California Ag Network

Tag: UCCE Farm Advisor

  • Roller Crimping – A Cover Crop Termination Option

    With the end of harvest comes the beginning of winter prep work. One practice that provides a bevy of positive impacts for orchards is planting a winter cover crop. Adding organic matter to the soil via cover crops can improve water infiltration and water holding capacity of the soil. Fixing nitrogen and breaking up compacted soil can also be achieved by selecting appropriate species for your cover crop mix.

    While the benefits of winter cover cropping in orchards are well-known, the particulars of when and how to terminate the cover crop can be overwhelming. Using herbicides to terminate a crop can be expensive, and achieving good coverage can be difficult with large amounts of biomass. Mowing or tilling can disrupt the soil, losing some of the accumulated carbon. Annie Edwards and Margaret Smither-Kopperl with the USDA recently released a report on termination using a roller crimper in orchard systems. Their findings show that, while this can be a great tool to maximize the benefits of cover cropping, roller crimping works best with certain cover crop species and growth stages.

    Figure 1. Roller crimper implement. Photo credit: Tracy Robillard, USDA NRCS.

    Roller crimping is a common practice in other parts of the country but had not yet been studied in California orchard systems. A roller crimper is a drum-shaped implement with blunt curved blades (Figure 1.) that rolls over and crushes down the vegetation without disturbing the roots. This creates a mat of biomass over the soil’s surface, similar to mulch. This has been shown to reduce soil temperatures, conserve soil moisture, decrease erosion, and reduce herbicide use. The timing of roller crimping is critical; too early and you won’t have enough biomass produced to adequately cover the soil and provide the desired benefits. Too late and the cover crop may have produced seed that can germinate in the current season, depending on irrigation practices and precipitation.

    Research completed at the USDA’s Lockeford Plant Materials Center compared four different cover crop seed mixes to determine how well the roller crimper terminated each. Additionally, each of the four mixes was crimped at 6 different times to evaluate which growth stage was most successful for termination. Data on “bounce back” (when plants stood back up and continued to grow) as well as regrowth, biomass, vegetative stage, and cover crop canopy height were collected.

    • Results showed that the brassica mix “bounced back” and regrew when roller crimped too early. Termination with the roller crimper was most effective when these species were at least a week into flowering, with termination remaining effective through “seed maturing” stage.This termination timing allowed for good biomass accumulation,  and the study notes that the brassica mix formed a vegetative mat when crimped, which is the ultimate goal of this termination technique.
    • The fava beans alone remained crimped after the plants had visible early pods reaching about 3/16”. The fava beans and the brassica mix were the only treatments that were completely successfully terminated with the crimper.
    • The “annual plow down” mix, consisting of oats, vetch, peas and fava beans, showed best results when crimped at early visible pods in the fava. This mix also had the largest biomass accumulation over the season. However, the oats in the mix had a high rate of “bounce back”. Crimping was not an ideal method of termination for this mix.
    • The triticale crop continued to regrow when crimped too early. Crimping was most successful for this species after the plants started to flower. However, even then, the triticale “bounced back” and was never fully successfully crimped.

    Ultimately, roller crimping is most effective for termination of cover crop mixes without grass species. Crimping brassica mixes and fava beans produces a vegetative mat that can protect soil and maximize the benefits of cover cropping. It is key to use this technique at the appropriate vegetative growth stage for the species in your mix, as crimping too early can necessitate repeating the practice.

    The entire USDA report, including seeding densities, seed sources, and photos can be found here. — By Becky Wheeler-Dykes, UCCE Farm Advisor, Glenn, Tehama and Colusa Counties

  • Prepare to Prevent an Autumn Freeze from Damaging Walnut Trees

    Autumn freeze damage in walnuts can occur when trees experience freezing temperatures prior to going into dormancy. In recent history, Sacramento Valley growers experienced this most severely in 2018, and since then, many have implemented management practices to prevent widespread damage.

    Symptoms of freeze damage can include:

    • Late leaf-out in affected limbs
    • Weak or dead terminal budwood and smaller upper limbs (mild/moderate damage)
    • Major limb death, mainly in the upper and exposed portions of the tree, and/or apparent whole tree death (severe damage)
    • Darkened or orange-tinted wood, with gray/black streaking in the cambium underneath
    • Sunburn on damaged limbs, usually on south- and west-facing sides

    This past spring (2024), growers in Sutter, Yuba, Yolo, and Tehama counties reported symptoms of freeze damage, likely from an early autumn freeze that occurred near Halloween. If you noticed any of these symptoms in your orchards this past spring or are aware of cold pockets in certain areas/fields, consider the following steps to prevent another year of freeze damage.

    Preventative management:

    Increase soil heat storage:

    1. For young trees, it’s best to cut off N applications by mid-August to prevent tender new growth that is most vulnerable to freeze damage. Also, withhold irrigation starting in early to mid-September to set a terminal bud on the trunk (Figure 2). After the terminal bud has set, resume irrigation to avoid tree stress and defoliation, without the fear of pushing tender new growth.

    2. For mature trees, cut off nitrogen (N) applications by the beginning of September. Terminal buds may set as a positive side effect of the water cutoff done ahead of harvest to avoid shaker injury.

