What Is Agrobacterium, the Bacterium Behind Crown Gall?

Crown gall in roses is caused by a soil-borne bacterium historically known as Agrobacterium tumefaciens, now often classified as Rhizobium radiobacter. It is famous as a “natural genetic engineer” because it can transfer its own DNA into plant cells and reprogram them.

The Life Cycle: How It Lives, Feeds, and Moves

• Survives for years in soil and plant debris.
• Waits for wounds on roots, crowns, or stems to enter the plant.
• Transfers tumor-inducing DNA into plant cells.
• Creates galls and uses them as a food source.
• Spreads through soil, water, tools, infected propagation materials.

How Agrobacterium Moves and Spreads in Roses

Agrobacterium requires a wound to infect a rose. Common entry points include graft unions, root wounds, pruning cuts, freeze cracks, insect damage and mechanical injury around the crown.

Systemic Movement Inside the Plant

Once inside, Agrobacterium can move through the rose’s vascular system, especially the xylem. This explains why one sample may test positive while another part of the same plant appears clean. Stress can trigger hidden infections, causing galls to form months or years later.

What Is a Crown Gall and What Actually Kills the Rose?

A crown gall is a hard, woody tumor created when Agrobacterium inserts DNA into plant cells and forces them to divide uncontrollably. The gall disrupts water flow, weakens the plant and opens pathways for secondary infections.

Why Clean Stock and Sanitary Practices Are Vital

Because Agrobacterium can persist in soil, live inside plant tissue, and spread through water, tools and propagation, prevention begins with clean stock and meticulous sanitation.

How to Tell Crown Gall Apart from Other Root & Crown Growths

Not every lump is crown gall. Some are natural healing tissue (callus), some are beneficial nitrogen nodules, and some are caused by entirely different organisms such as root-knot nematodes. Learning to distinguish them will help you act quickly — and avoid unnecessary worry.

Nitrogen Nodules: A Beneficial Partnership

While roses are not legumes, they can host beneficial nitrogen-fixing bacteria — especially Frankia or rhizobia-like microbes — in biologically rich soils. These microbes form tiny, smooth, round nodules on fine feeder roots and create a “mini nitrogen factory” that supplies your rose with usable nitrogen.

• Small, smooth, round structures on fine roots
• Sometimes pinkish inside from leghemoglobin
• Never woody; easily distinguished from corky galls at the crown
• Generally associated with healthy, vigorous plants

Root-Knot Nematodes

Root-knot nematodes (Meloidogyne spp.) are microscopic roundworms that cause multiple small swellings along roots. These bumps are internal distortions of root tissue — not external tumors — and they typically occur throughout the root system in infested soil.

• Many small knots, not one big mass at the crown
• Stubby, forked or “bearded” roots with fewer feeder roots
• Stunted, yellowing growth above ground

Nematodes do not spread through pruning tools and do not behave like crown gall. Management focuses on soil health, rotation, tolerant rootstocks and sometimes soil replacement in small beds.

What Is Callus Tissue on Roses?

Callus tissue is the rose’s natural healing response. Whenever you prune, graft, or the plant experiences a small injury, cells around the wound begin to divide and form a protective, cushioning layer that gradually closes and seals the exposed tissue.

On a rose, callus usually appears as:

• A smooth or bumpy, organized ridge or collar around a pruning cut or graft union.
• Tissue that starts slightly swollen and fleshy, then becomes firm and blends in over time.
• Growth that does not keep ballooning outward once the wound is sealed.

Healthy callus:

• Follows the shape of the original wound (like a neat ring over a cut).
• Is a sign that your rose is actively repairing itself — not a disease.

What Are Root Initiation Cells on Roses?

Bare Root roses often come with new roots beginning to form. The very first step in that process is the creation of root initiation cells — tiny clusters of actively dividing cells under the bark that will eventually emerge as new roots.

Root initiation cells typically look like:

• Very small, smooth “pimples” or bumps along a buried stem or cutting.
• Light-colored tissue that soon elongates into white, thread-like roots.
• Multiple tiny bumps, rather than one large, woody mass.

These are completely normal and highly desirable during propagation. They should not be confused with crown gall, which does not turn into neat, organized roots but remains a corky, chaotic tumor. Root initiation cells will always have the beginning of a point, or tip. When they are first emerging, they can look round like a nitrogen nodules, but you can distinguish them by looking closely at the rounded growth. If there is a beginning of a point, or tip - it will form a root. If not, it is a nitrogen nodule.

Preventative Measures Home Gardeners Can Take

• Start with clean, reputable stock.
• Handle bare roots gently.
• Sanitize tools between plants.
• Plant in well-drained, biologically rich soil.
• Avoid injuring the crown when working in the bed.
• Avoid replanting roses in soil known to have crown gall.

What to Do if You Suspect Crown Gall on an Established Rose

1. Isolate the plant.
2. Confirm using the comparison chart and descriptions above. If unsure - contact your  local extension office.
3. Remove the plant if crown gall is present.
4. Remove adhering soil and two wheelbarrows full of soil immediately surrounding the roots and do not reuse it on other roses. You may place the soil on grass well away from desirable landscape plantings.
5. Disinfect all tools, containers and work surfaces.
6. Avoid replanting roses immediately in the same hole.

If a Bare Root Shipment Appears to Have Crown Gall

If a bare-root rose arrives with suspicious swelling, immediate action helps prevent potential spread of Agrobacterium.

1. Do not plant or soak the suspect bare root with other bare root roses
2. Isolate the rose and all packaging materials. Anything touching the roots should be considered potentially exposed.
3. Contact your vendor with detailed photos and a description of what you see.
4. If other roses shared packaging, treat them as exposed.
5. Consider applying Ferti-Lome® Fire Blight Spray (streptomycin sulfate) to the external surfaces of exposed bare roots. 

According to plant pathologist Dr. Mark Windham, streptomycin can help reduce surface populations of Agrobacterium on:

• bare roots that shared packaging,
• paper and packing media,
• benches and tools used during handling.

This treatment reduces exposure risk but does not cure internal infections or remove existing galls.

Dr. Windham advises gardeners who receives a bare root rose with visible signs of crown gall and receives other bare root roses that do NOT exhibit signs of crown gall can feel confident that Ferti-Lome® Fire Blight Spray will significantly reduce agrobacterium populations that could have potentially been transferred to healthy bare root roses during transit. NEVER use bleach to soak your roses in. Any plant cells that come in contact with bleach will die. 

Disinfect all tools, work surfaces, and containers using:

70% isopropyl alcohol,

10% bleach (freshly mixed),

Lysol concentrate, or

commercial horticultural sanitizers.

If gall is confirmed, dispose of the plant. Do not compost; bag and discard.

How Grace Rose Farm Helps Prevent the Spread of Agrobacterium

• We partner only with growers who maintain documented gall-free production.
• We inspect every bare root by hand.
• We never use shared water vats—only clean sprayers.
• We disinfect pruners between every plant.
• We sanitize workstations daily.
• We train staff extensively in clean-handling horticulture.

Crown gall is a complex disease, but with clean stock, mindful planting, good sanitation and a bit of scientific understanding, your roses can thrive beautifully. Thank you for trusting us to help you Grow With Grace™.