California Walnuts: Flooded Orchards – What We’ve Learned From Past Floods

    In 1986 flood, some walnuts were flooded for 45 days. Photos by Janine Hasey

    Experiences with the 1986 and 1997 levee breaks in Yuba and Sutter Counties and other “high-water” events over the past thirty years afford us some expectations for the types and extents of tree damage that may result from orchard flooding in 2017.

    This article will first discuss some generalities of flooding damage and two key types of damage that tend to follow prolonged orchard flooding, i.e., waterlogging and Phytophthora diseases.

    Next, the article reflects on the floods in 1986 and 1997 and considers ways in which specific conditions associated with those floods seemed to affect orchard outcomes. Finally, we offer management steps to consider in dealing with orchard flooding in 2017.

    Flood Damage Generalities

    Tree damage from flooded soil is usually minimal if the flood occurs when the trees are dormant and the water continues to flow rather than remains standing in the orchard area.

    However, where water stands for extended periods in an orchard, two potential problems are of concern:

    • Waterlogging of the root system.
    • Diseases caused by Phytophthora.


    Waterlogging can be viewed as an “oxygen starvation” effect of flooding. When an orchard soil is flooded or otherwise saturated with water, the air in its pore spaces is displaced, removing the supply of oxygen to the roots. The terminal portions of new walnut roots can be killed within one to four days in a saturated soil.

    Click image to enlarge.

    As saturation is prolonged, damage to roots becomes greater and can lead to chlorosis, leaf wilting, and in extreme cases, entire root systems can be killed. As soil temperatures increase, the negative effects of waterlogging are accelerated. Walnut trees apparently survive waterlogging events during winter due to a combination of low soil temperature and a lack of active roots.

    Waterlogging risk tends to be greater in “heavy” or compacted soils because of slower reentry of oxygen after saturation. Following floods in previous years, we observed less injury to trees growing in deep, permeable soils than to trees growing on soils with impaired vertical drainage caused by a claypan or hardpan.

    Waterlogging damage to walnut trees can vary in severity, may not be immediately manifest, and can be difficult to discriminate from Phytophthora damage. For example, decline of walnut trees is sometimes not obvious until late spring or summer, when backhoe excavations can be used to confirm suspected waterlogging damage on walnut root systems. If you need to excavate a large portion of your land, you may have to rent a Commercial Dump Truck to collect the soil you’ve dug up.

    Boundaries between dead, waterlogged portions of major roots and crowns and living parts of the same roots tend to be poorly defined and occur in the absence of distinct cankers and lab diagnostics that indicate Phytophthora crown and root rot. Waterlogged tissues often have swollen lenticels (Photo 1).

    Phytophthora diseases.

    Several species of the “water mold” Phytophthora are found in surface water sources such as our rivers and canals, as well as in infested soils. These fungal-like organisms can be carried along by the water and swim in it. After a levee break, the river water is a potential source of spreading Phytophthora inoculum throughout the flooded area.

    Flooding can bring active Phytophthora inoculum in contact with the aerial portions of trees, resulting in infections of the tree trunk, scaffolds and shoots. Flooding also increases the risk of root and crown infections by Phytophthora.

    Some Phytophthora species preferentially infect roots, but many preferentially infect the root, crown, or aerial tree parts.

    When Phytophthora infects woody tissues of trees above or belowground, it typically causes cankers (i.e., continuous, often-elliptical areas of dead bark, ranging from less than an inch to more than a foot across). The cankers often “bleed”. Removing the outer bark from the margin of a canker typically reveals a distinct, zonate (with concentric lines) margin, which is in contrast to the typically diffuse, non-zonate margin and vascular streaking evident at the edges of dead areas of waterlogged roots.

    In general, cool to moderate temperatures and water-saturated conditions in or on soil favor tree infection by Phytophthora species. Some Phytophthora species are more virulent than others, and risk of disease also depends on genetic resistance of the rootstock or scion. Within a given rootstock or scion, susceptibility to Phytophthora can vary with time of year.

    For example, in experiments on seasonal variation in susceptibility of walnut, almond, and peach rootstocks to Phytophthora citricola (which can cause root and crown rot and trunk and branch cankers), the rootstocks were least susceptible to infection during wintertime and most susceptible during the growing season. In contrast, infection by Phytophthora syringae in almond and peach is most likely during the cool conditions of fall, winter, and early spring. These cankers cease development when summer temperatures become lethal to this “cool-temperature” pathogen.

    Reflections on 1986 and 1997 flooding


    A subtropical storm brought heavy precipitation to northern CA and moderately high snow levels lasting for nearly 10 days, from February 11-20. Rainfall in the northern Sierra Nevada at 5,750 feet registered almost 56 inches. The water roared down the Yuba River where the levee broke February 20 in the evening flooding Linda and Olivehurst and acres of orchards.

    Walnut and prune orchards were flooded for 45 days from late February through mid-April.