    Increase soil heat storage:

    3. Maintain short groundcover during autumn frost season, beginning mid-October. Keeping groundcovers cut to 2 inches or less during frost season allows sunlight to reach the soil surface, increasing soil heat storage for a warmer orchard through the night. Recently cultivated soil has many air spaces, low heat storage capacity, and low heat conductivity resulting in colder minimum temperatures. The ground surface must be moist for bare ground to be the warmest (see steps 4 and 5).

    4. Trees that remain water-stressed after the heat of summer are the most vulnerable to autumn and winter freeze damage. If there has not been adequate rainfall by mid-October for young orchards, or immediately after harvest for mature orchards, begin irrigating to refill the soil profile.

    5. Water-filled spaces in the soil conduct and store more heat than empty air spaces. Once the soil profile has been refilled, the top several inches of soil are the most important for freeze mitigation and should be kept at field capacity (not saturated/ponding) if possible. Either a dry surface crust on one extreme or a frozen sheet of ponded water on the other extreme will both hinder the re-radiation of stored daytime heat during the night.

    6. Continue to actively monitor soil moisture and freeze predictions in November and December (until trees are acclimated to frost – see note below). Since freeze events are variable depending on location, monitoring each block remotely with a freeze alarm makes good sense. If a freeze is predicted and the soil is dry, irrigate to wet the soil 2 to 3 days before a freeze event to fill the air spaces so the soil will store more heat. Light irrigation to moisten a dry soil surface the morning before a freeze will help obtain the greatest heat storage for re-radiation at night (as long as there is no standing water during the freeze event).

    Active irrigation during freeze event:

    7. Some growers with the ability to actively irrigate during sudden autumn freeze events have reported success in preventing damage. With active irrigation, heat is given off as water converts from liquid to ice. This occurs during the coldest hours of the early morning of the freeze event and is the reason higher flow rates are needed to more successfully manage frost events.

      • We know from work in almonds that active frost protection with solid set irrigation can achieve as much as 4 degrees protection if your system can run 40 gpm/ac.
      • With micro-sprinkler irrigation, 1-2 degrees of warming can be achieved with at least 30 gpm/ac. 25 gpm/ac has been shown to be enough to keep sprinkler heads from freezing.

    Painting:

    8. In addition to water management, painting young trunks and shoots white can minimize damage if applied as soon as possible after a freeze. Research by UC Walnut Specialist Bruce Lampinen has shown painting after leaf fall with white interior latex paint diluted 50% with water minimizes damage to shoots and buds, especially on the south-west side of the tree. The paint moderates large day-to-night temperature fluctuations after sunny winter days.

    Treatment:

    If you suspect freeze damage occurred, cut into the branches shortly after a freeze event and check the tissue for drying or browning. Swift action in the week after a freeze event can significantly decrease damage. If you don’t paint early, painting trees after a freeze can still help decrease severe damage. Wilbur Reil, Farm Advisor Emeritus, found that when trees were painted a week after a freeze event 18% showed damage, compared with 46% damaged in the unpainted trees. Painting any parts that may be damaged should improve recovery and protect against winter sunburn of affected tissue.

    New growth typically occurs below the damaged/dead locations. Dead wood should be pruned out later in the season (mid-summer), after the tree has fully leafed out, to reduce potential colonization sites by Bot or Phomopsis. In young trees with significant freeze damage, remaining limbs or new limbs may have to be trained as new scaffolds.

    Potential causes:

    In “normal” years, the combination of short days, gradually dropping fall temperatures, and rainfall pattern allows trees to go into dormancy in a stepwise pattern (2021 and 2022 in Figure 3 below). In years where we’ve received more reports of autumn freeze damage, there have been large differences in daily high and low temperature (2020), and sudden drops to freezing temperatures prior to trees reaching full dormancy (2020 and 2023).

    As temperatures drop into the low 30s °F and days shorten, carbohydrates move from leaves to wood and starch converts to soluble sugars. Slow cooling is needed to promote starch degradation into sugars, which act as “antifreeze” to protect cells from freezing, as does adequate soil moisture, which keeps cells hydrated. When temperatures suddenly and prematurely drop below 32°F before the acclimation process is completed, carbohydrate movement is likely disrupted and trees are susceptible to freeze damage.

    Unfortunately, we do not know how many frost/mild freeze events are required to sufficiently acclimate trees in autumn. So far, growers that were previously hit hard by fall freeze events have reported success in avoiding subsequent freeze damage by having frost alarms ready and turned on by October 15, completely rehydrating trees after harvest, and actively irrigating during freeze events. Eventually, once mature healthy trees are fully dormant, they can tolerate temperatures to the low 20s °F or below. — By Clarissa Reyes, Orchards Advisor, Sutter-Yuba, Butte & Placer Counties, and Luke Milliron, UCCE Farm Advisor Butte, Glenn & Tehama Counties

  • Minimize Potential Sudden Autumn Freeze Damage in Walnut

    Damage from a sudden autumn freeze can occur when trees experience freezing temperatures prior to going into dormancy. With sudden autumn freeze damage occurring in three of the last five years, preparing for these freeze events needs to be a regular part of every walnut grower’s summer and fall orchard operations. The best approach to escape damage from a fall freeze is to have frost alarms ready and turned on by October 15, completely rehydrate trees after harvest, and actively irrigate during frost events.

