Janine Hasey, UCCE Farm Advisor, Sutter/Yuba/Colusa Counties, and
Greg Browne, USDA Plant Pathologist, UC Davis

This is the third in a three part series looking at past experiences of flooded orchards. Part 1 discussed generalities of flooding damage, and Part 2 reflected on past flooding events. Finally, we offer management steps to consider in dealing with orchard flooding in 2017.

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.

• 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: Information on possible disaster relief resources for flooded orchards.