The unusually wet winter and spring of 2023 has had unprecedented impacts on our local orchard systems. However, the cumulative impacts of a barrage of atmospheric rivers and persistent standing water in some flooded areas may not be realized for another year. Cold and rainy conditions during bloom limited pollinator activity and impeded orchard access for spray operations, creating the ideal conditions for the onset of plant pests and disease issues. The timing of bud and shoot development has likely also been impacted, as the buds for next year's crop are formed during the current year; therefore, the unusually wet and cold spring of 2023 may also influence the development of the 2024 crop.  

The bloom period for many Prunus species (almond, peach, cherry, plum, prune, etc.) was unusually protracted due to the high frequency of rain and cool temperatures. Saturated soil conditions limited orchard access, resulting in delayed shaking of mummy nuts in almond, an essential sanitation practice for management of navel orangeworm, a key insect pest (Figure 1A). With few bee flying hours available for pollination, and wet, cold conditions conducive to disease development (Figure 1 B,C and D), the crop outlook for 2023 remains uncertain.

Pistachio, walnut, and pecan, all wind-pollinated crops, are experiencing delayed bud break and shoot development. The progression of bud break appears to be at least 10 days behind the norm and the timing and synchrony of male and female flower maturity is yet unknown.

Orchards that have sustained flooding may be physiologically impacted by roots persistence in anoxic conditions and may also be exposed to waterborne plant pathogens. Over time saturated soil will become depleted of oxygen by the roots and microbial communities resulting in anoxic conditions that can lead to root mortality. The extent of damage to the roots may not be realized until the season progresses, and root damage may manifest with general canopy decline and anchorage issues. Many of the tree crops grown in California are grafted onto disease resistant rootstocks. If flood water rises above the graft union, the benefit of these rootstocks is largely negated. Many rootstocks have been selected over time for resistance to Phytophthora spp., a group of soilborne pathogens that create motile spores that swim at the water surface. These pathogens are common in surface water, and incite canker diseases, particularly when the water level persists above the graft union. Symptoms of infection include general canopy decline and cankers, many of which ooze or ‘bleed' (Figure 1C). The full suite of symptoms may not manifest until further into the growing season when the heat imposes stress on the trees.

The rainfall and flooding have additionally influenced the nutritional status of orchard soils. Cool,  wet soil conditions slow the chemical and biological reactions that control the availability of nutrients for tree uptake. Micronutrients, present in only trace concentrations in soil, become particularly limited as  the wet and cold create reductive conditions that promote lime-induced iron chlorosis, a common nutritional deficiency  in prunes, almonds, and citrus (Figure 1 E).  In sites that have undergone whole orchard recycling with incorporation of woody biomass into the soil, saturated, anaerobic conditions may moderate the microbial activity needed to stimulate wood chip decomposition and further restrict nutrient availability until the soil warms. 

As rivers breach their banks, rushing floodwater may transport the less-soluble nutrients, such as sulphate of potash (SOP) that is often applied in the autumn, away from the tree rootzone. As water moves through the soil profile, other nutrients, such as nitrogen, may be lost from the rootzone by leaching. The leaching will, however, ameliorate salt accumulation after years of drought, particularly in sites that have relied on saline groundwater for irrigation.

As the season progresses, growers should work closely with their PCAs, crop consultants, and UCCE farm advisors to observe and document orchard changes in the 2023 season. Photographs should be taken of anomalies, such as delayed bud break, so comparisons may be made to past and future years. Additionally, photos of putative disease and insect issues may easily be shared with representatives from private industry as well as researchers in the UC system for both diagnosis and discussion of best management strategies. A suite of photos and management options for the most common pest and diseases of agricultural crops can be found at the UC IPM website (www.ipm.ucdavis.edu).