    Left: Withhold irrigation until a terminal vegetative bud sets on the trunk. Right: After a severe freeze event cut into the southwest-facing trunk, looking for dark brown discoloration of the cambium. (Photos: Janine Hasey)

    Steps to prepare:

      1. Tender new growth is most vulnerable to freeze damage: prevent trees from pushing late-season growth by cutting off N applications by mid-August (young trees) or early September (mature trees).
      2. For young trees, withhold irrigation starting in early to mid-September, waiting to resume irrigation until after a terminal bud (left photo) is set on the trunk. After the terminal bud has set, resume irrigation to avoid tree stress and defoliation. For bearing trees, terminal buds usually set as a side effect of the water cutoff done ahead of harvest.
      3. Keep groundcovers cut to 2 inches or less starting in mid-October. This allows sunlight to reach the soil surface, storing heat for a warmer orchard through the night.
      4. Rehydrate trees immediately after harvest, and actively monitor soil moisture and freeze predictions (usually from mid-October through December) until trees are acclimated to frost (see note below). Trees with adequate soil moisture are better able to withstand low temperatures without damage than trees in dry soil. This is because water-filled spaces in the soil conduct and store more heat than empty airspaces. If a freeze is predicted and the soil is dry, it should ideally be wetted 2 to 3 days before a freeze event. Light irrigation to moisten a dry soil surface the morning before a frost will help obtain the greatest heat storage for re-radiation at night (if there is no ponding going into the freeze event).
      5. Some growers with the ability to actively irrigate during sudden autumn freeze events have reported great success in preventing damage. We know from work in almonds that active frost protection can achieve as much as 4 degrees of warming with solid set, 1-2 degrees with micro-sprinkler, and maybe even some benefit running a drip system.

    When do you no longer have to worry about preparing for the next freeze?

    Fully dormant mature healthy trees can tolerate temperatures to the low 20’s (°F) or below. We believe walnuts acclimate by having the first mild frost events in autumn with lows near or just below 32° F. However, we do not know how many mild freezes are required to acclimate trees in autumn. If the soil is dry ahead of the third, fourth, and maybe additional freeze events – it’s better to irrigate and be safe if you can do so. In addition, consider keeping up your freeze response program longer into fall or early winter for younger orchards with lots of current season’s growth.

    If you suspect freeze damage occurred, cut into the branches shortly after the freeze event and check the tissue for drying or browning (right photo). Sunburn after freeze can further damage tissue on the southwest side of the tree. Paint the southwest side of damaged trees with 50% diluted (1:1 water to paint) white interior latex paint. Painting up to a week after the freeze event can reduce additional damage by half or more. You can learn more about freeze recovery in our article 2020 Walnut Freeze: Road to Recovery.

    — By Luke Milliron, UCCE Farm Advisor Butte, Glenn, and Tehama Counties; Jaime Ott, Orchard Systems Advisor, Tehama, Shasta, Glenn, and Butte Counties; Janine Hasey, UCCE Farm Advisor Emerita; and Joe Connell, UCCE Farm Advisor Emeritus

  • Considerations for Almond Varietal Selection

    Planting a new almond orchard? Watch this brief interview with UCCE Stanislaus County Director & Farm Advisor Roger Duncan as he shares some considerations for varietal selection based on his recent presentation at the Tree & Vine Expo. Read more about in Pacific Nut Producer Magazine.
    Please thank this video’s sponsor Trece for their industry support.

  • Preparing Your Orchards for 2022 in the Face of Drought

    Plan for the worst, hope for the best. That’s a tough, solid, strategy as the 2021 season winds down and almond growers and PCAs look to 2022. The following are some considerations when following this strategy. Every operation is different, and growers must decide what works best for their business. Final decisions may not need to be made until early 2022, but planning ahead, given the stakes, is recommended.

    The core issue is water, with both availability and quality of concern depending on local conditions.

    The region and state start the water year (Oct-Sept) way behind on water. As of the middle of September, major reservoirs (Shasta and Oroville) serving the region and state are at 22-25% capacity, less than half of the normal storage for this time of year. The current forecast for the rest of 2021 is for equal chances for normal precipitation in the Sacramento Valley with a 70-80% chance of La Niña winter. 2020-21 was a La Niña winter. These are all predictions, not certainties, but the current precipitation outlook for the winter ‘21-‘22 is not great.

    If the drought continues, more groundwater will be pumped to keep trees alive and, if enough water is available, productive. Using moderate to low quality water (see table) can risk decreasing yield from increasing rootzone salinity and/or toxic levels of the elements chloride, boron or sodium. For most of the Sacramento Valley, groundwater quality is good to very good. However, for parts of the Colusa and Sutter groundwater basins, water quality is not so good. Irrigation water quality levels are important to planning for next year, especially if similar quality groundwater was used in 2021.

    Thresholds for 3 important irrigation water quality components based on risk to almond growth or yield reduction.

    *For a more extensive information on water quality for almond irrigation see:

    https://www.sacvalleyorchards.com/almonds/irrigation/lower-quality-water/ 

    With a worst case scenario of low/no surface water deliveries and falling well water levels, here are some thoughts to consider in planning for 2022.

    Rank orchards by potential value (net return to grower) in 2022 and future years. Possible considerations and rankings for use in farming decisions are suggested in the following table. These groupings are just examples based on UC research and the author’s experience. Orchard rankings and farming decisions should be based on local conditions and grower experience with input from PCA/CCA and nut handler.

