Crown gall on Plants

Symptoms

Look first at the base of the stem and the root crown, where the disease produces soft, tumor-like galls. Galls can also form on other plant parts. On young plants they start tiny and smooth. On older plants they become rough and sometimes massive.

The gall surface and the wood inside are colored like healthy bark, but they are softer. The galled wood also lacks the normal pattern of annual growth rings. Spongy, rough swellings on crowns or canes can range from pinhead-sized to golf ball-sized, and they darken from light-colored to brown or nearly black.

Grapes and caneberries commonly show aerial galls, which form above the soil line. Young infected plants may become stunted and fail to grow properly. Galls severely impair the movement of water and nutrients through the plant. Stunted growth also comes with greater susceptibility to wind breakage and drought stress.

Causes

Many agricultural soils carry Agrobacterium tumefaciens, the bacterium behind this disease. The bacterium infects only through fresh wounds in roots, stems, or the crown. Wounds from nursery handling, transplanting, pruning, or cultivation can create infection sites. Insect and nematode feeding creates infection sites in the same way.

Once the bacterium is in a wound, it transfers a portion of its DNA into a plant cell. That plant cell is then induced to become a tumor cell. Bacteria reproduce in galled tissue and then slough off. Those bacteria can survive in soil for long periods. The bacterium is therefore present both in the soil and in galled tissue.

Pruning is one of the wounding events named, so a fresh cut is a possible way in. Common soil fumigants do not satisfactorily control the pathogen. Heat is the soil option that is named: steaming at 140°F (60°C) for 30 minutes, or solarizing.

Diagnosis

Galls set this problem apart from a general care fault, because they are tumor-like growths on the plant itself. Check the base of the stem and the root crown, where the galls are produced. A gall is softer than the surrounding bark, although its surface is colored like healthy bark. Galled wood also lacks the normal pattern of annual growth rings.

Size and surface change with the age of the plant. Galls are tiny and smooth on young plants, and rough and sometimes massive on older ones. Swellings on crowns or canes darken from light-colored to brown or nearly black. Stunted growth is part of the picture, but the galls are the feature to look for. Aerial galls above the soil line are common on grapes and caneberries, so do not limit your search to the roots.

The plant's history matters too. The bacterium infects only through fresh wounds, so recent transplanting, pruning, or cultivation, or insect or nematode feeding, fits this diagnosis. Plants such as euonymus, rose, willow, and fruit and nut trees are among the hosts.

Treatments

Cultural controls come first: plant only high-quality, disease-free nursery stock and avoid injuring plants, especially around the soil line or roots. Before planting, solarize moist, bare soil under clear plastic for 4 to 6 weeks during the sunny, dry season. This reduces bacteria near the surface. Soil can also be treated with heat, either by steaming at 140°F (60°C) for 30 minutes or by solarizing. Common soil fumigants do not satisfactorily control the pathogen.

If you remove galls, cut them out only during the dry season, take as little healthy tissue as possible, and avoid burning or charring the wound. On certain fruit and nut trees, you can briefly heat and sterilize the cut edges with a blowtorch after removing galls, without charring the tissue. If an extensively galled tree grows poorly after gall removal, replace it. Practice strict sanitation, especially where cuttings are produced, and clean and periodically disinfect tools and surfaces that touch propagative material.

The organic option is a biological control agent, Agrobacterium radiobacter strain K-84 or K-1026. In the nursery, seeds or roots are dipped in a solution of it before planting. This is effective against most strains, but not the strain commonly affecting grape. Strain K-84 can also be applied to fresh wounds within 24 hours of wounding or pruning, because it only prevents infection on fresh cuts.

The chemical option has the active ingredients 2,4-xylenol and meta-cresol, applied directly to existing galls from winter through spring to kill the gall tissue. The product label is the legal instruction and must be followed for the crop and site being treated.

Prevention

Because the bacterium infects only through fresh wounds, protecting roots, stems, and the crown from injury is the core of prevention. Nursery handling, transplanting, pruning, and cultivation all create wounds, so take particular care around the soil line. Choose high-quality, disease-free nursery stock.

Soil that was solarized for 4 to 6 weeks in the sunny, dry season before planting has fewer bacteria near the surface. Steaming soil at 140°F (60°C) for 30 minutes is another heat-treatment option. Where cuttings are produced, strict sanitation matters, and tools and surfaces that contact propagative material should be cleaned and periodically disinfected.

Timing matters after any wounding, because a biological control agent only prevents infection on fresh cuts, within 24 hours.

Plants most often affected

Many plants can be infected, especially euonymus, fruit and nut trees, rose, and willow. Herbaceous hosts include chrysanthemum, dahlia, geranium, marigold, peony, and snapdragon. The host list also names grape, broccoli, hazelnut, quince, walnut, mango, banana, apricot, cherry, plum, almond, peach, apple, pear, and blueberry.

• Callery Pear
• Common Filbert
• European Mountain Ash
• Sweet Cherry
• Weeping Willow