The recent flooding on the Central Coast has really put a lot of local strawberry growers in a bad position.  Some fields have been underwater several days to a week or more, which has deprived plants of oxygen and sunlight needed for respiration and growth.   As these plants try to recover, they will likely be severely set back for several weeks.  Other fields that were briefly flooded are silted over and  have completely saturated soils.

Looking beyond our current conditions of just too much water, we need think about what the plan of action is going to be going forward into the spring.  Since in many areas strawberry plants have been set back because they were in saturated, anaerobic soil, growers will need to turn their attention to jump starting growth in their fields once the ground dries out, and becomes aerobic again. 

It will be important at this time to think about nutrition.  It's a fair bet that even fields that received pre-plant fertilizer lost a significant amount of nitrogen from the soil during the flooding and unprecedented rains during the last few months.  We could be wrong about this, so the first thing growers and farm managers should do once they are back out in the fields is take a soil sample and check the mineral N levels, especially the concentration of nitrate, which is the form of nitrogen that strawberry readily take up.   Nitrate-N will also mineralize from the soil organic matter and any organic amendments that were previously incorporated in the soil as the soil becomes oxygenated again.  Soil samples should be collected from 8 to 12 locations in the field from the 0 to 12-inch depth in or near the plant row and composited together.  A subsample of the composited soil should be analyzed for mineral forms of nitrogen (nitrate and ammonium).  We recommend using the soil nitrate quick test to assess the soil nitrate status of your fields in a timely manner. Please refer to this previous article on the how to accurately measure soil nitrate using the quick test.  If you intend to send the soil to a laboratory that can quickly analyze the soil, we suggest shipping the soil sample with blue ice so that it stays cold to prevent mineralization of N in transit.  The laboratory should analyze the sample for both ammonium and nitrate, the two mineral forms of nitrogen that are in the soil.

If the soil nitrate values are below 10 ppm nitrate-N  then the plants will likely benefit from an addition of N fertilizer.  Fertilizers containing nitrate forms of N such as CAN-17, UAN32, or ammonium nitrate would be good to add so that the strawberry plants can immediately take up nitrogen, which should help jump start growth.  The urea and ammonium contained in fertilizer will also mineralize to nitrate, but due to the recent anaerobic conditions and cold soil temperatures, this process may be slower than normal. 

Since this might be the first time that you've run your irrigation system in a while, it'll be good to check before fertigating to ensure that everything is in good order.  Check for broken connections between drip lines and the submains (layflat, oval hose).  Check that valves and pipes are still connected and unbroken and that the pump is functioning well.  These will be all good items to check before putting this system to work after such a long hiatus. 

Dear Colleagues,

Through Wednesday, California is expecting rounds of heavy rain, mountain snow, high winds, with potential for flooding, downed trees/debris and power outages. Please stay safe.

If you live near a river, levee or area prone to flooding, gather your essentials so you'll be ready to evacuate at a moment's notice. You'll have some peace of mind if you have gathered items in advance to be away from your home if evacuation orders are given by authorities. Also, prepare for power outages by having electronic devices charged, vehicles fueled, and the ability to prepare food and stay warm.

This is a good time to review your procedures for preparedness, such as the ability to stay informed and to be able to communicate. See Safety Note #203 for winter storm preparedness tips.

Click here to view the Cal-OES weather threat briefing.

A Safety Note series in the category of “Disaster/Emergency Information” is available at https://safety.ucanr.edu/Safety_Notes/- recommended are notes #166-169, #189 and #203.

For current weather alert information, Cal/OSHA recommends the NOAA Weather Alerts page at http://alerts.weather.gov/cap/ca.php?x=1.

David Alamillo
Environmental Health & Safety Specialist

Erin Dicaprio, Associate Professor of Cooperative Extension, Department of Food Science and Technology, University of California Davis has summarized information and resources for assessing crop food safety after a flooding event.  There are links to the Leafy Green Marketing Agreement Flood fact sheet and also a presentation made by Trevor Suslow, emeritus UC Cooperative Extension Specialist.  Please follow the link below.