    Orchard conditions possibly influencing net grower returns (NGR) in a drought year.

    Fall to prebloom practices can influence production potential for 2022 and could be adjusted on a per orchard basis. For example, higher cost items such as winter irrigation/salt management, potassium fertilization, preemergent herbicide and dormant sprays could be prioritized to the orchards with higher net return potential. [Orchard sanitation is also a big cost, but lack of sanitation in one orchard can mean that increased NOW, there, can spread harm and reduced net return in adjacent orchards.] Lower yielding orchards could receive less inputs this fall and/or spring, further limiting potential net income next year, depending on what is cut out or limited. Limiting inputs to orchards considered for removal could be further savings to growers.

    Hopefully, adequate rain and snow will mean that these hard choices to remove or limit orchard yield will not need to be made. In the meantime, planning ahead will make springtime decision making simpler if the weather stays dry. — By Franz Niederholzer, UCCE Farm Advisor, Colusa and Sutter/Yuba Counties

  • Farming Tips for Enduring Historic Drought in Northern CA

    It has been heart breaking to seeing wheat and alfalfa fields dry up the last few weeks.  The recent hot temperatures caused many fields to wither, while placing greater irrigation needs on the few crops being irrigated with well water.   It’s the driest year I have experienced in my 10 years in the Klamath Basin!

    Several people asked me about strategies moving forward this summer and what yields they can expect from crops with limited irrigation.  I do not know all the answers, but I tried to highlight some of my past experiences from research and farming below.  Please keep in mind every field situation is unique and one strategy rarely works in all situations.  I want to commend the farming community for being resilient in a terrible situation.  I also want to thank all those that have spent countless hours trying to improve the water situation today and in the future.

    Irrigation tips

    Almost all fields started this year with very little residual soil moisture.  At IREC, not a drop of water flowed through our tile drains this winter and spring.  Darrin and I noticed the first irrigation on many fields did not even refill the soil profile especially when irrigating during strong winds.  This made 2021 even more difficult as winter precipitation did not refill the soil profile naturally.  As a result, many alfalfa and grain fields are starting to show unusual patterns from drought such as wind strips and tall growth under sprinkler drains.  If you can water fields with well water, it is very important to check the sprinkler heads to make sure they are the proper size, check water pressure on the ends of irrigation lines, and make sure your irrigation is providing good coverage.  Offset your wheel-lines one roll the next irrigation in fields with wind strips.    As many of us are irrigating out of drains, it is important to make sure you have adequate sprinkler pressure.  If you do not have adequate pressure, you need to reduce the size of your sprinkler heads and extend the duration of the irrigation set (16 hrs vs 12 hrs) or run less lines.

    Many alfalfa producers are in the position to harvest 1st cutting and cannot water for the next couple weeks until the hay is picked up.  In this situation, producers should consolidate limited water on their most productive fields.  Once an alfalfa field wilts from drought it will typically go dormant.  This is good as the alfalfa will likely survive until next year, but it also means the alfalfa is resistant to green-up and grow again in mid-summer.  As such, I’d recommend using limited water to irrigate healthy fields instead of trying to stretch it across all fields especially those fields that are already wilted with poor growth going into first cutting.

    Potato producers will need to apply long sets to refill the soil profile when the potatoes emerge.  Most field have very little residual soil moisture, and it is important to avoid drought stress during vegetation growth and tuber initiation to maximize tuber set and tuber quality.  I advise potato growers to check soil moisture frequently throughout the potato rooting zone to make sure water reached the bottom of the hill and is uniform throughout the field.

    Most farmers should be done irrigating winter grain, and I see a few producers irrigating spring grain.   If you decide to irrigate spring grain, I recommend irrigating the crop fully until you run out of your water allocation instead of trying to space irrigation throughout the summer.  Irrigating spring grain fully to meet the crop water demand during tillering and elongation will maximize forage yields, and once wheat and barley fields show signs of drought stress, they will start to head producing low forage and grain yields.

    Fertilization tips

    Producers that decide to irrigate grain and grass fields should make sure to apply adequate nitrogen.  The cost of the fertilizer will easily pay for itself with high forage prices and nitrogen can double grass forage yields compared to leaving fields unfertilized.  Granular nitrogen fertilizer applied as a topdress (over the top of the crop) should be watered into the soil with at least an inch of water within a few days of application to prevent volatility losses.  Don’t fertilize fields if you do not have adequate water to irrigate crop!

    Pest Management tips

    2021 is shaping up to be a very bad pest year.  We have already witnessed problems with blue aphid and weevils in alfalfa, and I am starting to see armyworms in grain and alfalfa.  We had a very high population of seedcorn maggot in onions as all the maggot flies seemed to congregate in the few irrigated fields throughout the basin.  Thrip populations are also very high.  Onion producers should start scouting for thrips earlier than normal and be ready to start insecticide applications if thrip numbers build on young plants.  Potato producers should expect higher than normal pest populations.  Aphid populations are high, and many insects will be moving into potato fields after alfalfa and grain harvest.  Mites may be more problematic than normal as fields are receiving less overhead irrigation and we have a lot of dust in the air.  I hope the dry weather will help reduce foliar crop diseases, but the hot temperatures and stagnant water sources may make my prediction incorrect.   Potato and onion farmers need to apply frequent irrigation to keep up with crop water use, so make sure to scout for foliar diseases and try to avoid keeping leaf surfaces wet for extended periods. — By Rob Wilson, UCCE Farm Advisor, ANR Intermountain Research & Extension Center

