By Linda Workman Smith
Once again crown gall has reared its ugly head in one of my blackberry patches. Like an unwelcome guest who drops in unexpectedly, I spotted its dreadful visage a couple of weeks ago while doing spring clean-up; thus began my strategy for getting it to “move along.” I plan to use biofumigation and soil solarization.
What is Biofumigation?
Biofumigation is the suppression of soil-borne pests and pathogens by the use of plants that contain inhibitory chemicals. The plants can be harvested as rotation crops or cultivated back into the soil as green manure.
Biofumigation uses certain Brassica cultivars, which are grown, pulverized and incorporated into the soil prior to crop planting. The biofumigation process creates a powerful soil sterilizing agent, known as Isothiocyanate (TC) a natural gas released from Brassicaceous plant tissues. The ITC gas is released once the plant cells are damaged (by pulverization), and when the crop is rapidly incorporated into the soil the sterilizing Biofumigation effect takes place. By ploughing under green plant tissue, also known as green manure, a large amount of organic matter is also returned into the soil profile.
Successful Biofumigation offers the following benefits:
- Reduction in soil borne pests and diseases
- Increases soil organic carbon levels
- Provides a nutrient catch crop to retain leachable nutrients
- Improved soil structure
- Increases the soil’s nutrient and soil moisture holding capacity
- Weed suppression
- Improved crop productivity
- Offers an alternative to existing agricultural chemicals
The Biofumigation effect is limited to only certain Brassica cultivars, so it is important to only use improved and recognized varieties to achieve the expected benefits.
What is soil solarization?
Soil solarization is an environmentally friendly method of using solar power for controlling pests such as soil borne plant pathogens including fungi, bacteria, nematodes, insect and mite pests along with weed seed and seedlings in the soil by covering soil with a sheet of transparent polyethylene, to trap solar energy. It may also describe methods of decontaminating soil using sunlight or solar power. This energy causes physical, chemical, and biological changes in the soil.
The following is information that I gathered concerning:
Crown Gall on Blackberries
Crown Gall disease is caused by a soil borne bacterium that results in plant tumors in the crown of the plant. It spreads by entering open wounds. Crown gall reduces stand productivity; once a field is infected, it could stay infected for many years. Do not take cuttings from any plant that shows evidence of crown gall, and remove infected plants. Also be careful to disinfect pruning tools after use on plants where crown gall may be present.
Agrobacterium tumefaciens is a bacterium that lives in the soil and infects plants through wounds. Common methods of entry are through injuries made by cultivation, pruning or insect feeding; also can occur through natural causes, such as wind and hail. Once the bacterium enters the plant it inserts a portion of its DNA (called T-DNA) from the tumor-inducing portion of the bacteria's chromosome, into the chromosome of a healthy plant cell. The expression of the T-DNA causes the overproduction of plant hormones that lead to undifferentiated cell growth in that region of the plant; thus forming a gall. Small galls usually develop within 2 to 4 weeks after infection when temperatures are 20 degrees C/68 degrees F, or above. The expanding tissues can sometimes disrupt the flow of nutrients and water. Bacteria are released from the gall into the soil as the gall decays. The bacteria are spread to new plants and areas through splashing rain, irrigation water, insects, tools and plant parts used for propagation. The bacteria can survive up to two years in the soil without a plant host.
Swellings or galls may form above ground on stems or branches or below ground on roots. The galls are usually rounded with a rough surface and a spongy texture and may darken and crack with age. The galls can be confused with galls made by insect or mites and also by physiological responses to wounding or grafting. However, the interior of the gall caused by an insect will have chambers or cavities where the insect developed. The interior of a gall due to crown gall will have a mass of disorganized vascular tissue. It is often necessary to isolate the bacteria through diagnostic methods in order to make a positive identification. The effect that crown gall may have on a plant ranges from little to no impact on the growth and production to declining plant vigor or even death. The disease has a greater impact on young plants and is of great concern to the nursery industry.
The most effective method to manage crown gall is through prevention and sanitation. Plant disease-free, healthy stock from reputable nurseries. Do not plant in areas where crown gall has been seen during the previous three years. Destroy infected plants immediately, and do not plant back into the same spot. Use care during propagation to avoid transmitting bacteria to healthy plants. Sterilize pruning tools frequently using a disinfectant such as 70 percent alcohol or 10 percent household bleach. Avoid mechanical wounding to young plants by cultivation, weed eating, etc.
Galltrol®, the biological control agent, has been very successful in preventing crown gall in blackberries. Galltrol® is a nonpathogenic strain of Agrobacterium that is antagonistic to the crown gall bacterium. It is very effective when used as a preventative, but it will not cure infections after they have occurred. Dip the roots of new planting stock in Galltrol® prior to planting and follow label directions closely.
Note: I’ve been unsuccessful in finding the aforementioned product—L.W.S.
Ellis, M.A., A.H. Converse, R.N. Williams and B. Williamson. 1991.
Compendium of raspberry and blackberry diseases. APS Press:
St. Paul, MN. 100 p.
Back to my tale:
Last spring and summer—in an attempt to eradicate the curse of crown gall from one of my blackberry patches—I removed a substantial number of canes. Then following instructions from information gathered on soil fumigation with mustard, I:
- Obtained Pacific Gold mustard seed— one among several varieties recommended for soil fumigation
- Planted seed in infected area and waited for it to grow
- When mustard began to bloom I chopped well and tilled into soil
- Then with instructions from Oklahoma Cooperative Extension Service fact sheet #7640—Solarization of Soil for Control of Plant Disease—I covered tilled area with transparent polyethylene, sealed edges and let the sun do its work for a couple of months.
Combination of these two practices could, theoretically, increase the effectiveness of disease control on a given plot—at least that’s my opinion, for what it’s worth. Another benefit is killing lots of weed seeds.
Now to the current plan of attack.
- I first arranged plastic around the infected cane, excavated the cane with as much of the surrounding soil as possible—without disturbing neighboring canes—then immediately placed excavated cane and soil in my burn pile.
- Set aside all tools used in this process, to disinfect—still in a plastic bag in the shop
- Filled excavated area with fresh soil from my dirt pile
- Over-seeded entire blackberry bed with Pacific Gold mustard seed
Now I shall have to wait for the next steps which will be:
- Just as mustard begins to bloom I’ll chop it well--the fumigating gas is produced when plant cells are damaged (by crushing, mowing or chopping)and compounds called glucosinolates (present in all brassicas) come into contact with an enzyme (myrosinase) in the presence of moisture in the soil
- And since I can’t till in mustard without damaging the other blackberry cane roots, I’ll gently work it into the areas where I removed infected canes
- Add 1 to 2inches of fresh soil to whole bed —over the desiccated mustard
- Mulch entire blackberry bed with 2 to 3 inches of wood chips—bought in bulk from my good friends at Roller Dirt Farm at Tecumseh
My hope is that the added soil and wood chips will hold in enough of the isothiocyanate (ITC)--which is a natural gas released from all brassicaceous plant tissues—to help mitigate, if not eliminate the crown gall bacterium in the soil.
Also am hoping to not kill this entire blackberry patch with my experimentation.
And what is life without hope?
As always, happy gardening,
Linda Workman Smith
Multi-County Master Gardener Association