Diaporthe vaccinii Shear 1931

California Pest Rating for
Diaporthe vaccinii Shear 1931
Pest Rating: C

Initiating Event:

On February 3, 2017, CDFA was requested by the USDA APHIS for information on the export of Vaccinium plants from California to the EU, in preparation of a federal risk assessment of the introduction of a quarantine fungal pathogen, Diaporthe vaccinii into the EU through USA-originated Vaccinium spp.  Subsequently, the status and risk of this pathogen in California is assessed here and a permanent rating is proposed. 

History & Status:

BackgroundDiaporthe vaccinii is also known by its asexual/anamorph name, Phomopsis vaccinii, and causes stem cankers, twig blight, leafspots, and fruit rot of Vaccinium spp. (blueberries and cranberries) (EFSA, 2014).  While D. vaccinii is considered the prominent species of Diaporthe on Vaccinium spp. worldwide, there are other species within Diaporthe and Phomopsis that attack Vaccinium spp. causing diseases that include stem cankers, twig blight, and fruit rot similar to D. vaccinii  (EFSA, 2014).  Also, as symptomless (latent) infections of D. vaccinii may occur, diagnosis of the disease based on symptoms alone is not reliable but can be obtained through molecular analysis.

Diaporthe vaccinii is regarded as native to North America and has been reported from Vaccinium-growing regions in the USA and Canada (Lombard et al., 2014). During the 1960-70s twig blight disease of blueberries became a serious problem in blueberry-growing regions of Wisconsin, Indiana, and southern Michigan, and in the 1980-90s increased tremendously in prevalence and severity in the southeastern USA, particularly North Carolina (Milholland, 1995).  However, D. vaccinii is not known to be present in California and Vaccinium spp. originating in California and shipped under certification to international trading partners, continue to test free of the pathogen by CDFA (Heaton, 2017).

In California, blueberry production has been increasing over the past decade.  Blueberry cultivation is done mostly in cool northern coastal regions, however, southern cultivars with low chilling-hour requirements needed to break dormancy are also farmed in the San Joaquin Valley and southern coastal regions Bremer et al., (2008).  In 2015, blueberries were cultivated on 5,700 harvested acres in California yielding a production of 624,000 cwt for a total value of $116,979 (CASR, 2015-2016).

Disease Development The epidemiology of the fungus has been studied in the USA.  The pathogen overwinters in infected and dead twigs, and possibly on plant debris (fallen twigs, leaves and fruits).  Ascospores and conidia are disseminated in the crop under wet and humid conditions.  In North Carolina, rain-dispersed conidia of the anamorph Phomopsis vaccinii have been trapped throughout the growing season with the largest numbers trapped between blossom budbreak to bloom (EPPOa, 2016; Milholland, 1982).

The pathogen enter host tissues mainly through wounds and to a lesser extent directly into the tips of young, succulent shoots,  Healthy unwounded blueberry plants were not infected even after one month of exposure to natural field inoculum (EFSA, 2014). Once the fungus enters the stem through the vascular tissue, it progresses downwards towards the base, girdling the old branches at their junction and killing the part of the plant above the girdle (CABI, 2017).  The fungus also enters host vascular tissues through open flower buds and, it is believed that blueberry blight develops primarily from infection of flower buds at budbreak through bloom in North Carolina (Milholland, 1982).   Conidia on germination enter berries throughout the growing season and remain dormant until maturation causing soft rot and leakage of juice at harvest (Milholland & Daykin, 1983).  Diaporthe vaccinii has been reported to be an endophyte of apparently healthy blueberry and cranberry stems (CABI, 2017).

Germ tubes of germinating conidia enter leaves producing spots.  About 2-3 weeks later, pycnidia with conidia are apparent on stems and leaf spots.  The pathogen has been isolated from fruiting bodies found on overwintered cranberry leaves in New Jersey, but not  in Wisconsin from vines collected in the spring from beds in which dieback had been very severe in the late summer of the preceding year (Friend & Boone, 1968).  Overwintering was indicated to be necessary for ascocarp (sexual fruiting body) development, completing the life cycle, perpetuating the species, and producing a source of inoculum for infection in the next season.  However, in the southeastern USA, the pathogen is reported to overwinter in infected blueberry twigs and produce conidia from pycnidia (asexual fruiting body) in the following year (EFSA, 2014).  A correlation has also been indicated between vine dieback and dry conditions, with the latter predisposing the plant to dieback (Friend & Boone, 1968).

The pathogen grows well in a wide temperature range of 4-32°C and optimum pH 5-6.  In experiments, the most favorable temperature range for conidium germination and growth was 21-24°C wherein 95% conidia germinated and either entered plants through wounds or directly at the tips of young succulent blueberry shoots held inside a damp chamber.  About 71% shoots became blighted four days after inoculation. In artificially inoculated plants, the fungus caused cankers and dieback symptoms above 30°C (Weingartner and Klos, 1975).

