It is not often that we observe cankers caused by Phytophthora on pistachio in CA; however, the unique situation observed on a recent farm call is worth recounting. The decline and mortality (Figure 1A) of approximately 20 trees in a Tulare County pistachio block of ‘Golden Hills' on PG1 rootstock (a selection of Pistacia integerrima) was unusual simply because we generally find PG1 rootstock to be resistant to Phytophthora. In fact, the rootstock did not exhibit any symptoms of cankering and the cambium appeared healthy (Figure 1B).  Rootstock suckers were actively growing, suggesting that the pathogen was not infecting the roots or crown of the tree (Figure 1A). The scion, however, exhibited extensive bleeding (Figure 1C) and gummosis (Figure 1D), which is typical of Phytophthora infection on Pistacia vera.

The diagnosis of this problem was possible only after gathering information from the grower on orchard management history. Recently the grower had changed the microsprinkler nozzles to a higher flow rate nozzle (14 gallon/hr). The stream of water from the microsprinkers was wetting the trees above the graft union. Additionally, the grower had recently utilized surface water (not ground water) for irrigation. Surface water has a high probability of Phytophthora contamination. The combination of the change to higher flow rate sprinklers, wetting of the scion wood, and risk of introduction of Phytophthora to the orchard, all suggested that the cankering, canopy decline, and mortality is related to Phytophthora infection. After sampling the canker tissue and incubating it in selective medium, the presence of Phytophthora sp. was confirmed in the tissue.

To prevent further incidence of Phytophthora in the orchard, the irrigation system needs to be changed to prevent water from splashing on the tree trunks. This may include both changing the nozzles to a lower flow rate, and moving sprinklers further from the base of trees. As long as the water does not touch the susceptible P. vera scion, the introduction of Phytophthora to the orchard in surface water is of minimal risk to orchard health.  In order to preserve the rootstock and enhance regeneration of productive, economically viable trees, the grower may opt to cut the scion off to a point below the graft union and re-graft onto the mature rootstock.

The grower may also opt to run products through the irrigation system that may assist with disease management.  Two examples of products that have been found to prevent Oomycete infection on pistachio and other crops include phosphites and Ascophyllum nodosum extract. Replicated trials using K-PHITE 7LP (Plant Food Company, Inc) and Acadian (Acadian Seaplants) have demonstrated that both product types offer protection of pistachio from infection by Phytopythium helicoides (another Oomycete causing root rot on pistachio and almond), and therefore may have value in mitigating future infections within the orchard. K-PHITE 7LP is labeled for management of Phytophthora on pistachio. Acadian, an extract of A. nodosum, is registered as a fertilizer; however, the efficacy of A. nodosum extract in mitigating plant diseases, including those caused by Oomycetes, is well documented in the scientific literature.

For more information on management of Phytophthora, visit UC IPM Online (www.ipm.ucdavis.edu).  Always read the label of the product being used, and note that all registered pesticides are not necessarily listed on the UC IPM Online website (http://www.ipm.ucdavis.edu) or in this newsletter.  Always check with certifier to determine which products are organically acceptable.

Evaluation of flowering and fruit set on almond allows for within-season assessment of orchard productivity; however, understanding the vegetative growth dynamics of almond allows growers to consider parameters affecting productivity years into the future.  Vegetative growth of almond has two main components: vegetative shoot growth and spur production. Vegetative shoot growth provides the overall architecture of the canopy, and spur production generates the tissues that give rise to the majority of fruit in subsequent seasons.  Both vegetative shoot growth and spur production are key components to the development of an economically sustainable and productive orchard.

Timing of vegetative growth.  All buds (vegetative and flower) are formed during the prior season. Because almond has one of the lowest chill requirements of permanent crops grown in California, the chill requirement is generally fulfilled by January 1. As temperatures increase, growth initiation is induced, and bud break ensues, with flower buds breaking in advance of vegetative buds. Vegetative shoot growth proceeds at a somewhat uniform rate throughout the season on young trees, but the durationof spur elongation is short and generally complete by April or early May.

Vegetative buds. On almond, vegetative buds can be distinguished from flower buds by shape. Flower buds are thick and oval; vegetative buds are pointy and triangular. On shoots, flower buds are generally formed on either side of a vegetative bud. On spurs, the apical bud is always vegetative (Figure 1A), and this bud can give rise to either further spur growth (Figure 2) or a vegetative shoot (Figure 1B). Spurs in positions with high light interception are more likely to give rise to vegetative shoots than new spur growth. 

Vegetative shoot growth. Vegetative buds may give rise to long vegetative shoots that support future spur production. During the early years of orchard establishment, long shoot growth is the main component of vegetative development on almond (Figure 3A, B, C). On mature trees, vegetative shoot growth occurs under conditions of low crop, high vigor, and in regions of the canopy where there is excessive light interception. Canopy regions with excess light include external/exposed areas and empty spaces resulting from broken limbs.

