Photo Credit: Kathy Coatney
BEST PRACTICES
Almonds in bloom.
Orchard Management Monthly: February Kathy Coatney Editor Walnuts he Crown gall bacterium (Agrobacterium tumefaciens) is a soil-borne bacterium, and it can cause significant economic loss to California walnut orchards. Because the crown gall pathogen is a common soil-borne bacterium, there are several approaches to preventing the disease. Research suggests:
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Fumigate fields before planting with Telone® C-35, or, in heavily infested crown gall sites, Telone® C-35 followed by chloropicrin Limit the time between nursery or cold storage and planting, and keep
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West Coast Nut
February 2017
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nursery stock cool prior to planting Sterilize grafting tools frequently Avoid planting too deep and mounding soil around the trunk Keep crown of tree as dry as possible
Once Agrobacterium tumefaciens is in a field it can survive for at least two years in the orchard soil, and at least one and a half years in non-irrigated fallow soil. Methyl bromide has been phased out, but there are alternative products available that will reduce the disease: Vapam, Telone® C-35, and Telone® C-35 followed by an additional application of chloropicrin. It is also advised to use “clean” black walnut seeds for Paradox Production. In Paradox production, black walnut seeds are shaken to the ground, and they may lay on the ground for six to 24 hours.
The seeds can become contaminated with crown gall pathogen from the soil. Using a catch frame or tarp to catch the seeds can decrease crown gall on susceptible walnut rootstocks. The best form of disease control has been the identification and development of disease resistant hosts. Texas black walnuts have shown resistance to crown gall. New Texas crossed with English hybrids are currently being examined under various field conditions. The clonal Paradox rootstock (RX1) has also shown moderate resistance to crown gall. Pistachio Botryosphaeria panicle and shoot blight remains one of the major diseases of pistachios in California. This disease has the highest potential for crop destruction because it kills buds and fruit