Crown gall is one of the most common diseases observed in commercial walnut orchards in California.  The disease, caused by a plant pathogenic bacterium, is easy to identify based on symptomology alone.  Agrobacterium tumefaciens causes crown gall (Figure 1), but the disease name is a misnomer because the pathogen also induces galls on roots (Figure 1B) and stems (Figure 2). Another related bacterium, Agrobacterium rhizogenes, causes hairy root (Figure 1B) a disease that can easily be identified based on symptomology (root proliferation) alone. Crown gall is more prevalent in commercial walnut orchards than hairy root; however, hairy root incidence may be under-estimated simply because symptoms are below ground.  Both pathogens may be established in the same orchard, and occasionally may be observed on the same tree (Figure 1B).

The infection process. Agrobacterium tumefaciens is a soilborne pathogen that requires a wound to infect plants.  The bacterium survives in soil and is a somewhat ubiquitous soil inhabitant. Although the bacterium may be prevalent in orchard soil, only a fraction of the population is pathogenic.  Pathogenic isolates contain a circular piece of extra-chromosomal DNA called a plasmid.  The plasmid inserts into plant DNA, thus genetically transforming plant cells to proliferate and form a tumor.

Location of symptoms assists in assessing origin of infection. The most commonly asked question about crown gall is “where did it come from?” Unfortunately, it is often difficult to pinpoint the original source of inoculum in an orchard, particularly if tumors are present on roots or at the crown. The pathogen may be present in nursery or orchard soil and it may take months for symptom development after introduction of the pathogen to a wound. As a consequence, it is difficult to determine the timing of initial infection and whether the pathogen was introduced in the nursery or the field, or perhaps even both. 

The location of aboveground (aerial) galls may offer some indication of inoculum source and provide lessons to prevent disease spread.  When aerial galls form above or below the graft union, the most probable method of pathogen introduction is on infested tools utilized for pruning or removal of suckers (Figure 2A).  Tools may become contaminated with the pathogen upon contact with infested soil or by cutting through infested plant material. The removal of rootstock suckers close to the crown may bring loppers or other pruning tools in contact with infested soil or crown gall tumors.  The wounds caused by removal of suckers at the base of the tree may also serve as infection courts for inoculum residing in adjacent soil.  Infested soil near the base of the tree may be splashed by rain or microsprinklers to the cut surfaces, resulting in infection and future symptom development.

When galls are observed at the graft union (Figure 2B), the most common thought is that the pathogen was transmitted on a dirty grafting knife.  There are other potential sources of inoculum, however, that may be responsible for galls formed at the graft union. Budwood may become contaminated with the pathogen upon collection. If the budwood shoot falls on contaminated soil after cutting from the mother tree, the cut surface may become infested.  As a result, an infection may form at the graft union. When walnut rootstocks are field grafted at an older age (ie. two-year old), suckering may be more prevalent at or near the graft union (Figure 3A).  The removal of these suckers provides opportunities for infection near the graft union and gall formation long after the graft was made (Figure 3B). Last, asymptomatic seedlings have been found (experimentally) to contain endophytic populations of A. tumefaciens.  These endophytic populations have the potential to lead to gall formation at secondary stem wound sites (Yakabe, et al. 2012).

Rootstock selection.  Another common question is whether the use of a clonal ‘Paradox' selection offers some protection from crown gall.  ‘Paradox' rootstock is susceptible to crown gall, regardless of whether the rootstock was produced from a seed or via micropropagation (clonal).  Plants produced by micropropagation are less likely to become infested with the pathogen in the nursery than seedlings, simply because the clones are produced in axenic (sterile) culture and plantlets are grown up in pots containing sterilized potting medium.  The potted clonal plants could still become infected in the nursery if the pathogen is introduced via contaminated tools/boots, etc.  or from the splashing of water from contaminated soil. Additionally, clonally propagated plants that are sold bare-root may become infected if grown out in contaminated soil.  ‘Paradox' seedlings may become infested with the pathogen if the seed contacts contaminated soil upon collection, or if the nursery block is planted in contaminated soil.  To mitigate potential for contamination of seedling trees, nurseries tend to shake rootstock seed source trees onto tarps, disinfest the seed, and plant seed in ground with no prior history of infestation.  Regardless of rootstock source (seed vs. clone), a low level of crown gall incidence may be anticipated in new plantings simply due to the endemic nature of the pathogen and ease of transmission, despite the vigilance in sanitation at the nursery level.

