Solubilization of Copper and the Mode of Action of Bordeaux Mixture

Abstract

SINCE the discovery of Bordeaux mixture over 80 years ago the question has often been posed as to how the copper reaches the fungal cell in toxic concentrations when the solubility of dried Bordeaux deposits in water is so low. Of the three possible agencies for solubilizing copper, namely the atmosphere, the spore, and the host plant, the last two have been regarded as the most important and it is established that spore^1,2 and leaf^3,4 exudates can dissolve copper from Bordeaux spray deposits. The assumption from this work is that a soluble copper complex is formed, possibly with the amino-, hydroxy-, or dicarboxylic acids in the exudates, and this complex is accumulated by the fungal cell where it may dissociate to give free cupric ion. However, most workers have shown that the copper complexes which could possibly be formed with leaf or spore exudates are much less toxic than the free ion^5–10. In fact, under conditions of high spore/copper ratios, copper can stimulate germination by leaking growth factors in the spore exudate^11,12.