ABSTRACT
I. INTRODUCTION Copper (Cu) is an essential trace mineral for the growth of plants and livestock. Before the turn of this century [as reviewed by Reuther and Labanauskas (1966)], Cu was regarded as a plant poison, as indeed it may be when applied to soil in excessive concentrations as high as those commonly used for such fertilizer elements as N, P, K, and S. A large number of researchers, working independently, developed evidence and made observations which indicated that Bordeaux (a fungicide containing CuS04) sprays occasionally had stimulating effects on plant vigor and yield that were not associated with the control of fungus diseases. Grossenbacher (1916) and Floyd (1917) in Florida reported that dieback of citrus could be controlled by spraying affected trees with Bordeaux or by soil applications of CuS04• They indicated that Cu acted in some indirect way to stimulate citrus growth or to alleviate an unknown disease factor.
