Replant Strategies to Cope
with Fumigation Regulations
Choosing a resistant rootstock when replanting an almond orchard will help improve chances of tree vigor whether fumigation is an option or not. If possible, allowing the land to remain unplanted for a season after the old orchard is pulled will eliminate the food source for pathogens. In addition, with some diseases the presence of nematodes worsens the disease problem; in these instances, rootstocks resistant to both the disease and nematodes should be considered.
Resistant rootstocks are not stand-alone disease prevention tools; however, in conjunction with good cultural practices, they are the first line of defense in managing replant diseases. Common replant disease pathogens and the most resistant rootstocks available are:
- Oak root fungus (Armillaria) – Marianna 2624
- Crown Gall – Nemaguard
- Bacterial Canker – Lovell, Viking
- Phytophthora – Marianna 2624, Krymsk 86
Soil samples should be taken before making any planting decisions. Soil should be tested for presence of disease pathogens, nematodes and soil characteristics. Sands and loams carry a higher risk for both replant disease and nematodes than fine-textured soils.
Fumigant choice and application method
Once armed with detailed information about the risks of replant diseases, plant parasitic nematodes, and more aggressive pathogens and pests, growers can evaluate options for fumigation. For replant disease complex, chloropicrin and chloropicrin mixtures are generally used, while methyl bromide and 1,3-D are preferred for nematodes.
In University of California trials supported by the Almond Board of California, spot fumigation treatments of chloropicrin using the irrigation system installed for the replant trees was effective in preventing replant disease. The drip emitters effectively treated the root zone of the future tree. Successful spot fumigations were also made using a GPS-controlled shank injection.
Spot fumigations have the added advantages of reducing VOC emissions, reducing fumigant costs, and reducing the size of required buffer zones.
Strip applications were also successful in UC trials, significantly reducing the amount of product applied per orchard acre. Researchers are also considering water seals, different tarp materials and soil amendments to minimize fumigant emissions.
Seasonal Guide to Environmentally Responsible Pest Management Practices in Almonds