New Approaches to Control Walnut Blight

Research this year in the laboratory and field continued to show that the bactericidal action of copper compounds could be greatly increased by amending them with iron. There are at least three different mechanisms which cause the increased efficacy: 1) When soluble iron is added to fixed coppers at amounts ranging from 10 to 100 ppm, the ferric iron interacts synergistically with the copper ion causing the ~acterium to have increased susceptibility to the toxic action of the copper ion. 2) The deposition of iron on walnut leaves helps to prevent chelation of copper by other substances that naturally occur on leaves, thus leaving more free copper ions to kill blight bacteria. 3) The addition of ferric chloride to suspensions or solutions of copper compounds such as the fixed coppers results in a reduction of pH causing increased solubilization of copper ions. This is in part because the ferric ion has a greater affinity to anions such as the hydroxide ion (Cu(OH)2) in the Kocide molecule thereby releasing free copper ions on walnut parts. On walnut leaves, the addition of iron to fixed coppers results in up to a 30 fold increase in the concentration of the free copper ion. Field experiments showed that the combination of iron with fixed coppers resulted in a statistically significant reduction of walnut blight bacteria in buds. So far, no phytotoxicity has been noted by the addition of iron to copper compounds but this needs more research.