Clonal selections of ‘Paradox' are available in the nursery trade. These include ‘Vlach,' ‘VX211,' and ‘RX1,' which are regarded as vigorous, highly vigorous, and moderately vigorous, respectively.  All are susceptible to crown gall, but ‘RX1' may have low to moderate resistance, making it a potential choice rootstock for replant holes contaminated with A. tumefaciens.

For information on rootstock terminology utilized in the walnut nursery trade, please visit the following article posted on the UC Frit and Nut Information Center website:  https://ucanr.edu/datastoreFiles/391-536.pdf

Influence of crown gall on tree health and productivity.  Many walnut trees live to maturity even with crown gall infection; however, infections that girdle the tree may cause early mortality. Crown gall is associated with reduction in tree size and yield; the higher the severity of the disease (ie. percent of circumference of the tree affected), the smaller the tree diameter and yield (Yaghmour, et al. 2016; Olson and Buchner, 2001). Crown gall may also predispose trees to future damage by pests and diseases (Fichtner, 2011; Yaghmour, et al. 2016).

Treatment of crown gall in the field. Removal of galls from infected trees is time-consuming and expensive. A decision on whether to rogue infected trees and replant or remove the tumors is determined by the extent of galling and age of the tree. The decision can be aided by exposing the gall with compressed air to better judge the extent of galling (Figure 4A). If a decision is made to remove the gall it can be surgically removed (Figure 4B), and surrounding tissue can be disinfected. On trees with galls colonizing three-fourths the perimeter of the tree, heat treatment has been found to provide better control than surgery followed by chemical treatment (Olson and Buchner, 2001). Unfortunately, the exact amount of heat required to kill the pathogen while preserving cambium tissue is not known.  Excess heat may damage the tree and inhibit recovery (Figure 4C).Guidelines for assessing the value of replanting vs. treating affected trees, as well as the efficacy of various methodologies implemented for gall removal, can be found in the following article:  http://ceglenn.ucanr.edu/files/185675.pdf. 

Chemical and biological treatments for managing crown gall. First and foremost, tools (ie. pruning tools, grafting knives, etc.) should be sanitized between trees to prevent transmission of the pathogen. Sodium hypochlorite solution (bleach) is an inexpensive disinfectant with an LD₁₀₀ of 0.5 ppm for A. tumefaciens; however, it is corrosive to tools, may be phytotoxic, and exhibits reduced efficacy in the presence of dissolved and suspended solids.  In order to maintain the efficacy of sodium hypochlorite solution for tool sanitization, fresh solution would have to be continually replenished in the container to prevent the buildup of solids. Cationic surfactants, such as quarternary ammonium compounds, disrupt cell membranes of the pathogen.  In a USDA ARS research study, a commercially available cationic surfactact, Physan 20® (Maril Products Incorporated, Tustin, CA), exhibited an LD₁₀₀ of 2 ppm. In this study, the presence of solids in solution had less impact on the efficacy of the cationic surfactants than on sodium hypochlorite, another benefit that these products have over bleach.

Strains of A. tumefacians (Strain K84) (ie. Galltrol A®, AgBioChem, Los Molinos, CA) are sold as biological control agents for protection of plants from pathogenic strains of A. tumefaciens. The product is sprayed on the roots prior to planting to ensure colonization of wounds by the biocontrol agent prior to exposure to the pathogen.  Research studies have demonstrated the efficacy of Strain K84 for preventing crown gall; however, efficacy of the product may vary based on pathogen population dynamics and environmental conditions.

Another registered product, composed of a mixture of two phenols (ie. Gallex®, AgBioChem, Los Molinos, CA), can be utilized as a post-plant treatment of galls.  The product may be applied directly to small galls, or as a disinfectant on exposed areas after gall excision.  

For more information on historic research conducted on crown gall, visit the Walnut Research Reports, which can be searched by topic, author, or year on the UC Fruit and Nut Research and Information Center website:  https://ucanr.edu/sites/cawalnut/.  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.

Select References

Fichtner, E.J. 2011. Association Of Tenlined June Beetle with Crown Gall in a Tulare County Walnut Orchard. Walnut Research Reports.

Hasey, J., Leslie, C., Hackett, W., McGranahan, G., Brown, P.J., Westphal, A., McKenry, M., Browne G., Kluepfel, D. 2018. Walnut Trees in the Nursery Trade: Understanding Terminology, How They are Propagated, Availability and Clonal Rootstock Pest Interactions. UC Fruit and Nut Research and Information Center

Olson, B. and Buchner, R. 2001. Field treatment of crown gall on walnut: Different options' affects on growth and productivity. Pacific Nut Producer. July/August 2001.

