A recent resurgence of galls on apple trees has sparked new interest in an old disease. By Anna Mouton.
Plant galls are abnormal growths formed by cell enlargement or proliferation — or both — resulting from infections and other causes. Several species of bacteria can produce root galls. Crown gall refers to galls caused by Agrobacterium at the base of the trunk and on the roots of woody plants.
Descriptions of crown gall go back to 1853, and the bacteria responsible for the galls were first isolated from grapevines in 1897, but scientists were at a loss to explain how a bacterial infection could provoke tumours in plants.
The breakthroughs came in the 1960s and 1970s when biologists discovered that certain Agrobacterium species and strains are natural-born genetic engineers. Researchers were quick to repurpose Agrobacterium for creating the first transgenic plants, and Agrobacterium has been keenly studied ever since.
Despite the intense focus on Agrobacterium, crown gall remains a largely insoluble problem worldwide. Agrobacterium can infect about 600 plant species, but members of the grape, walnut, and rose families (including pome and stone fruit) suffer the most.
Several species of Agrobacterium are associated with crown gall, and some also cause other diseases, including hairy root in apple trees.
Crown gall damages the root system. Growing galls compress plant vessels, limiting water flow to above-ground organs by as much as 80%. Young plants are worse affected, showing poor vigour, fewer leaves, and water stress. Older trees can suffer yield losses and poor growth, and may sucker excessively.
How do I recognise crown gall?
Crown gall starts as small, round nodules on the trunk and roots, usually near the soil surface. Galls are initially soft and whitish or light green, darkening and hardening as they grow. The galls consist of disorganised cells running riot, so galls eventually develop convoluted, irregular surfaces.
Galls on apples tend to become woody and hard, while on other plants, they may be spongy and crumble, or detach from the host. Sometimes the galls decay during tree dormancy, only to enlarge again when the plant resumes growth. Individual galls are generally 0.5–10 centimetres in diameter, although they can become much larger.
Callus may be mistaken for galls. A callus is also hard, round, and knotty, but the surface of the callus remains smooth, whereas the surface of galls becomes rough and warty. When cutting through callus, bark and woody tissues are visible, whereas galls contain no identifiable tissue layers.
Because callus forms around wound edges, it can take any shape, while galls are mainly spherical.
Pests such as root-knot nematodes and woolly apple aphids also stimulate abnormal root growths. However, root-knot nematodes do not attack apple trees, and woolly apple aphids tend to be visible near the galls.
Testing for crown gall is challenging. Culturing Agrobacterium from galls is not enough to confirm a diagnosis because non-disease-causing Agrobacterium species and strains are widespread in soils.
Unfortunately, not culturing the bacteria is also not enough to exclude a crown-gall diagnosis. For reasons explained below, Agrobacterium is often absent from galls.
Various PCR tests for crown gall are used internationally, and other molecular-based detection assays are under development.
How does Agrobacterium cause galls?
Agrobacterium species are ubiquitous in soil and often found closely associated with roots. Disease-causing Agrobacterium strains have genes that enable them to genetically engineer their host. These genes exist as plasmids — small circular DNA molecules separate from the rest of the bacterial genome.
The bacteria are attracted to injured tissues, for example, wounds on roots. Plants respond to wounding by ramping up cell division and DNA replication, which makes it easier for the Agrobacterium genes to insert themselves into the host DNA. Once the Agrobacterium genes have been incorporated, the plant genome is said to be transformed.
The new genes in transformed cells have two functions. One, they influence the production and effects of auxins and cytokinins, leading to uncontrolled cell proliferation and the appearance of galls. Two, they drive the cellular production of opines — combinations of amino acids and sugars — that feed the bacteria.
In the pathogen-host arms race, plants have evolved several mechanisms to defend themselves against Agrobacterium, but once plant cells are transformed, they will continue to grow abnormally whether or not the bacteria remain present.
Secondary invasions of galls by microbes and insects are common. However, most of the diseased tissue in older galls is dead, so Agrobacterium itself is no longer present — this is why Agrobacterium culture is best attempted from new galls.
Control of crown gall
Biological control with non-pathogenic strains of Agrobacterium has not been consistently effective. It only works for some strains of pathogenic Agrobacterium and seems less successful in apples than in stone fruit.
Trees are most susceptible to infection in the nursery, so every effort should be made to prevent exposure to pathogenic Agrobacterium.
Agrobacterium can be introduced into the nursery through infected soil or rootstocks from stool beds. Thereafter, it can spread within the nursery through the soil and with contaminated tools, as well as through contact between infected and healthy trees when these are bundled, stored, or transported together.
Young trees are at the most risk when planted in potentially infected soil, especially if they have sustained wounds during lifting, handling, and planting. Fields used for cereal crops are less likely to harbour crown-gall bacteria than fields used for fruit or nut crops. Fumigation has not proven effective at eradicating crown-gall bacteria from infected sites.
Mechanical or insect damage to roots increases the opportunities for Agrobacterium infections. The bacteria can also infect through pruning and grafting wounds.
Nursery stock suffering from crown gall should be destroyed. The trees are a potential source of Agrobacterium strains that can infect healthy trees.
For more about the recent outbreak of galls on apple nursery trees, consult issues 197 and 200 of the Hortgro freshNOTES.
Image: A confirmed case of crown gall in a plum tree.
Supplied by Sonja Coertze | Stellenbosch University Plant Disease Clinic.