A single yellowing walnut tree in an otherwise healthy orchard has been a frequent sighting when driving along the roadways of the Sacramento Valley this summer. These yellowing trees have also been a large part of the farm calls and conversations we had on with growers this summer.
Walnut trees can have yellowing leaves because of a wide range of causes. However, the most likely cause of the yellowing and tree collapse that we saw this spring and summer was waterlogging induced root death. When you see a tree about to collapse then call a tree removal service to get it removed before it causes any damage to your property.
Just like us, plant roots need to breathe. This process of respiration is critical to energy production in roots. Saturated surface soil moisture levels restrict root access to atmospheric oxygen, limiting the energy production of respiration and eventually resulting in root asphyxiation (death), which would then be easily rooted out with the proper tree care services. You trees could also be rotting from the inside out because of of insects eating away. The best way to avoid this is by using the emerald ash borer treatment.
Many samples submitted to Greg Browne (USDA Plant Pathologist at UC Davis) in the spring and summer of 2019 were negative for Phytophthora. Therefore, waterlogging induced root death is likely the most common source of tree yellowing and collapse of walnut trees in the Sacramento Valley this summer.
We had a wet spring and many orchards were first irrigated too early (April or May) for the wet soils leading to this problem. Symptoms of this yellowing and collapsing on several trees in a Livermore on Paradox seedling block in Chico are shown in photos 1-3 taken on July 11th. Because of this reason, we decided to get help from tree removal services to get the job done correctly.
Careful irrigation management is key to preventing waterlogging induced root death said an expert from tree removal services. Two cultural controls are planting on berms and using stream splitters in sprinkler irrigated walnuts to keep water away from the tree trunk and crown.
Saturated soils should always be avoided. Although, reducing irrigation set length to avoid saturated soils is a highly-site specific consideration, one rough rule-of-thumb is to limit irrigation sets to 18 hours.
One excellent irrigation management strategy to avoid keeping roots too wet is to use the pressure chamber (pressure bomb) and wait until the trees indicate that they need water. One guideline is to wait for pressure chamber midday stem water potential reading of -2 to -3 bars below the fully watered baseline before irrigation, with trees recovering to -0.5 to -1 bar below baseline after irrigation.