248 PHYSIOLOGY OF THE FUNGI 



of the copper compound to dissolve until the spore was no longer able to 

 take more copper from the solution. This theory is attractive because 

 of its simplicity, but there seems to be no very good evidence for it 

 (McCallan, 1929). 



In practice, Bordeaux mixture and other "insoluble" copper sprays 

 act as if they were more soluble than is indicated by chemical tests. 

 However, in practice the fungicide is exposed to the action of the atmos- 

 phere, the host plant, and the fungus spores. This is a more complicated 

 situation than that found in the chemical determination of solubility. 

 Barker and Gimingham (1911) found that intact leaves increased the 

 soluble copper from Bordeaux mixture to some extent but were of the 

 opinion that the host plant had only a slight influence on the solubility 

 of such sprays. However, if the leaves were injured, they were quite 

 effective in bringing copper into solution. The possibility that the spores 

 exert a solvent action on "insoluble" copper compounds has long been 

 considered by plant pathologists. The spores of at least some species 

 do exert a solvent action on Bordeaux mixture. McCallan and Wilcoxon 

 (1936) showed that the amount of copper brought into solution by the 

 soluble materials washed from or excreted by 100 million spores of some 

 species varied as follows: Uromyces caryophyllimis, 1.01 mg.; Sclerotinia 

 fructicola, 0.76 mg.; Neurospora sitophila, 0.12 mg.; Botrytis paeoniae, 

 0.10 mg.; Glomerella cingulata, 0.046 mg. ; Aspergillus niger, 0.023 mg.; 

 and Alternaria solani, 0.013 mg. Enough spores of Neurospora sitophila 

 were collected so that the nature of the soluble materials from the spores 

 could be identified chemically. Malic acid was isolated and identified. 

 The presence of amino acids also was detected. Both malic acid (or 

 malates) and various amino acids dissolve "insoluble" copper compounds 

 under nevitral or alkaline conditions with the formation of soluble complex 

 copper compounds. McCallan and Wilcoxon showed that sodium 

 cuprimalate and a copper-glycine compound were about as toxic as copper 

 sulfate. On the other hand, Goldsworthy and Green (1936) were of the 

 opinion that spore secretions played a minor role in increasing the solu- 

 bility of Bordeaux mixture, but the evidence of McCallan and Wilcoxon 

 seems quite conclusive. 



Basic copper carbonate (malachite, Cu(OH)2-CuC03) and cuprous 

 oxide (CuiO) are used in treating seeds. Since the seed covered with 

 these materials is planted in soil which contains a variety of protein 

 degradation products, it is easy to understand how these substances are 

 made sufficiently soluble to be fungicidal. Marten and Leach (1944) 

 studied the effect of various nitrogenous compounds upon the solubility 

 of cuprous oxide. Gelatin and peptone were less efficient in dissolving 

 cuprous oxide than were glycine, aspartic acid, asparagine, or cystine. 

 Ammonium hydroxide was also a solvent for cuprous oxide. With all 