Yaghmour, M., Facundo, H., Fichtner, E. 2016. Effect of crown gall on thousand canker incidence.  Walnut Research Reports.

Yakabe, L. E., Parker, S. R., and Kluepfel, D. A. 2012. Cationic surfactants: Potential surface disinfectants to manage Agrobacterium tumefaciens biovar 1 contamination of grafting tools. Plant Dis. 96:409-415.

Yakabe, L.E., Parker, S.R., Kluepfel, D.A. 2012. Role of systemic Agrobacterium tumefaciens populations in crown gall incidence on the walnut hybrid rootstock ‘Paradox.'  Plant Disease 96:  1415-1421.

Over 80% of the almond crop is borne on short, compact vegetative shoots called spurs. Each season, however, only a portion of the spur population on a given tree supports fruit production. Because of their role in supporting productivity and yield, maintenance of a healthy spur population contributes to the economic sustainability of an orchard. Understanding the dynamic states of spurs between seasons and the conditions promoting spur productivity and survival may enhance orchard management practices to maintain or increase yields in future years.

What are spurs? Spurs are short, compact vegetative shoots (approximately 0.5-2 inches long) that are borne on the prior season's wood. Spurs are either formed from lateral buds on vegetative shoots (Figure 1A) or from vegetative buds on spurs (Figure 1B). When spurs give rise to further spur growth over sequential years, it may be difficult to visually evaluate the age of a spur due to the compact nature of growth (Figure 2). The apical bud on a spur is always vegetative (Figure 1B); however, spurs can also support up to 6 flower buds in a season (Figure 3B).  The duration of spur growth on almond is short and generally complete in April or early May.

Spurs exhibit a localized carbon economy. Spurs are considered semi-autonomous with respect to carbon supply, meaning that spurs serve as both the main source and sink of carbohydrates utilized in vegetative and reproductive growth. As a result, spurs remain vegetative (Figure 3A) for 1-2 years prior to flowering.  Although not immediately productive, vegetative spurs with adequate leaf area produce and store carbohydrates for support of future flowering and nut development.  In fact, the leaf area of spurs is a better predictor of potential for flower bud development than the number of leaves per spur. Spurs with less than 10 cm² leaf area are unlikely to support viable buds (floral or vegetative); spurs with 10-12.3 cm² leaf area are likely to support only vegetative buds; and spurs with >12.3 cm² have a higher probability of supporting flower buds. Due to the carbohydrate demand of setting fruit, few spurs flower the year after bearing. 

Spur leaf area influences flower bud development. Flower buds can be differentiated from vegetative buds by both shape and position. Flower buds are generally positioned on either side of a vegetative bud on shoots (Figure 1A), or in lateral positions on spurs (Figure 1B). Vegetative buds are triangular and pointy, whereas flower buds are thicker and more oval than vegetative counterparts. In early summer, buds manifest in leaf axils, but it is impossible to differentiate between floral and vegetative buds until late August or early September. Even in late summer, identification of flower versus vegetative buds may require bud dissection and microscopy.

Flower bud development does not proceed at a uniform rate in a given block or tree, but varies dramatically between spurs. The rate of floral bud development is positively related to leaf area. Consequently, spurs supporting high leaf areas exhibit more rapid flower bud development than spurs with lower leaf areas.

Prior year spur leaf area affects flowering and nut set. Spurs supporting high leaf area in a given year have enhanced potential to support flowering and nut set in the subsequent year. In fact, non-bearing spurs with >50 cm² leaf area have over an 80% probability of flowering the following year.   Non-bearing spurs in lower light positions in the interior of the canopy may require more years in a vegetative state prior to supporting flower and fruit production.

Spur survival is influenced by prior year leaf area and exposure to light. The literature suggests that spurs remain viable for 3-5 years; however, the survival potential of individual spurs is related to light exposure, bearing status, and prior season leaf area. We have found that spurs in well managed orchards in outer canopy positions can remain productive for more than 10 years.  Regardless of bearing status, spurs with higher leaf areas are more likely to survive into the following season. Bearing spurs are more likely to survive into the following season when occupying light-exposed positions in the canopy. Conversely, the mortality of non-bearing spurs is generally not influenced by light interception in the canopy. These relationships are all explained by the reliance of spurs on a localized carbon economy.

Orchard management for enhanced spur survival and productivity. Following best orchard practices, particularly in irrigation scheduling and nutrient management, will allow for canopy development and maintenance of tree health. However, consider that practices supporting excessive growth may cause shading, which may be limiting to spur survival. Promotion of modest annual growth will allow for production of new spurs, but be patient because new spurs may take 2 years to support flowers.  Last, when managing the tree canopy, overlapping branches and dead wood should be removed to prevent shading and promote spur survival and productivity.

