclerotinia fructicola, 

 brought into solution 1.01 and 0.76 mg of copper respectively, whereas 

 the least active fungus, Alternaria solani, liberated 0.013 mg of copper. 

 Determinations of total solids excreted by the spores showed that those 

 species excreting the greatest amount of solids were also the most active 

 in bringing copper into solution. It was also found that those spores most 

 sensitive to the toxicity of Bordeaux mixture were in general the ones 

 most active in solubilizing copper. 



By means of ultrafiltration tests it was further found that the active 

 solubilizing material in the spore excretions is all in true solution. Also, 

 practically all of it may be removed in the first washing of the spores. A 

 quantity of spore excretion was obtained from approximately 360,000,000,- 

 000 spores of Neurospora sitophila, and an analysis indicated 3.1 per cent 

 of malic acid present in the solid matter. There was also found 0.75 per 

 cent of amino nitrogen, thus indicating the presence of amino acids. Since 

 the spore excretions are practically neutral, their action cannot be due to 

 any acidic properties. It is well known that certain amino acids form soluble 

 complexes with copper oxide, and it was found that glycine and aspartic 

 acid as well as neutral sodium malate will dissolve large amounts of copper 

 from Bordeaux mixture. Finally, comparative toxicity tests of sodium 

 cuprimalate and of a copper glycine derivative showed that these forms of 

 copper exert substantially the same action as copper sulfate. Thus, in 

 view of these findings, it is believed that the salts of hydroxy acids such as 

 malic and perhaps others, as well as of amino acids present in the spore 

 excretions, act on Bordeaux mixture to form soluble toxic copper hydroxy 

 and copper amino salts.^^ 



Weathering action. The action of the spores in liberating soluble copper 

 from Bordeaux mixture was of course demonstrated on glass in the labora- 



