Xanthomonas arboricola pv. pruni (Smith) Vauterin, Hoste, Kersters & Swings

California Pest Rating for
Xanthomonas arboricola pv. pruni (Smith) Vauterin, Hoste, Kersters & Swings
Pest Rating: B


Initiating Event:

In September 2013, CDFA plant pathologist, Luci Kumagai, identified Xanthomonas arboricola pv. pruni associated with symptomatic almond seedlings that were submitted by Sierra Gold Nursery in Sutter County to the CDFA Plant Pathology Laboratory.  David Marion, CDFA environmental scientist, surveyed the nursery shade house where the trees were housed and determined that only Monterey Almond trees exhibited symptoms of bacterial canker.  Subsequently, in line with CDFA’s current Q rating of X. arboricola pv. pruni and ‘Nursery Standard of Cleanliness’, the lot of affected almond trees was destroyed and other Prunus spp. in the nursery were protected from further potential infection. Near about that time, the pathogen was found in commercial almond orchards in a few counties in northern California, thereby marking its first non-official detection in the State.  The detection of the associated disease was reported by the University of California Cooperative Extension Farm Advisor for Stanislaus County.  In view of the recent finds, the current temporary rating is herein assessed for the proposal of a permanent rating.

History & Status:

Background:  Xanthomonas arboricola pv. pruni is a bacterial pathogen that attacks only Prunus spp. causing disease commonly known by various names: bacterial canker of stone fruit, bacterial leaf spot of stone fruit, bacterial shot-hole of stone fruit, and black spot of stone fruit.  The bacterium belongs to the family Xanthomonadaceae of the order Xanthomonodales.  No strains have been reported, however, difference in virulence to peach, plum and apricot have been noted (Du Plessis, 1988).  The species was first described in North America (Michigan) in 1903 on Japanese plum, but it not clear if it spread from there throughout the world or if it naturally has a wide geographical range.  In California, it is a relatively new disease of almonds (UCIPM, 2013).

Disease cycle:  On Prunus species, the pathogen overwinters in plant tissues such as buds, protected areas (cracks in the bark), and in leaf scars. On almonds it overwinters on fruit mummies and twig cankers.  On plum and apricot, cankers formed during the preceding season continue to develop in spring and provide a source of inoculum.  During late winter as temperatures warm, peach leaf and flower buds swell, and as new tissue growth initiates, bacteria multiply and cause the epidermis to rupture, forming a lesion or spring canker.  Bacteria are spread from cankers or mummified fruit to newly emerging leaves by dripping dew and splashing and/or wind-blown rain.  Infection takes place through natural openings or wounds.  High moisture conditions favor leaf and fruit infections.  Severe infection is favored by warm temperatures (19-28°C), light frequent rainfall and fairly heavy winds and dew.  Following foliar infection, cankers develop in the green shoot tissue, but usually become sealed off by formation of a periderm barrier layer.  Also, cankers tend to dry out during the summer months thereby reducing viability of bacteria.  For that reason, twig cankers produced in plum and peach during the summer are not considered important overwintering sites or sources of inoculum for spring infections.  Generally, late shoot infections that occur just before leaf fall in autumn provide the primary inoculum source for the following spring (CABI, 2014; UCIPM, 2013).

HostsXanthomonas arboricola pv. pruni attacks only Prunus species, in particular fruit crops such as almonds, peaches, cherries, plums, apricots, P. salicina (Chinese/Japanese plum), and ornamental species of Prunus including P. davidiana (Chinese wild peach), Japanese apricot (P. mume), and P. laurocerasus (cherry laurel).  Generally, species of the Sino-Japanese group (P. japonica and P. salicina) are more susceptible than European plums (CABI, 2014; EPPO, 2013)

Symptoms: Symptoms may vary depending on the infected plant host and plant part.

On peach leaves, infection is first apparent on the lower leaf surface as small, pale green to yellow, circular or irregular areas with a light tan center. These spots become apparent on the upper surface as they enlarge, becoming angular and darken to deep-purple, brown or black.  Tissue immediately surrounding the diseased spots becomes yellow.  The spots may darken before they drop out giving a shot-hole appearance. Usually, spots are concentrated toward the leaf tip as bacteria accumulate in that area with droplets of rain or dew. Bacterial ooze may exude from the spots.  In severe infections defoliation may occur.  On peach fruit, small, sunken circular spots with frequently water-soaked margins or light green halos appear on the surface. Pitting and cracking occur near the spots as the fruit enlarges.  Gum may exude from bacterial wounds, especially after heavy rains.  Spring cankers appear on the top part of overwintering twigs before green shoots are produced.  These cankers initiate as small, water-soaked slightly darkened superficial blisters that extend 1-10 cm along the length of the twig or girdle it causing tip death or “black tip injury’.  The area below the dead tip harbors the bacteria.  Twigs that get infected late in season result in ‘summer cankers’ which are dark purple spots surrounding lenticels that later dry out and become limited, dark, sunken, circular to elliptical lesions.