Select References:

Lamp, B.M., Connell, J.H., Duncan, R., Viveros, A.M., Polito, V.S. 2001. Almond flower development:  Floral initiation and organogenesis. Journal of the American Society for Horticultural Science. 126:  689-696.

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

Polito, V.S., Pinney, K., Heerema, R., Weinbaum, S.A. 2002. Flower differentiation and spur leaf area in almond. Journal of Horticultural Science and Biotechnology. 77:  474-478.

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

The first step in assessing the cause of canopy chlorosis and decline in an orchard is mapping the distribution of the symptoms. If a pattern of chlorosis is similar across irrigation lines, then the cause of the problem may be related to over- or under-watering. Two scenarios present themselves regularly during summer farm calls: a) terminal tree chlorosis, and b) within row tree chlorosis (Figures 1 and2). 

Terminal Tree Chlorosis.  In some orchards, the terminal tree along the irrigation line may become chlorotic and decline in advance of mortality. If terminal tree chlorosis is a trend throughout the orchard, it is worth assessing the sprinkler distribution at the end of the irrigation lines. In some orchards, the terminal tree is outfitted with a sprinkler that is not shared with a neighboring tree (Figure 2A). This terminal tree receives 1.5 x the amount of water as the other ‘healthy' trees down the irrigation line. In an otherwise adequately-irrigated orchard, these terminal trees are over-irrigated and develop chlorosis and decline. Sometimes the terminal sprinkler is positioned adjacent to the trunk (Figure 3), resulting in direct wetting of the trunk, a condition that predisposes the tree to Phytophthora infection, particularly when surface water is utilized.

Correcting terminal tree chlorosis:  To correct the over-irrigation of the terminal tree, the microsprinkler head can be changed to a lower flow rate.  Sprinklers should be placed away from the base of trees to prevent direct contact of the trunk with the stream of water. Additionally, when replanting dead or declining trees at the end of rows, consider that the irrigation needs of the replant are considerably lower than that of the neighboring older tree in the row.

Within-row chlorosis. If canopy chlorosis is consistent throughout the orchard, but terminal trees appear healthy, assess the distribution of sprinklers around the terminal tree in comparison to the trees along the irrigation line. If the terminal tree receives less water (Figures 1B and 2B) than adjacent chlorotic trees, consider the potential that the orchard, as a whole, is over-irrigated. To test this hypothesis, growers and orchard managers can use a pressure chamber to assess the midday stem water potential of the trees. Almond trees maintained from ^-6-^-10 bar are under low water stress, but may be more susceptible to disease. Maintenance of almonds at ^-10-^-14 bar (mild stress) from mid-June through hull split, minimizes risk of disease (ie. hull rot) and supports shoot growth. For information on use of a pressure chamber for enhanced irrigation management of almond, walnut and prune, download UC ANR Publication #8503 (http://ucanr.edu/datastoreFiles/391-761.pdf).

Correcting within-row chlorosis:  If the orchard at large is over-irrigated, a change in the overall irrigation strategy is warranted. A combination of pressure chamber use to measure tree water stress, and consideration of weekly crop transpiration may enhance irrigation scheduling. The California Department of Water Resources and UCCE 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. To learn how to use these reports, please refer to the following article:  http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=26858.  Crop ET reports can be found online (ie:  http://cetulare.ucanr.edu/Agriculture782/Custom_Program911/). 

Over the past decade, combined nut acreage in Tulare and Kings Counties has more than doubled, resulting in an expansion of the labor force needed to harvest the >250,000 acres of nuts locally. The annual Nut Harvest Safety Training co-hosted by the Kings County Farm Bureau, the Tulare County Farm Bureau, and UCCE Tulare and Kings Counties attracted over 150 attendees to the Kings County Fairgrounds on July 11, 2018. The program was conducted in both English and Spanish, with Walter Martinez and Gabriel Torres, UCCE Tulare County, serving as translators. The program was additionally sponsored by the American Pistachio Growers.

Dr. Farzaneh Khorsandi Kouhanestani, a new CE Specialist at UC Davis in Biological and Agricultural Engineering joined the program with a talk on equipment safety. Martha Sanchez, Department of Pesticide Regulation presented on chemical safety. Officers Chris Tuttle and Gabriel Perez, California Highway Patrol, discussed road safety, and Gumaro Castellanos, Nationwide, presented on heat illness prevention. Francisca Camarena and Mireya Gutierrez of Family HealthCare Network were available during registration to measure attendees' blood glucose level and blood pressure.