    A polar system left several feet of snow in the Sierra on December 21-22, 1996. From December 26, 1996 through January 3, 1997, the weather pattern shifted to warmer and wetter storms of tropical origin that brought relentless precipitation, excessive runoff, and significantly melted the snow pack leading to widespread flooding. Oroville dam was spilling 160,000 cfs by January 1 and water was again roaring down the Yuba River. Late on January 2, a levee broke on the Feather River at Country Club Road in Yuba County flooding thousands of acres of orchards. Three weeks later, another levee gave way on the Bear River, causing more flooding in some of the same areas. In Sutter County, the west side of the Sutter Bypass levee broke in early January flooding the Meridian basin.

    Based on what we learned from orchard responses to the widespread flooding in 1986 and 1997 and less-extensive flooding of river bottoms in several additional years, we have some expectations of the problems growers will face in orchards flooded this year. A major difference between the 1997 flood and that of 1986 was that the flood occurred in later February in 1986, when many tree crops were becoming active.

    In 1997 by March, we observed waterlogging damage on collapsing peaches on heavier soil that had only been flooded a short time in January. Where trees are actively growing and then flooded, as in the river bottoms during our wet springs such as in 1995, most of the trees that died were in low areas where the water sat for prolonged periods. Those trees that were only in the fast-moving, cold water did fine, except for the submerged shoots.

    After the 1986 levee break, water sat for several weeks in certain low lying areas. In these areas, as the water receded, symptoms of aerial infection by Phytophthora were apparent in some orchards.

    Aerial infections by Phytophthora were indicated by bleeding cankers in aboveground parts of trees, i.e., on trunks, scaffolds, branches, limbs, and hangers, as opposed to crown and root infections, which originate in soil. The aerial Phytophthora cankers on walnuts and prunes in 1986 tended to occur in orchards that had been flooded for about 45 days, from late February through mid-April (Photos 2A &2B).

    After the 1997 levee breaks, we sampled flood waters in several orchards in the Arboga area (Yuba Co.) and Meridian (Sutter Co.) in early February. Using pear fruit to “bait” the Phytophthora, we found that every orchard location had species of the pathogen present.

    By mid-February 1997, we observed the first symptoms (amber colored gum) of aerial infections by Phytophthora on peaches, and soon afterwards the symptoms were seen on prunes. Cutting into the tissue below the gumming revealed reddish brown cankers. Many of these cankers, more on peach than on prune, continued to look active. With peaches in the flood zones and the river bottoms, there were multiple aerial Phytophthora infections on every tree in every orchard we surveyed, regardless of orchard age.

    Many of these orchards were flooded less than a week. By March 1997, we still had not seen aerial Phytophthora in flooded walnuts (bleeding in limbs and branches), suggesting that dormancy had offered them some protection from Phytophthora.

    2017 flooding and management considerations

    Photos 2A and @B.
    Click To Enlarge.

    Flood overview. From a series of tropical storms, we have experienced record-breaking rainfall events in January and February which have resulted in very high river flows for many weeks.

    At this writing, orchards outside the levees along the rivers either are or have been flooded from river seepage due to high flows and/or overflowing ditches/canals. With the record-breaking snow pack, there is uncertainty as to how long the rivers will have high flows or if we have a flood event during springtime. It likely will take months before the extent of losses from waterlogging and/or disease can be assessed.

    As the trees become active and the weather warms up, we can expect to start seeing losses that could continue through the hot summer when trees with damaged root systems or crowns typically collapse. From recent research, substantial root growth in walnuts begins about a month after leaf-out and peaks in the summer. This should work in our favor for orchards that are flooded or saturated this spring.

    Management guidelines to consider

    • Drain or pump standing water out of orchards.
    • Reduce vehicle/farm equipment traffic. Wet soils are easily compacted. Delay all operations that can wait until soil is dry enough to crumble at a depth of five to six inches rather than slick over or pack.
    • Apply sprays by air. The Department of Pesticide Regulation issued the emergency application of several fungicides to orchards with standing water only including Butte, Colusa, Glenn, Sutter, Tehama, Yolo, and Yuba Counties This will be in effect until June 1, 2017. See your local Ag Commissioner for more information on allowable fungicides.
    • Apply a phosphonate spray in May where Phytophthora is found or suspected. A summer and early fall application may also be needed.
    • Ridomil application is another option but is considered by some to be less cost effective than phosphonates for some Phytophthora diseases.
    • Remove deposited silt and debris around the root crowns when possible to decrease the chances of root/crown decay.
    • Plants are effective in drying waterlogged soils. Encourage the growth of cover crops or even weeds that will help dry the soil after flooding.
    • Fill in eroded areas in orchards if soil is available. Deposited materials in many instances are beneficial.
    • Check for salts (chloride and sodium); a continued high water table saturating surface soil may result in these salts accumulating or alkalization of certain spots which have shown these troubles before. Neither leaching nor gypsum treatments will be effective until the water table is lowered and good drainage can be achieved.
    • For new orchards or replants where seepage is problematic, consider using clonal Paradox RX1; it has high resistance to Phytophthora, but more observations are needed to determine how it performs under prolonged waterlogging. 

    Additional resources

    For additional resources, see Hasey’s blog here.  The blog covers several flooding-related issues not covered here, including guidance on documenting and reporting damage and losses, and locating possible disaster relief resources for flooded orchards outside and inside the levees, including riverbank sloughing, such as occurred extensively along the Feather River.

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