Dispersal and spread: Long distance dispersal occurs through movement of infected plant vines (EPPOa, 2016). Other modes for spread include infected plant debris, leaves, twigs, and fruit, rain/irrigation water splash.

Hosts: Principal hosts include Vaccinium macrocarpon (cranberry), V. oxycoccos (small cranberry), V. oxycoccos var. intermedium (Americana and European cranberries), V. corymbosum (highbush blueberry), V. ashei (rabbiteye blueberry).  While D. vaccinii is restricted to Vaccinium species, the wild European species, V. oxycoccos which usually occurs in mountain bogs could be a reservoir host for the pathogen (EPPOa, 2016).  Other hosts are Gaultheria shallon (salal), Rhododendron sp. (Farr & Rossman, 2017).

Symptoms:  In North Carolina, the predominant symptom was blighting in one-year-old susceptible blueberry cultivars. Systemic invasion has also been reported (Milholland, 1982).  Infected succulent, current-year shoots wilt in 4 days and are covered with minute lesions.  Major branches and frequently entire plants are killed as the fungus continues to travel downwards through the stem at an average rate of 5.5 cm in 2 months.  Regardless of the stem, cankers are long and narrow and are covered by the bark or epidermis.  On blueberry stems over two-years-old, a brown discoloration of the stem xylem below wilt symptoms can be observed.  However, inoculated stems only produce localized lesions.  Infected leaves develop spots which enlarge to 1 cm with pycnidia/conidiomata appearing in two weeks.  The pathogen may remain dormant until favorable conditions allow it to continue growing.  Infection of crowns frequently end in the death of stem originating from the crown.  Infected fruits turn reddish-brown, soft, mushy, often split and leak juice at harvest (CABI, 2017; EPPOa, 2016).  The fungus penetrated blueberry fruit at all stages of development and remains latent until maturation (Milholland and Daykin, 1983).

In cranberry, Diaporthe vaccinii does not cause twig blight disease similar to blueberry, but occurs on shoots and leaves without causing significant damage (CABI, 2017).  It is also a storage rot pathogen of cranberries, mainly causing a viscid rot of fruit, which becomes soft and discolored.  Also, infected upright stems turn yellow then orange and brown before dying back (Milholland, 1995).

It is important to note that in blueberry, symptoms similar to twig blight disease can be caused by other fungal pathogens such as Godronia cassandrae, Colletotrichum spp., Fusarium spp., and Botryosphaeria dothidea.  In cranberry, upright dieback is also caused by the fungus, Synchronoblastia crypta (EPPOc, 2009; CABI, 2017).

Damage Potential:    Fruit loss of two to three pints per bush with twig blight of blueberry disease were reported in North Carolina (Milholland, 1982).  Fruit loss of 0.5% out of 15.2% defective fruit were accredited to D. vaccinii (Milholland & Daykin, 1983).  The pathogen is not considered to cause appreciable economic loss in cranberry, except in Massachusetts.  Fruit loss has been considered minor due to D. vaccinii, being attributed more to the presence of other accompanying pathogens than to D. vaccinii (CABI, 2017).

Worldwide Distribution: Asia: China; Europe: Latvia (present with restricted distribution); North America: Canada, USA; South America: Chile (CABI, 2017; Farr & Rossman, 2017; EPPOa, 2016).   The pathogen was eradicated or no longer present in most of the EU (EPPOa, 2016).  In the USA, it has been found in Arkansas, Illinois, Indiana, Maine, Maryland, Massachusetts, Michigan, New Jersey, North Carolina, Oregon, Washington, and Wisconsin.

Official Control: Diaporthe vaccinii is listed as a quarantine pest for the European Union (EPPOb, 2016).  The pathogen is on the ‘Harmful Organism’ lists for Argentina, China, Ecuador, Guatemala, India, Israel, Mexico, Morocco, New Zealand, Norway, Peru, and Taiwan (USDA PCIT, 2017).

California Distribution: Diaporthe vaccinii is not known to be established in California.

California Interceptions: None.

The risk Diaporthe vaccinii would pose to California is evaluated below.

Consequences of Introduction: 

1) Climate/Host Interaction: Blueberry is the main host for Diaporthe vaccinii in California.  Blueberries are grown in northern coastal and southern coastal regions and in the San Joaquin Valley.  The pathogen grows well within a wide temperature range (4-32°C) and requires wet and humid conditions for spore dispersal and germination and fungal growth. Humid conditions along the coast may be more conducive for the pathogen than the drier environments of the San Joaquin Valley.  A ‘Medium’ score is given for climate-host interaction.

Evaluate if the pest would have suitable hosts and climate to establish in California.

Score: 2

– 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.

2) Known Pest Host Range: Vaccinium are the main host for Diaporthe vaccinii.

Evaluate the host range of the pest.

Score: 1

Low (1) has a very limited host range.

