Is it crown gall or leafy gall?

Research at Oregon State University aims to call a spade a spade

By Melodie L. Putnam and Marilyn Miller

Humphrey Gifford, an early English poet said, “I cannot say the crow is white, But needs must call a spade a spade.” To call a thing by its simplest and best-understood name is what is meant by calling a spade a spade. We have found confusion around the plant disease typified by leafy galls and shoot proliferation, and we want to call a spade a spade.

The bacterium Rhodococcus fascians causes fasciation, leafy galls and shoot proliferation on plants. These symptoms have been attributed variously to crown gall bacteria (Agrobacterium tumefaciens), virus infection, herbicide damage or eriophyid mite infestation. There is also confusion about what to call the symptoms caused by R. fascians. Shoot proliferation and leafy galls are sometimes called “fasciation,” a term also used to refer to tissues that grow into a flattened ribbon-like manner (Figure 1). The root for the word fasciation come from the Latin fascia — to fuse — and refers to a joining of tissues. We will reserve the term fasciation for the ribbon like growth of stems and other organs.

The terms “leafy gall” and “shoot proliferation” are unfamiliar to many people, but they are a good description of what is seen on affected plants. A leafy gall is a mass of buds or short shoots tightly packed together and fused at the base. These may appear beneath the soil or near the soil line at the base of the stem (Figure 2). They may also form in leaf axils (Figure 3), and in fewer cases, near leaf veins.

Shoot proliferation is a loose collection of shoots that are larger than the shoots in leafy galls, but smaller than uninfected shoots, which may or may not be fused at the base (Figure 4). Leafy galls and shoot proliferations are quite different than the galls caused by the crown gall bacterium, A. tumefaciens (Figure 5). Infection with A. tumefaciens causes swelling of tissue into tumors or galls on stems or roots, but these galls do not differentiate into buds or stems. In contrast, leafy galls are well differentiated into easily recognized plant parts.

Both R. fascians and A. tumefaciens are known to infect herbaceous and woody plants. Both bacteria have a wide host range (over 60 species for R. fascians and hundreds for A. tumefaciens). In addition, R. fascians infects monocots as well as dicots, unlike A. tumefaciens, which infects only dicots. There have been field observations that imply that populations of R. fascians may persist for one or two years in soil in which diseased plants have been growing. R. fascians will also move in water, although this is a passive process as these bacteria have no ability to move on their own.

On infected plants, bacteria are primarily limited to the surfaces of the leaves, petioles and stems, although some underlying cells may become infected. There is no evidence that R. fascians can systemically infect plants. The disease is primarily spread by taking cuttings from infected plants, and it is difficult to know if the plants you have are clean, because the bacteria can be present on plants for months before symptoms develop.

Plants affected by R. fascians often grow with less vigor, have an abnormally short stature, may produce fewer flowers and may have less root growth, although this varies with the plant species. There appear to be distinct differences in both the ability of various strains to infect various hosts and in host response. We have found that, within a genus, some plant species are resistant, while others are susceptible. We have also found that some cultivars are susceptible and others apparently are not.

We first started finding R. fascians in plants with shoot proliferations and leafy galls that were submitted to the Oregon State University Plant Clinic for diagnosis. The growers asked us to check for crown gall, and we could easily isolate the crown gall bacteria from affected plants.

We collected 536 isolates of Agrobacterium from dozens of herbaceous perennials. We inoculated 229 of these Agrobacterium isolates onto indicator plants to test for pathogenicity. Only 26 isolates of Agrobacterium were found to be pathogenic, and they were all isolated from plants with fleshy tumors typical of crown gall; they produced only tumors upon inoculation.

If Agrobacterium had been causing these leafy galls, then these inoculations should have produced leafy galls. It was interesting how frequently we recovered Agrobacterium from so many plants, yet most of these strains were incapable of causing disease. Table 1 lists the plants submitted to the clinic with leafy galls or shoot proliferation. Obviously, there is a wide range of plants affected by these symptoms.

On most of the plants we received for diagnosis we were making dual isolations, using one method for Agrobacterium and a different method for R. fascians. Interestingly, we recovered R. fascians much less frequently than we did Agrobacterium. However, we found that nearly all the R. fascians isolates we obtained were pathogenic. This was demonstrated by the ability of the R. fascians isolates to cause shoot proliferation when inoculated onto pea seedlings.

We were interested to see if we could reproduce the original symptoms of leafy galls on plants used in the trade, so we inoculated our R. fascians isolates onto 16 different ornamental plants. Pathogenic Agrobacterium isolates were also inoculated onto seven plant species for a comparison of symptoms. The Agrobacterium isolates produced crown gall-like tumors on the inoculated plants in all cases (Figure 6) except for Echinacea, which produced multiple shoots arising from crown gall-like tumors. In no case did Agrobacterium inoculation result in leafy gall production.

In contrast, inoculation with R. fascians isolates produced only leafy galls (Figure 7) or shoot proliferation. The division of symptoms was clear: Agrobacterium caused crown gall-like tumors and R. fascians produced leafy galls and shoot proliferations.

Disease management

What can be done about leafy gall (due to R. fascians) and crown gall (due to Agrobacterium)? For one thing, it is important to get an accurate diagnosis. Leafy gall production from R. fascians can be mistakenly attributed to other causes, and the control for a bacterial infection will be different than for an eriophyid mite infestation, for example.

Unfortunately, there is no treatment for either R. fascians or Agrobacterium infection at this time, therefore steps must be taken to prevent disease. There is a biological control product for prevention of crown gall called NoGall. When woody cuttings are dipped into this before planting, the biocontrol bacteria present in the formulation prevent tumor formation by preemptively colonizing the cutting, and by producing an antibiotic that is toxic to Agrobacterium tumefaciens.

We wanted to know if NoGall would work to prevent crown gall in herbaceous perennials. In research funded through the Oregon Association of Nurseries and the Oregon Department of Agriculture, we treated plants of Bryophyllum daigremontianum with NoGall, then applied various strains of Agrobacterium tumefaciens. We also tested BlightBan, another biocontrol product that produces a toxin effective against Gram negative bacteria. Neither product was effective in reducing the incidence of crown gall tumors in our test plants.

We have found that the predominant strains of Agrobacterium recovered from herbaceous plants to be different than those recovered from woody plants. This may explain why the NoGall was less effective in herbaceous perennials than it was in woody plants. We also tested NoGall and BlightBan against R. fascians infection, but again neither product offered any control.

We have not tested other products for control of R. fascians. R. fascians colonizes mainly plant surfaces, but it can also penetrate several cell layers below, which means surface treatment will not kill all the bacteria. An effective product would have to have systemic activity. We hope to try other control products in the future. Although there are no good chemical options at this time, good old-fashioned sanitation measures are still effective in preventing leafy gall and crown gall. Control measures for both diseases are listed in the “Get control” information box accompanying this article.

Meoldie L. Putnam is the director of the Oregon State University Plant Diagnostic Clinic and has been diagnosing plant diseases for over 20 years; Marilyn Miller is a faculty research assistant at OSU in Corvallis, Ore., and has worked with crown gall for over 25 years. Putnam may be reached at (541) 737-3472 or at putnamm@science.oregonstate.edu.

Digger magazine is produced by the Oregon Association of Nurseries Publications Department. publications@oan.org