  • Hive Strength and Bee Health/Safety for Successful Almond Crop

    For a successful harvest, start the season strong. A large crop at harvest requires good bee activity at bloom in the orchard. The current UC general recommendation for bee hive stocking rates is 1-3 strong hives per acre. A strong hive contains at least 8 frames covered with bees, an actively laying queen, and one to two frames of brood. Where cold, rainy and/or windy conditions limit bee flight (remember the 2019 bloom?), two to three strong hives may be needed to supply enough bees to set a decent crop when narrow windows of good bee weather open up. Less than two hives per acre may be sufficient to set a good crop in extended good bloom weather (2020 bloom). Good bee weather is at least 59oF, no rain and less than 10 mph wind speed. [In general, bees begin to forage when temperatures reach 55oF, winds less than 15 MPH and it’s not raining.]

    Hive strength makes a difference in pollination activity (see graph below). The more frames covered with bees in a hive means more foraging bees and more flowers pollinated. The best possible start to the season begins with strong hives in the orchard at the start of bloom. 

    Average pollen collected per hive for a range of hive strengths based on frames of bees per hive over a 7 or 10 day period. Data from Sheesley and Bernard, Cal Ag, 1970 

    To ensure strong hives in the orchard as bloom starts, pollination contracts should include 1) language stating hive strength and 2) an inspection clause stating that some fraction of the hives will be opened and frames inspected by a third party at or soon after delivery to confirm if the contracted hive strength was delivered. The hive strength check should happen at or close to delivery because, as almond flowers are an excellent food source for honeybees, a four-frame hive at delivery to the orchard can become stronger as the pollination season progresses. Assessing colony strength at the end or close to the end of pollination season is not an accurate measure of the pollination activity at the start of the season when strong hives are most needed.

    Growers using lower bee stocking rates (1-2 hives/acre) in an effort to save money are the most in need of contract language stating hive strength and a hive inspection. A single 8-frame hive collects 2.5x the pollen as a 4-frame hive.

    Hive health. Where hives are located and bees treated in an orchard can impact hive health and potentially pollination performance. Hive location, availability of clean water and spray programs (materials and timings) all should be considered by growers and communicated with beekeepers. Hive placement plays a role in good bee activity across the orchard. Hives should be placed in locations where early morning sun will warm the hives and in groups in or around the orchard no more than a quarter of a mile apart.

    Bees need water and will go find it (somewhere else) if not available in your orchard. Check-in with your beekeeper to decide on location and responsibility for providing watering stations for bees in your orchard. The water stations should be protected from pesticides by covering or moving the station or changing the water, or changing after spraying. Bees can’t drink while flying and can drown trying to get to water if there is no landing site at the water source. A 5 gallon bucket with clean water and an old towel or piece of burlap draped over the bucket lip and into the water works as a bee watering site. The Almond Board of California’s most recent Honey Bee Best Management Practices is available at almonds.com/sites/default/files/2020-12/BeeBPMs_12212020.pdf

    Bees can be harmed by pesticides. Certain pesticides and practices can be particularly harmful. In particular, all/any insecticides (except B.t. products such as Dipel) should not be used at bloom. Adjuvants, particularly organosilicones, can harm bees directly and/or increase the impact of pesticides on bees and should be left out of bloom sprays. Foliar nutrients may also harm bees. Protect your bee investment; put only fungicide(s) in the spray tank at bloom.

    While both bees and fungicides are needed during bloom in Sacramento Valley almond orchards in most years, the best practices for bee health and crop set require dividing the day between time for bee activity and time for spray activity; a split shift for bees and sprayers (on spray days). This approach lets bees work and then flowers can be protected. Here’s how that works.

    The key to good hive health is keeping sprays off the daily pollen load that forager bees carry back to the hive and fed to the brood. Almond flowers release some pollen every morning as humidity drops after sunrise. This occurs for several days after the flower opens. In an orchard with good bee activity, pollen released that morning is stripped from flowers by early afternoon. Fungicide spraying shouldn’t start until then; when pollen available for the day is gone (collected by bees and flown back to the hive). There are a couple of ways to check if the pollen is gone from flowers. If the pollen gathering bees (the ones with yellow lumps of pollen on their hind legs) are just doing touch-n-go landing on flowers, those flowers don’t have pollen left and it’s OK to spray. Another method is to rub the flower anthers (the spikey structures in the center of the flower) between your thumb and fingers and then check for yellow pollen on your hands. If there is little to no pollen on your fingers, the bees have been there and gone. (Wash your “pollen– check” fingers before rubbing your eyes. Don’t ask me how I know.)

    Almond flowers provide pollen (and nectar) that build strong hives while providing pollination leading to nut set and a good harvest for growers. The continued success of this annual win/win relationship relies on consideration of the needs of both partners. Growers need strong hives at the beginning of bloom and beekeepers need strong hives at the end of bloom.

