Biological Control of the Crown Gall Diseases

Six different strains of crown gall bacteria were tested to determine whether the commonly used biocontrol strain, K-84, would present them from infecting the paradox hybrid. Previous research showed that K-84 was highly effective in preventing crown gall infections of English root stock. There was considerable variability among the six strains in ability to cause disease. One of the strains did not infect at all. Another was weakly virulent, and the other strains readily produced large galls. Surprisingly, the biocontrol strain, K-84, did not prevent galls from forming on the paradox hybrid. All five strains produced galls even when the wounds were treated with K-84. This may explain why the hybrid is frequently infected with the crown gall disease. It is possible that K-84 does not multiply and protect the wounds in paradox hybrid as it does with other varieties. Research is going on to determine if greater cohcentrations of K-84 could possibly prevent galls from forming on the hybrid. In other work, the mechanism of action by which K-84 protects plants from crown gall bacteria was studied. By a process of mutagenesis, the ability of K-84 to control crown gall was directly related to its production of an antibiotic. Whether this antibiotic is made in wounded tissues of paradox hybrid has not yet been determined. Temperature directly influences how long a wound is susceptible to infection by crown gall bacteria. At temperatures above 70of, root stock such as English walnut must be treated almost immediately for strain K-84 to protect against the crown gall disease.