Spurs. Spurs are short, compact vegetative shoots, approximately 0.5-2 inches long (Figure 2). Spurs arise on vegetative shoots or on spurs produced in the prior season (Figure 2). Within a season, the duration of spur growth is generally short, with spur extension completed by April or early May. Spurs are always formed on the prior year's wood, and remain vegetative for 1-2 years prior to flowering. As a consequence, the process from vegetative shoot growth to spur production and flowering may take 4 seasons.

Spurs support approximately 80% of the total almond yield in a given year, yet only about 20% of the total spur population on a tree supports nut production each year.  The fact that only 1 in 5 spurs bear fruit in a season is explained by the dynamic status of spurs between years. A portion of spurs remain only vegetative in a given year (Figure 1C), whereas others may support 1-5 flowers that may develop into single fruit-bearing spurs or multiple fruit-bearing spurs (Figure 1D). Due to the reliance on a localized carbon economy, individual spurs tend to alternate bear, meaning that spurs that bear fruit one year tend not to flower or bear fruit the following year.

Comprehensive view on vegetative growth. In new almond plantings (Figure 3A), growers should expect the mainstay of vegetative growth to be production of long vegetative shoots (Figure 3B and C). Although the majority of the future crop is produced on spurs, it will take time for bearing spurs to be represented in the canopy.  Consider that spurs are produced on the prior year's wood and will remain vegetative for 1-2 years before entering productivity. Patience is needed as these vegetative spurs store carbohydrates to support future nut development.

Select References

Tombesi, S., Lampinen, B.D., Metcalf, S., DeJong, T.M. 2016. Yield in almond related more to the abundance of flowers than the relative number of flowers that set fruit. California Agriculture 71: 68-74. Online: https://doi.org/10.3733/ca.2016a0024

Lampinen, B.D., Tombesi, S., Metcalf, S.G., DeJong, T.M. 2011. Spur behavior in almond trees:  relationships between previous year spur leaf area, fruit bearing and mortality. Tree Physiology 31:  700-706. Online:  https://doi.org/10.1093/treephys/tpr069

Kester, D.E., Martin, G.C., and Labavitch, J.M. 1996. Growth and Development. In: W.C. Micke, Editor, Almond Production Manual. Oakland, California, University of California, Division of Agriculture and Natural Resources (pp 90-97).

The California Department of Water Resources and the University of California Cooperative Extension have teamed up to provide Weekly ET Reports to agricultural water users to assist with irrigation scheduling. The reports include water use information for a variety of crops including almonds, pistachios, walnuts, grapevines, citrus, and stone-fruit of mature bearing age. Adjusted on a weekly basis, water use estimates account for the changing growth stage and weather conditions at the Madera, Parlier, Lindcove, Stratford, Panoche, and Five-Points CIMIS weather stations. Each report gives crop-specific evapotranspiration (ETc, total crop water use including soil evaporation) estimates for the previous and coming week.

Beginning the Irrigation Season

One of the objectives of the weekly ET report is to help managers decide when to initiate the first irrigation.Irrigating too much and too early can lead to reduced growth and yield due to loss of fine feeder roots as well as root disease. Stored soil moisture from winter and early spring precipitation will reduce the need to irrigate early in the season. As ET rates increase and the seasonal crop water use exceeds accumulated rainfall, water managers should consider if there is adequate soil moisture to supply the difference or to begin to irrigation. The weekly ET report provides “Accumulated Precipitation” since January 1^st and the “Accumulated Seasonal Water Use” since leaf-out. When water use exceeds precipitation, it may be time to irrigate. Irrigation decisions should be confirmed with field monitoring of soil moisture and plant water status.

Irrigation Frequency and Duration

Managers need to know the application rate of their irrigation system in either inches/hr or gallons/hr and the effective wetted volume (surface wetting and sub-surface lateral subbing) of the root zone as a % of the orchard floor (Figure 1).Different soils will have larger or smaller wetted zones as soils with different texture hold different levels of moisture. This will influence when seasonal irrigation should begin, the appropriate set duration, and frequency of irrigation events throughout the season.