    Finally, hives should be removed once 90% of the flowers in the last pollinizer variety have shed their pollen. By this time, the colonies have done their job in the orchard and most bees working the flowers will be foraging for nectar, not pollen. The majority of the pollen gathering bees will be foraging off-site and not providing pollination services to the grower who rented the hives. — By Franz Niederholzer, UCCE Farm Advisor, Colusa & Sutter/Yuba Counties

  • Broccoli Rotations Lower Pathogen Populations and Reduce Disease Incidence of Verticillium Wilt

    In 1999, several UC researchers published foundational research in a paper titled, “Evaluation of broccoli residue incorporation into field soil for Verticillium wilt control in cauliflower.” Since this publication more than 20 years ago, many studies have further investigated this concept and many coastal growers, especially organic producers, have adopted broccoli rotations as a strategy for Verticillium wilt control. Today, typical implementation of this strategy is two broccoli plantings back to back prior to the crop for which Verticillium wilt suppression is desired. While California coastal vegetable production has been the framework for much of this work, the adaptability of this practice to the Sacramento Valley is very promising for management of Verticillium wilt in warm and cool season crops.

    Verticillium wilt is caused by the soilborne fungal pathogen Verticillium dahliae. Microsclerotia, the fungal inoculum that causes infection, dwell in the soil until root exudates stimulate germination and direct the fungal hyphae towards the root. In susceptible plants, infection occurs when hyphae enter the roots right behind the root tip, and continue growth into the water-conducting vascular tissue, the xylem. Once in the xylem, hyphal growth and sporulation can move the fungus into the upper plant tissue. Plant death triggers the fungus to a reproductive stage, prompting microsclerotia formation. When infected crop residue is incorporated into the soil, microsclerotia in the crop residue are incorporated, too. Management is particularly challenging because the pathogen host range is over 300 crops and the inoculum survive upwards of 13 years. To establish control of the pathogen, the key is to reduce inoculum—the number of microsclerotia, below levels damaging to susceptible crops.

    BROCCOLI SUPPRESSES VERTICILLIUM WILT AND DECREASES PATHOGEN PROPAGULES

    Broccoli is one of the few non-host vegetables and member of the Brassicaceae family. Bok choy, broccoli raab, Brussels sprouts, cabbage, cauliflower, Chinese cabbage, and rapini are susceptible to V. dahliae, as are black mustard, Indian mustard, oilseed rape, and turnip. In broccoli, no infection to minor infection from V. dahliae has been observed. In the case of minor infections, the pathogen does not progress beyond the roots and microsclerotia formation in the roots is repressed. Apart from the importance of selecting a non-host as a rotation crop, the glucosinolate profile of broccoli, the secondary compounds responsible for the toxic effect, differs from other brassicaceous crops

    Following broccoli residue incorporation, research out of Japan demonstrated Verticillium wilt incidence of eggplant decreased by 53% compared to eggplant without broccoli rotation. In California Cauliflower production, disease incidence and severity were both reduced approximately 50% following broccoli residue treatments.

    Broccoli did not just decrease disease incidence, but decreased the amount of pathogen inoculum, showing promise for longer term management. In a California study, overall reduction in the number of propagules in V. dahliae-infested plots after two broccoli crops was approximately 94%, in contrast to the five-fold increase in the number of propagules after two cauliflower crops. These findings corroborate earlier studies showing reductions in the numbers of soilborne microsclerotia of V. dahliae and incidence of wilt on cauliflower that were comparable to reductions caused by chloropicrin and metham sodium treatments. Importantly, following broccoli rotations, microsclerotia continue to decline through-out the following cropping season and remain low during the following season. In contrast, propagules in soil fumigated with chloropicrin and metham sodium declined initially but later returned to pre-treatment levels by the end of the cropping season.

    MECHANISM OF SUPPRESSION

    Shetty et al. (2000) reported that the effects of broccoli in reducing microsclerotia and suppressing disease may be associated with the following mechanisms: production of volatile antifungal substances such as allyl-isothiocyanate (ITC) by broccoli residue, increase in antagonistic microorganisms, and degradation of microsclerotia melanin by ligninase/melaninase produced by soil microorganisms in the presence of broccoli lignin. ITCs are chemically similar to methylisothiocyanate, the active agent from the chemical fumigant metam sodium. Likely associated with the ability to generate these conditions, fresh broccoli residue was shown to be more suppressive than dry residue. During tissue decomposition, the glucosinolates in crucifer crops, the characteristic sulfur-containing constituents of the members of Brassicaceae responsible for their inherent pungent odor, break down to produce sulfides, isothiocyanates, thiocyanates, and nitriles that have either fungistatic or fungicidal properties. In addition to release of toxic compounds and microbial activity provided by broccoli residue, the plant may be serving as a ‘decoy’, ‘trap crop’ or ‘dead end host’, further driving population numbers down. As described earlier, some V. dahliae infection is observed in broccoli roots, but it does not result in microsclerotia formation. By stimulating inoculum germination and preventing fungal reproduction, the number of viable microsclerotia decrease in the soil.

    GROWER IMPLEMENTATION OF RESEARCH FINDINGS

    To facilitate greater adaptation of rotations with broccoli in other crops susceptible to V. dahliae, Bhat and Subbarao asked the question whether isolates of V. dahliae originating from different susceptible hosts could cause wilt on broccoli. They evaluated 15 different host isolates against multiple broccoli varieties. This included tomato, eggplant, bell pepper, lettuce, potato, watermelon and strawberry, and found that only isolates from cabbage and cauliflower were weakly pathogenic. Broccoli cultivars Baccus, Greenbelt, Parasol, Patriot, and Symphony showed resistance to Verticillium infection. This provides some evidence for the usefulness of this method in other cropping systems.