– Medium (2) has a moderate host range.

– High (3) has a wide host range.

3) Pest Dispersal Potential: The pathogen has high reproductive capability resulting in production of numerous ascospores, and conidia, however, these are primarily dependent on water splash for dispersal. Long distance spread occurs primarily through movement of infected plants. A ‘Medium’ rating is given to this category.

Evaluate the natural and artificial dispersal potential of the pest.

Score: 2

– 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.

4) Economic Impact: Under suitable environmental conditions, Diaporthe vaccinii may infect blueberries causing storage rot of mature fruit causing significant losses in crop yield, value and market.

Evaluate the economic impact of the pest to California using the criteria below.

Economic Impact: A, B, C

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.

Economic Impact Score: 3

– 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.

4) Environmental Impact: No significant impact on the environment is expected.

Evaluate the environmental impact of the pest on California using the criteria below.

Environmental Impact:  None

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.

Environmental Impact Score: 1

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.

Consequences of Introduction to California for Diaporthe vaccinii: Low (9)

Add up the total score and include it here.

-Low = 5-8 points

Medium = 9-12 points

-High = 13-15 points

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.

Evaluation is ‘Not established’ (0).

Score: (0)

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.

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 = 9



Conclusion and Rating Justification:

Based on the evidence provided above the proposed rating for Diaporthe vaccinii is C.


Bremer, V., G, Crisosto, R. Molinar, M. Jimenez, S. Dollahite, and C. H. Crisosto.  2008.  San Joaquin Valley blueberries evaluated for quality attributes.  California Agriculture, 62 (3): 91-96.  http://CaliforniaAgriculture.ucop.edu

CABI.  2017.  Phomopsis vaccinii (Phomopsis twig blight of blueberry) full datasheet.  Crop Protection Compendium.  http://www.cabi.org/cpc/datasheet/18747

CASR.  2015-2016. California agricultural statistics review 2015-2016.  California Department of Food and Agriculture.  https://www.cdfa.ca.gov/Statistics/PDFs/2016Report.pdf

EFSA.  2014.  Scientific opinion on the pest categorization of Diaporthe vaccinii Shear.  European Food Safety Authority (EFSA), Parma, Italy.  EFSA Journal 12: 3774.

EPPOa.  2016.  Diaporthe vaccinii data sheets on quarantine pests.  Prepared by CABI and EPPO for the EU under contract 90/399003.   https://www.eppo.int/QUARANTINE/data_sheets/fungi/DIAPVA_ds.pdf

EPPOb.  2016.  EPPO A2 list of pests recommended for regulation as quarantine pests (version 2016-09).  https://www.eppo.int/QUARANTINE/listA2.htm

EPPOc.  2009.  Diaporthe vaccinii Diagnostics.  OEPP/EPPO Bulletin 39, 18-24.

Farr, D. F., and A. Y. Rossman.  2017.  Fungal Databases, Systematic Mycology and Microbiology Laboratory, ARS, USDA. Retrieved March 13, 2017, from http://nt.ars-grin.gov/fungaldatabases/

Friend, R. J., and D. M. Boone.  1968.  Diaporthe vaccinii associated with dieback of cranberry in Wisconsin. Plant Disease Reporter, 52:341-344.

Heaton, J.  2017.  J. Heaton, CDFA, email to D. Schnabel, cc: T. Walber and J. Chitambar, CDFA, sent Friday, March 10, 2017 9:01:02 am.

Lombard, L., G. C. M. Van Leeuwen, V. Guarnaccia, G. Polizzi, P. C. J. Van Rijswick, K. C. H. M. Rosendahl, J. Gabler, and P. W. Crous.  2014.  Diaporthe species associated with Vaccinium, with specific reference to Europe.  Phytopathologia Mediterranea 53: 85-97.

Milholland, R. D.  1982.  Blueberry twig blight caused by Phomopsis vaccinii. Plant Disease, 66:1034-1036.

Milholland R. D. 1995.  Phomopsis twig blight and fruit rot.  In Compendium of Blueberry and Cranberry Diseases.  APS Press, The American Phytopathological Society, pg. 13-14.

Milholland, R. D., and M. E. Daykin.  1983.  Blueberry fruit rot caused by Phomopsis vaccinii.  Plant Disease 67: 325-326.

USDA PCIT.  2017.  USDA Phytosanitary Certificate Issuance & Tracking System. Retrieved March 13, 2017. 6:04:39 pm CDT.  https://pcit.aphis.usda.gov/PExD/faces/ReportHarmOrgs.jsp.

Weingartner, D. P., and E. J. Klos.  1975.  Etiology and symptomatology of canker and dieback diseases on highbush blueberries caused by Godronia (Fusicoccum) cassandrae and Diaporthe (Phomopsis) vaccinii. Phytopathology, 65(2):105-110

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.

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Pest Rating: C

Posted by ls