Example: If the effective wetted volume for a sandy loam soil is 25% of the whole orchard floor and crop ET for the week is given as 1.5” (0.21”/day), then the moisture extraction from the wetted area for the week = 1.5”/25% = 6”. A sandy loam soil at field capacity that has approximately 0.8 inches plant available water per foot of soil would have 5” of available moisture in a 5-foot rooting zone. The weekly water use (6” every 7 days) will exceed soil plant available water in about 5 days. In this example, irrigation sets need to be more frequent than once a week, roughly every 3-4 days. The application time for a system with one gallon per hour (gph) emitters that deliver about 1”/day would be calculated as:

Application time = (0.21”/day ET * 3 days)/ 1”/day irrigation * 24 hr/day = 15 hours every 3 days

Irrigation recommendations for common crop spacings are shown in the report in units of inches or gallons of water needed per week. To convert inches per tree to gallons:

Gallons = (Inches * 27,154 gal/acre inch water) / # plantings per acre  

Adjusting for System Efficiency

Different irrigation systems can vary greatly in efficiency. Those with high uniformity such as drip micro-irrigation, are roughly 80-95% efficient and require less output to meet crop needs than other systems like flood-furrow, with efficiencies as low as 50%. The recommended amount of water to apply is corrected for a range of irrigation system efficiencies. If the mature almond orchard in the example was 90% efficient, you'd find the 90% column in the second table of the Weekly ET Report, and put in the almond value at 90% (1.67 inches or 0.24 in/day) into the equation above, instead of 1.5 acre-inches.

If you would like to receive weekly reports, have questions or need more assistance contact:

Mae Culumber UCCE Fresno County Nut Crop Advisor, cmculumber@ucanr.edu, 559-241-7526

Elizabeth Fichtner UCCE Tulare County Orchard Systems Advisor, ejfichtner@ucanr.edu, 559-684-3310

Phoebe Gordon UCCE Madera County Orchard Systems Advisor, pegordon@ucanr.edu, 559-675-7879 George Zhuang UCCE Fresno County Viticulture Advisor, gzhuang@ucanr.edu, 559-241-7506

Steve Ewert, California Department of Water Resources, sewert@water.ca.gov, 559-230-3334

Cold winter temperatures can reduce populations of leaffooted bug, Leptoglossus zonatus (Dallas), by ~50 to 80%.  But unfortunately, it takes a cold year much like occurred in January of 2007 where daytime temperatures remained low and nighttime temperatures reached about 20° F for several hours.  In other words, ouch for the citrus crop.  Fall and winter temperatures of 2016 / 2017 were ideal for leaffooted bug and the growing season started out with large populations.  I need to add that we do not fully understand if the wet winter positively affected populations - it certainly did not have a negative impact.

Monitoring and managing leaffooted bug presents an IPM challenge.  In the fall between September and mid-November, the species produces a full generation; certainly, on pomegranate and although I have not observed it, also on desert willow.  In most years, adults have moved from those host plants by late December to protected overwintering sites such as Mediterranean fan palm and Italian cypress trees.  In early March leaffooted bug leave overwintering sites to feed on what happens to be available at the time i.e. almonds and subsequently pistachio.

The IPM challenge is that we do not have an effective monitoring tool to detect the bug when they make their initial entrance into almond in spring and pistachio in early summer.  And moreover, once leaffooted bug is detected, pyrethroids offer the best management option – not necessarily the best IPM option. 

Given the importance of pomegranate in the life cycle of leaffooted bug, PCAs and growers need to concentrate monitoring efforts on that crop during September through October, especially focusing on unmanaged orchards and hedgerows near almond and pistachio orchards.  If populations are found they will consist mainly of immature stages and there are two management options, clothianidin and pyrethrins.  The caveat is that those compounds have only contact activity; coverage must be good and the insecticides will likely not have a great impact on adults because they will spook and fly away before being sprayed. 

At this time, pistachios have passed the most vulnerable stage for leaffooted bug damage but can still suffer some damage and should be monitored and treated if bugs are found.  The UC Statewide IPM Program provides a few monitoring recommendations focusing on damaged nuts and nymphs.  Adults can be monitored for by visually searching tree canopies for about 15 – 20 seconds each.  Concentrate searches to the sunny side of the canopy and focus attention on the nut clusters.

              Unfortunately, the treatment options with any residual activity are limited to pyrethroids.  Producers have begun gearing up for “hull split” navel orangeworm sprays.  Note however, that non-pyrethroid compounds such as methoxyfenozide, chlorantraniliprole, spinetoram do not have any activity against leaffooted bug or other large bugs.     

Insecticide use should occur only if monitoring indicates the presence of leaffooted bug and/or its feeding damage.  Apply insecticides only after considering the potential risks of the compound to beneficial organisms, including bees and biological control agents, and to air or water quality.  For more information on these topics please consult the UC IPM Pest Management Guidelines for Almonds at http://ucipm.ucanr.edu > Agricultural pests > Almond

Photo Caption:  

Fig. 1.  Aggregation of leaffooted on pomegranate in early October of 2016.  The aggregation is comprised mostly of fifth instar.