    Implementation of broccoli rotations for Verticillium wilt management is optimized when two successive broccoli crops are grown immediately prior to desired Verticillium wilt reduction. Higher amounts of glucosinolates, specifically glucobrassicin, are found in older plants. Research has reported a complete absence of glucobrassicin in broccoli seedlings, 50% of the total in immature heads (5-10 cm diameter) and the highest levels at fully developed Packman broccoli heads (15-20 cm diameter). These results suggest that glucobrassicin synthesis is active during later stages of broccoli development. Plants should be mowed and finely chopped in order to disrupt the plant cells as much as possible. The greatest reductions in microsclerotia occur at soil temperatures above 68°F, and most of this reduction occurs within 15-30 days of incorporation. Variation in efficacy of this method is attributed to multiple factors: fluctuation in climate and cultivation conditions, physical and chemical properties of the soil, soil microbial properties, the type of broccoli cultivar used, differences in pathogen density, and variance in the susceptibility of the following crop host. The types and amounts of glucosinolates vary with the crucifer species and determine the level of plant pathogen growth reduction.

    This practice could also have other potential benefits and drawbacks. Growers in California have observed for many years that where broccoli residues from processing plants are dumped onto a field, weed populations are reduced the following year. Thus, rotations with broccoli may have multiple pest management benefits. However, in recent years in the Sacramento Valley, crop damage from bagrada bug has been significant. Although these outbreaks have largely occurred in fall, outbreaks have occurred in the spring in this region. Members of the Brassicaceae family are the host plants for bagrada and under favorable environmental conditions would support this pest population.

    This management strategy is specific to Verticillium dahliae and is not transferrable to other soilborne pathogens such as Fusarium spp.. Because these two pathogens are common in the Sacramento Valley and above ground symptoms are similar, diagnosis is important. Contact me at any time for disease diagnostic support. All visits and sample analyses are provided free of charge.

    Thousands of microsclerotia, small, black propagules of V. dahliae, formed on susceptible crop residue and remained intact post residue incorporation (Photo by M. Lloyd).

    SUMMARY

    • Two broccoli plantings immediately prior to growing the verticillium-susceptible crop is recommended for best protection
    • Fresh broccoli residue has greater reduction in V. wilt than dry residue
    • Field tarping following fresh residue incorporation did not increase (or decrease) efficacy
    • Suppression of V. dahliae is specific to broccoli and not provided by other Brassicaceae crops.
    • V. dahliae isolates from 15 host crops, including tomato, eggplant, bell pepper, lettuce, potato, watermelon and strawberry, were effectively suppressed by 5 broccoli cultivars
    • The most significant reduction in V. dahliae occurs 15 days post-incorporation, and continues to decline over the season.
    • More mature broccoli plants have higher levels of volatile antifungal substances
    • The mechanisms of action are hypothesized to include: volatile antifungal compounds, changes in the soil microbial communities and serving as a ‘dead-end host’.
    • Broccoli has been shown to reduce pathogens causing Verticillium wilt and lettuce drop, but not other soilborne pathogens such as Fusarium spp. — By Margaret Gullette Lloyd, UCCE Small Farms Advisor

    SUGGESTED READING

    Koike S, Subbarao K. 2000. Broccoli residues can control Verticillium wilt of cauliflower. Calif Agr 54(3):30-33. https://doi.org/10.3733/ca.v054n03p30.

    http://calag.ucanr.edu/archive/?type=pdf&article=ca.v054n03p30

  • Why Aren’t my Blackeye Bean Pods Filling Out?

    Recently, I received a call about a blackeye bean field in the San Joaquin Valley with a lot of bean pods that did not fill out at the tips (photo). I contacted the UC Riverside blackeye bean breeders Drs. Phil Roberts and Bao Lam Huynh and they shared that this problem is primarily caused by heat, which affects pollen viability and thus fertilization. Here’s their response:

    It [lack of pod fill] is the typical male-sterility symptom [lack of pollen viability] associated with extreme temperatures (heat or cold). Based on the planting date you gave, we just checked the temperature in Denair, CA [farm location] and noted that it was quite warm (~100) during the flowering time (40-50 days after planting) and recently during the pod filling stage, so heat must have been a main cause. The symptom could also be more severe if water is limiting.

    Always be prepared with good irrigation management practices for all crops going into heatwaves, like the one we’re having now.  The minimum seasonal irrigation needed to produce a blackeye bean crop being managed for full yield from one pod set is 16 to 18 inches. This estimate includes a pre-irrigation of 4-inches, and irrigations of 4-inches when floral buds first appear, and 8 to 10 inches during 5 to 6 weeks of flowering and pod filling. If additional irrigations are needed during the vegetative stage, one could increase the total irrigation requirement to 20 or more inches. Irrigating for a second flush of pods could require an additional 8 to 12 inches of water. Irrigation requirements are further increased by any water required to leach salts or to compensate for an inefficient irrigation system.