On plum leaves: the shot-hole effect is more pronounced than on peach leaves.  On plum fruit symptoms vary from large sunken, black lesions to small pit-like lesions.  On plum and apricot, twig cankers are perennial developing on 2-3 year old twigs.  As a result, deep-seated cankers are formed in the inner bark thereby deforming and killing twigs.

On cherry leaves symptoms develop similar to peach but are rarely of importance.  Fruit may be distorted and bacteria usually internally inhabit fruit pulp.

On almond:  In California, damage has been predominant on the ‘Fritz’ variety however similar damage has been observed by researchers on ‘Monterey’, ‘Padre’, and ‘Nonpareil’ varieties (Holtz et al., 2013).  Symptoms on leaves, twigs and fruit are similar to those produced on peach.  Symptoms on infected almond nuts include the production of amber colored gum from spots on the hull which internally reveals a lesion. Lesions may enlarge, become sunken and orange in color, or exude an orange slime.  Furthermore, infected nuts may stick on spurs and be close to mummified, lesion nuts of the previous year.    Leaves may have spots, turn yellow and drop prematurely.  Twigs may have lesions or cankers.

Damage Potential:  The pathogen is capable of causing severe defoliation thereby weakening trees.  The leader (i.e., the vertical stem at the top of the trunk) dies and fruit is reduced in size and often not marketable.  Serious losses in peach (25-75%), plum and apricot production are reported from Australia, New Zealand, and the USA (CABI, 2014; EPPO, 2014).  Damage to stone fruit is more severe where the latter are grown in light, sandy soils than in heavier soils (UCIPM, 2013).

Transmission:  Local spread of the bacterial pathogen from cankers and mummified fruit is limited and dependent on dripping dew and splashing and/or wind-blown rain.  Long distance spread, as in international trade, is through infected plantings, budwood, and fruit (except seeds).

Worldwide Distribution: Asia (China, India, Iran, Japan, Korea DPR, Korea Republic, Lebanon, Pakistan, Saudi Arabia, Taiwan, Tajikistan); Africa (South Africa, Zimbabwe); Europe (Bulgaria, France, Italy, Moldova, Montenegro, Netherlands, Romania, Russia (Far East, Southern), Slovenia, Spain, Switzerland, Ukraine); North America (Bermuda, Canada, Mexico, USA); South America (Argentina, Brazil, Uruguay); Oceania (Australia, New Zealand).

In the USA it is present in Alabama, Arkansas, California, Connecticut, Florida, Georgia, Idaho, Kentucky, Louisiana, Maryland, Michigan, Mississippi, Missouri, New Jersey, New York, North Carolina, Oregon, Pennsylvania, South Carolina, and Texas.

Official Control: Ten countries list X. arboricola pv. pruni on their “Harmful Organism Lists’ namely, Canada, Chile, Ecuador, Israel, Madagascar, Mexico, Morocco, New Caledonia, Peru, and Turkey.  Whereas, 41 countries worldwide list X. campestris pv. pruni (synonym of X. arboricola pv. pruni) on their lists (USDA PCIT, 2014).  Xathomonas arboricola pv. pruni is listed as an A2 quarantine pest by EPPO and of little economic importance in EPPO countries where it is present.  Also, it is of quarantine significance for the Inter-African Phytosanitary Council/IAPSC (EPPO, 2014).

California Distribution:  The bacterial spot pathogen is relatively in California.  It has been found on almonds (mainly Fritz cultivar), in Colusa, Merced, Stanislaus, and San Joaquin Counties, as well as sweet cherry and other stone fruit crops in San Joaquin and Stanislaus Counties (UCIPM, 2013).

California Interceptions:  The pathogen was recently intercepted in a nursery in Sutter County.  The plants were destroyed (see ‘Initiating event’.)

The risk Xanthomonas arboricola pv. pruni would pose to California is evaluated below.

Consequences of Introduction: 

1)  Climate/Host Interaction: Evaluate if the pest would have suitable hosts and climate to establish in California. Score:

– Low (1) not likely to establish in California; or likely to establish in very limited areas
Medium (2) may be able to establish in a larger but limited part of California
High (3) likely to establish a widespread distribution in California.

Risk is High (2) The pathogen is limited to high moisture and warm temperature conditions and regions for establishment.

2)  Known Pest Host Range: Evaluate the host range of the pest:

– Low (1) has a very limited host range
– Medium (2) has a moderate host range
– High (3) has a wide host range.

Risk is Medium (2): The host range is limited to Prunus spp. stonefruit which is cultivated in vast acreage within California.  

3)  Pest Dispersal Potential: Evaluate the dispersal potential of the pest:

– Low (1) does not have high reproductive or dispersal potential
– Medium (2) has either high reproductive or dispersal potential
High (3) has both high reproduction and dispersal potential.

Risk is High (2): The pathogen increases and tends to cause infections in spring and its spread to non-infected tissue is dependent on warm temperatures and wet conditions brought about by wind-driven rainfall, water-splash and dripping dew.

