Declining walnuts, what’s the deal?

Pic Bubbles.png — Kari Arnold, Orchard Systems Advisor, Stanislaus County

There are a number of reasons walnuts turn yellow and begin to decline. Sometimes the root system becomes root bound, due to improper planting or pockets of heavy soil. Other times walnuts can be infected with phytophthora, a fungus like organism that grows well in pro-longed periods of standing water and cooler temperatures. Irrigation can also play a role in yellowing walnut trees, not in the sense of under watering, but instead over watering. Walnuts are known to do best in fertile, deep, well-drained, loamy soils-really what doesn’t? Walnuts do not like “wet feet” and begin to yellow, bronze and decline when exposed to inadequately irrigated soils, which growers might be observing in their orchards now. The question stands, what is adequate?

A recent study performed by Allan Fulton (UC Cooperative Extension Farm Advisor, Tehama County), Ken Shackel (UC Professor, Department of Plant Sciences, Davis, CA) and Bruce Lampinen, (UC Integrated Orchard Management\Walnut Specialist, Davis, CA) show that over aggressive, early irrigation may saturate the soil, depriving roots of oxygen. This condition is easily observed during the summer to fall months, when trees yellow above ground and roots are blackened on the inside, sometimes showing a metallic blue hue on the outside, which can be seen in Figure 1. The crown may appear rotten when cutting into the vascular tissue and phytophthora is often isolated from the tree.

The rotten crown and roots then limit the transportation of water and nutrients thus leading to a yellowed, stunted appearance above ground. Fortunately, ongoing research shows a delay of irrigation in the spring regulated by the weekly repeated measurement of midday stem water potential (SWP) can benefit walnut orchards. The idea here is that irrigation is not applied in the spring until the trees begin to express a certain level of stress. This stress level is measured using a pressure chamber like that shown in Figure 2, is expressed in negative bars of pressure and referred to as stem water potential, or SWP.

So, what is SWP? To answer that question, a discussion of basic plant physiology is required. Water moves from the soil to the atmosphere through plants. Plants, or walnuts in this case, suck water from the soil, utilize some for photosynthesis (a process by which plant cells convert light energy into chemical energy) as well as nutrient transport and cellular turgidity, but eventually lose quite a bit to the atmosphere when stomates are open. Stomates are like very tiny doors on the underside of most leaves, that open to allow carbon dioxide in (for sugar production, via photosynthesis), and oxygen out (a by-product of photosynthesis). While those stomates are open, water also exits the plant leaf as water vapor, this is called evapotranspiration. This water vapor is being pulled from the leaf at a certain level of negative pressure, which is dependent upon the level of suction required for the roots to pull water out of the soil. This atmospheric pull changes based on the level of moisture in the soil because of the ease, or lack there-of, for roots of the plant to extract water from the soil. This atmospheric pull, or tension, is reflected as stress on the plant, and can be measured in negative bars of pressure, using a pressure chamber. Because the measurement reflects the atmospheric pull’s potential, the term stem water potential, or SWP, provides a proper description.

So, how is SWP measured? SWP is measured during hottest, driest periods in the day when the tree is under the most stress, between the hours of 1:00 pm and 3:00 pm. Leaves are wrapped inside mylar bags

which are typically provided with the pressure chamber, and available in multiple sizes, although the smallest bags tend to fit the chamber best. The terminal leaflet of a walnut is bagged and allowed to hang enclosed for at least fifteen minutes. During this time, the tension of the water in the leaf becomes equal to the tension of the water in the tree. For most trees the best leaves to measure are on branches closest to the trunk in the lower canopy, Figure 3. The idea here is to reduce the distance from root to shoot so that the value properly represents the tree. The further away from the roots, the more variable the value becomes. The stem of the bagged leaf is then inserted into the top piece, tightened down, and the stem is cut flush with the chamber top with a sharp razor blade (be careful!). The top is tightened down by either a clockwise motion for screw top models, or sturdy push pins provided in box chamber models, and pressure is applied either by pump action, or nitrogen gas canisters. Always read all safety precautions and perform safe practices when operating this equipment. Pressure is applied until a small amount of fluid is excreted from the cut stem. The value shown on the dial is recorded, and compared TO BASELINE, which is the most important, and often easily misunderstood, part.

So, what is baseline? Baseline is a value that generally represents a fully irrigated tree. This value varies depending on the crop, relative humidity and temperature. Please see the table provided for walnuts or visit http://informatics.plantsciences.ucdavis.edu/Brooke_Jacobs/index.php for date specific baseline values calculated using CIMIS weather stations.

So, what does this all mean? Typically, over-irrigated trees will show symptoms in mid-summer, when the tree is experiencing high levels of stress not due to a lack of water, but instead due to over irrigation resulting in rotting roots and crowns. Although irrigation can still be mitigated at this time, quite a bit of damage has been done. To try to avoid this damage, a delay in irrigation in the springtime is expected to improve tree health.

Further studies by Allen Fulton, Ken Shackel, and Bruce Lampinen have observed recent improvement in tree health related to a delay of irrigation in the spring. Initial field trials compared control trees to delay trees. The control treatment consisted of irrigation initiation based on a grower standard typically related to soil moisture meters and/or leaf out. The delay treatment in turn utilized SWP measurements of approximately 2 to 3 bars below baseline to initiate irrigation, which didn’t trigger until the first week of June (site dependent). Yield measurements between treatments showed no significant differences, yet trees in the delay treatments appeared to improve in general overall health over time. Field trials have been extended to a few other counties, including Stanislaus, to incorporate variation across walnut growing regions in CA. Research is ongoing, and currently funded by the California Walnut Board.

So, how much do I water? Weekly updates of crop water use are available. This information is provided by UC Cooperative Extension (UCCE) and the Department of Water Regulations (DWR). Evapotranspiration is recorded for a limited number of different crops at multiple sites throughout California and appropriated in finer detail to specific locations using CIMIS weather stations. These tables can then be used as guides to know how much water to apply. SWP can be used to fine tune how often irrigation is applied and how much by measuring tree stress before and after irrigations. For more information on any of these topics, please contact your local UCCE Farm Advisor. Additionally, for a video demonstration on how to use a pressure chamber, please follow this link:

https://www.youtube.com/watch?v=M6ZkmB1ld60.