    Additional water may need to be applied during extreme heat events which drive plant transpiration rates to the limit. Make sure to check the soil moisture in the top 12 to 24 inches of the soil profile and apply additional water if the soil is dry. If in doubt about how much additional water is needed, check the reference evapotranspiration (ETo) and make sure to irrigate to replace at least 120% of your daily ETo in your area.  The current (mid to late August) daily ETo in the San Joaquin Valley ranges from 0.25 to 0.30 in/day; make sure your applied irrigation replaces 120% of these values.

    More information on growing blackeye beans can be found in the publication, UC ANR Blackeye bean production

    in California, http://beans.ucanr.edu/files/226601.pdf. – By Rachael Freeman Long, UCCE Farm Advisor

    Lack of pod fill in blackeye bean tips caused by heat, which affects pollen viability and fertilization; water stress can add to this problem, especially during heatwaves. San Joaquin Valley, 2020.

  • Irrigation Strategies to Avoid Heat Damage to Cool Season Vegetables

    Currently, we are experiencing a prolonged heatwave on the central coast.  Heatwaves have become a recurring phenomenon in recent years, especially in late summer. With thousands of acres of cool season vegetables in the ground, irrigation will be critical for keeping crops cool and for supplying enough moisture to meet their water needs.

    Crops can be kept cool by maximizing evapotranspiration (ET).  As liquid water vaporizes heat is lost from the surfaces of leaves and soil and from the surrounding air, which cools the temperature of the crop.  Under water stress leaf stomates close during the hottest period of the day (11 am to 4 pm) and the temperature of the plant tissue can rise above the temperature of the surrounding air.  If the temperature becomes too great leaves and other plant parts may become scorched.

    Since most ranches have a limited number of wells and personnel to irrigate, it is challenging to assure that each field has adequate soil moisture to prevent plants from overheating.   A good strategy is to irrigate just enough to refill the soil profile to the rooting depth of the crop.

    To prioritize which fields to irrigate one should consider the water holding capacity and existing level of moisture of the soil, as well as rooting depth and developmental stage of the crop.  For example, a lettuce crop near maturity with a high ET demand, growing on a sandy textured soil that feels dry, should probably be irrigated soon.  A young lettuce crop with a low ET demand, growing on silt loam soil that still feels moist, likely can be irrigated later without suffering heat damage.

    Another consideration for prioritizing which fields to irrigate are recent field operations.  A recently transplanted vegetable field may need to be irrigated first but may not need a long irrigation to re-saturate the soil around the roots.  A crop that was recently cultivated may have pruned roots, and therefore may need water soon to prevent wilting under these hot conditions.

    Table 1 estimates how much moisture is available to a vegetable crop between saturation and moderately dry or dry conditions for different soil textures.  This table can be a guide for how much water should be applied to re-saturate the soil.  For example, applying 0.42 inches per foot of rooting depth will bring a moderately dry silty clay soil back to saturation.  Applying more than this amount of water will likely over-saturate the root zone.

    Table 1. Estimated plant-available moisture for different textured soils.

    Also, estimating the cumulative crop ET since the last irrigation can guide how long to irrigate. Reference ET values between south Salinas and Soledad during this hot spell have been as high as 0.27 inches per day.  If the crop has a full canopy, 0.25 to 0.3 inches for each day since the last irrigation would be a good rule of thumb for how much water to apply as long as the total does not exceed the water holding capacity of the soil.

    Lastly, one needs to convert the amount of water to apply to an irrigation run-time.  To make this calculation one needs to know the application rate of the irrigation system. For impact sprinklers, the application rate can be estimated using Tables 2-4.  Note that pressure and nozzle size have a significant effect on application rate.  For drip, the irrigation time will depend on the tape discharge rate and pressure, as well as the spacing of drip lines.  Assuming that the drip system is operated at the pressure recommended by the manufacturer (usually 8 to 10 psi) one can use Table 5 to approximate the application rate.  For example, for one drip line of medium flow tape (0.45 gpm/100 ft) on 40- inch wide beds the application rate of the drip system is 0.13 inches per hour. If there are several drip lines per bed then multiply the application rate in the table by the number of drip lines.

    The appropriate run-time can be estimated by dividing the amount of water to apply by the application rate of the irrigation system.  For example, to apply 0.6 inches of water to a field with drip using medium flow tape the water would need to run for 4.6 hours:

    Hours to operate the irrigation system = 0.6 inches of water/0.13 inches per hour = 4.6 hours

    Summary

    Irrigating the right amount of time to bring the soil back to saturation will maximize crop ET during these hot days, and hopefully prevent any heat damage to crops.  Also, consider visiting the CropManage website (cropmanage.ucanr.edu) for further guidance on scheduling irrigations. This online tool can assist growers in quickly estimating how much water to apply to meet crop water needs.

    Table 2.  Sprinkler application rate for varying pressures and nozzle diameters for a solid set spacing of 30 × 30 feet (Rainbird 20JH).
    Table 3.  Sprinkler application rate for varying pressures and nozzle diameters for a solid set spacing of 30 × 33.3 feet (Rainbird 20JH).
    Table 4.  Sprinkler application rate for varying pressures and nozzle diameters for a solid set spacing of 30 × 40 feet (Rainbird 20JH).
    Table 5.  Drip application rates for varying bed widths and tape flow rates estimated for 1 drip line per bed.  Multiply the rate in the table by the number of drip lines per bed to determine the actual application rate.  (For 3 drip lines on an 80-inch bed multiply by 3)
    “— By Michael Cahn, UCCE Farm Advisor, Monterey County”