4)  Economic Impact: Evaluate the economic impact of the pest to California using these criteria:

A.   The pest could lower crop yield.
B.   The pest could lower crop value (includes increasing crop production costs).
C.   The pest could trigger the loss of markets (includes quarantines).
D.   The pest could negatively change normal cultural practices.
E.   The pest can vector, or is vectored, by another pestiferous organism.
F.   The organism is injurious or poisonous to agriculturally important animals.
G.   The organism can interfere with the delivery or supply of water for agricultural uses.

– Low (1) causes 0 or 1 of these impacts
– Medium (2) causes 2 of these impacts
High (3) causes 3 or more of these impacts.

Risk is High (3): Infection of Prunus spp. could lower crop yield and value thereby resulting in a loss of market.

5)  Environmental Impact: Evaluate the environmental impact of the pest on California using these criteria:

A.   The pest could have a significant environmental impact such as lowering biodiversity, disrupting natural communities, or changing ecosystem processes.
B.   The pest could directly affect threatened or endangered species.
C.   The pest could impact threatened or endangered species by disrupting critical habitats.
D.   The pest could trigger additional official or private treatment programs.
E.   The pest significantly impacts cultural practices, home/urban gardening or ornamental plantings.

Score the pest for Environmental Impact:

– Low (1) causes none of the above to occur
– Medium (2) causes one of the above to occur
High (3) causes two or more of the above to occur.

Risk is High (3): Infection of ornamental Prunus species, in particular could impact residential and commercial cultivation of ornamental and fruit trees, requiring cultural practices to remove infected plant parts and mummified fruit.  In addition, official and private treatment programs may be needed to manage the pathogen.

Consequences of Introduction to California for Xanthomonas arboricola pv. pruni:

Add up the total score and include it here. (Score)

Low = 5-8 points
Medium = 9-12 points
High = 13-15 points

Total points obtained on evaluation of consequences of introduction to California = 12

6)  Post Entry Distribution and Survey Information: Evaluate the known distribution in California. Only official records identified by a taxonomic expert and supported by voucher specimens deposited in natural history collections should be considered. Pest incursions that have been eradicated, are under eradication, or have been delimited with no further detections should not be included. (Score)

-Not established (0) Pest never detected in California, or known only from incursions.
Low (-1) Pest has a localized distribution in California, or is established in one suitable climate/host area (region).
-Medium (-2) Pest is widespread in California but not fully established in the endangered area, or pest established in two contiguous suitable climate/host areas.
-High (-3) Pest has fully established in the endangered area, or pest is reported in more than two contiguous or non-contiguous suitable climate/host areas.

Evaluation Low (-1):  Xanthomonas arboricola pv. pruni has been detected on almond, sweet cherry and other stone fruit in four counties within the Central Valley of California.

Final Score:

7) The final score is the consequences of introduction score minus the post entry distribution and survey information score: (Score)

 Final Score:  Score of Consequences of Introduction – Score of Post Entry Distribution and Survey Information = 11


To date, Xanthomonas arboricola pv. pruni has been detected mainly in almond orchards in four California counties.   Targeted surveys for the detection of this relatively new pathogen may result in a wider distribution and range of host plants than currently known for the State.  If that occurs, then a lower rating than that proposed here is probable.  Therefore, diligent screenings and management of planting stock in nurseries will remain critical to mitigate risk of introduction of the pathogen to new, non-infected commercial production sites.

Conclusion and Rating Justification:

Based on the evidence provided above the proposed rating for Xanthomonas arboricola pv. pruni is B.


CABI.  2014.  Xanthomonas arboricola pv. pruni datasheet report.  Crop Protection Compendium.  www.cabi.org/cpc/

Du Plessis, H. 1988.  Differential virulence of Xanthomonas campestris pv. pruni for peach, plum, and apricot cultivars.  Phytopathology, 78 (10):1312-1315.

EPPO, 2014.  Xanthomonas arboricola pv. pruni (XANTPR).  PQR database.  Paris, France:  European and Mediterranean Plant Protection Organization.  http://newpqr.eppo.int

Holtz, B., D. Doll, R. Duncan, J. Edstrom, T. Michailides, and J. Adaskaveg.  2013.   http://www.ipm.ucdavis.edu/PDF/MISC/168605.pdf

UCIPM.  2013.  Bacterial spot (Xanthomonas arboricola pv. pruni) University of California Agriculture & Natural Resources, UC IPM Online, Statewide Integrated Pest Management Program.  http://www.ipm.ucdavis.edu/EXOTIC/bacterialspot.html

USDA PCIT.  2014.  USDA Phytosanitary Certificate Issuance and Tracking System.  Phytosanitary Export Database.  https://pcit.aphis.usda.gov/PExD/faces/ReportHarmOrgs.jsp

Responsible Party:

John J. Chitambar, Primary Plant Pathologist/Nematologist, California Department of Food and Agriculture, 3294 Meadowview Road, Sacramento, CA 95832. Phone: 916-262-1110, plant.health[@]cdfa.ca.gov.

Comment Period:  CLOSED

The 45-day comment period opened on June 1, 2015 and closed on July 16, 2015.


Posted by ls