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Crown gall Management

Preplanting Management Options

Planting stocks. Visual examination for crown gall tumors has been the conventional primary screen for diseased material. The method is limited for complete disease control because A. tumefaciens can reside on the rhizoplane and systemically in certain host plants such as grapevines, chrysanthemums, and marguerite daisy.

Site selection. Fields that have grown cereal crops for a long period are favored as crown gall-free sites. Fields previously used for growing fruit and nut crops can remain infested with A. tumefaciens. Certain weeds such as morning glory (Ipomoea leptphylla) can serve as natural hosts of A. tumefaciens and therefore perpetuate the survival of this pathogen in field soils.

Crop rotation. A crop rotation program employing cereal crops followed by green manuring helps reduce the population size of A. tumefaciens.

Chemical eradicants. Eradication of crown gall using creosote-based compounds, copper-based solutions, and strong oxidants such as sodium hypochlorite are transiently effective. The chemical eradicant application procedure is labor intensive and therefore costly both monetarily and to the environment. The superficial treatments are ineffective against systemically infected plants. Generally, chemicals are rarely used for control of crown gall.

Biological control. Certain strains of A. tumefaciens are sensitive to the antibiotic agrocin produced by A. radiobacter, a closely related soil-borne bacterium that does not infect plants. An example of an antibiotic produced is Agrocin-84, which is an analog of the opine agrocinopine A. Agrocinopine A is produced in crown gall tumors induced by A. tumefaciens strains whose Ti plasmid encodes for nopaline and agrocinopine A. Agrocin-84 mimics agrocinopine A and therefore is taken up by the same transport system used by A. tumefaciens to utilize agrocinopine A. Inside the A. tumefaciens cell, the antibiotic Agrocin-84 inhibits DNA replication and cellular growth.

Plants are protected against Agrocin-84 sensitive strains of the pathogen by dipping the root system into a suspension of A. radiobacter K84 before planting in infested fields. Biological control of crown gall has been a very effective method to control crown gall in several locations. In many other regions where agrocin-insensitive strains of A. tumefaciens (strains that do not acquire agrocinopine A) reside, this biological control strategy is ineffective.

Genetically engineering. Transgenic crop plants harboring one or more unique genes tailored to protect the plant from crown gall have been developed. Genes encoding products that degrade or inactivate the T-DNA strand complex when it enters the host cell, that prevent the expression of T-DNA genes encoding indoleacetic acid and cytokinin biosynthesis, and that prevent A. tumefaciens attachment to its target are some examples currently being tested. Biotechnology companies, such as DNA Plant Technology
(Oakland, CA), are applying sense strand messenger RNA or small-interfering RNAs to develop crown gall resistant fruit and nut crops.

Management in Established Fields

Trees in fruit and nut orchards can be maintained over long periods if the trees became infected at maturity. Diseased trees will bear crop, but with age the trees will become unthrifty and suffer dehydration as their root system becomes progressively infected. The removal of infected trees and vines is costly in loss in time and in money. Annual row crops such as sugar beets and cotton occasionally will have a few plants with crown gall, but the disease is considered of low economic importance. Perennial field crops such as alfalfa will occasionally become infected with A. tumefaciens where the organism is spread by the mowing equipment. Usually, rogueing of the diseased plants is sufficient to minimize further spread of crown gall.