Adapted from the article “Points to consider in the prevention of crown gall” by Daniel A. Kluepfel; USDA-ARS Crops Pathology and Genetics Research Unit, Dept. of Plant Pathology, Univ. of CA. Davis, Lani Yakabe, Dept. of Plant Pathology, Univ. of CA. Davis, and Janine Hasey, UCCE Farm Advisor, Sutter, Yuba, Colusa Counties in the October 2012 Sacramento Valley Walnut News.
Crown gall caused by the bacterium Agrobacterium tumefaciens can cause significant economic loss in both commercial walnut orchards and nursery operations in California. Paradox hybrid is extremely susceptible to infection by the crown gall causing bacterium. Since the crown gall pathogen is a common soil-borne bacterium, we need to approach disease prevention on multiple fronts. Laboratory and field research suggest the following are key to success:
- Fumigate planting sites with Telone® C-35, or, in heavily infested crown gall sites, Telone® C-35 followed by Chloropicrin.
- Limit time between nursery or cold storage and planting; keep nursery stock cool prior to planting
- Eliminate exposure of seeds and graft wood to field soil prior to planting or grafting/budding
- Surface sterilize grafting tools frequently
- Limit wounding of plant material
- Avoid planting too deep or mounding soil on the trunk
- Keep crown of tree as dry as possible; Agrobacterium is favored by wet environments
A fallow rotation does not appear to be an effective approach to reduce A. tumefaciens populations and limit crown gall formation. Once Agrobacterium tumefaciens is introduced into a field site it has the ability to survive for at least 2 years in the orchard soil and at least 1.5 years in non-irrigated fallow soil, and still induce crown gall formation.
Methyl bromide (MeBr), the previous standard pre-plant soil fumigant, has been phased out worldwide. The MeBr alternatives, Vapam, Telone® C-35, and Telone® C-35 followed by an additional application of chloropicrin, all reduce soil populations of A. tumefaciens. When applied alone, 1,3-dichloropropene (Telone® II) was not effective at controlling A. tumefaciens. Chloropicrin and 1,3-dichloropropene together in Telone® C-35 dramatically reduced A. tumefaciens populations in soil, but not in buried gall tissue. Additional chloropicrin applied after Telone® C-35 was needed to reduce A. tumefaciens in gall material.
Based on our laboratory data, Telone® C-35 is an effective preplant alternative to MeBr for the control of A. tumefaciens in soil. In sites with a history of high crown gall incidence, fumigation with Telone® C-35 plus chloropicrin combined with extensive gall removal from the soil should be considered.
Using “Clean” Black Walnut Seeds for Paradox Production
In Paradox production, black walnut seeds are shaken to the orchard floor where they may lay for 6 to 24 hours. During this time, seeds can become contaminated with crown gall pathogen from the soil. Contaminated seeds are then planted in fumigated soil, with little remaining native microbial community to suppress the invading A. tumefaciens. The crown gall pathogen establishes in soil, ready to infect the emerging walnut seedling. Eliminating soil contact by seeds with catch frames or tarps will most likely decrease crown gall incidence on susceptible walnut rootstocks.
Avoiding Contaminated Grafting Tools and Graft Wood
Agrobacterium contamination can come from improperly sanitized grafting and cutting tools, and even contaminated graft wood. Grafting tools and graft wood should never be left on soil where they can become contaminated with A. tumefaciens. The crown galls themselves are also sources of contamination and should be removed from the orchard after being cut from the tree.
Bleach is an effective disinfectant of water and solid surfaces. However, it is corrosive and rapidly inactivated by solids such as organic matter. Surfactants/detergents are potentially effective alternatives for high levels of organic matter. Surfactants known as quaternary ammonium compounds effectively reduced populations of A. tumefaciens in solutions and on solid surfaces. The detergents, benzalkonium chloride (BC), Cetyl trimethylammonium bromide, (CTAB) and Physan 20 rapidly reduced populations of A. tumefaciens. The activity of these detergents was only reduced by 16% in the presence of organic material, compared with bleach efficacy reduced by 64%. These detergents are also less phytotoxic than bleach.
The best form of disease control is the identification and development of disease resistant hosts. Texas black walnuts (Juglans microcarpa) have been found to exhibit elevated resistance to crown gall. New Texas x English hybrids are currently being examined under various field conditions. The clonal Paradox rootstock ‘RX1’ also has shown moderate resistance to crown gall in the field and in screening trials.