Didymella bryoniae (Auersw.) Rehm [teleomorph] (Auersw.) Rehm

California Pest Rating for
Didymella bryoniae (Auersw.) Rehm [teleomorph] (Auersw.) Rehm
Pest Rating:  B

Initiating Event:

None.  The risk of infestation of Didymella bryoniae in California is evaluated and a permanent rating is herein proposed. 

History & Status:

BackgroundDidymella bryoniae is the fungal pathogen that causes gummy stem blight of cucurbits disease affecting members of the family Cucurbitaceae.  Gummy stem blight was first reported in 1891 in France, Italy and the United States and affects the leaves, stems, fruits and seeds of all cucurbits.   The disease is most common in subtropical and tropical regions globally but also found in temperate regions, especially on winter squash and pumpkin and greenhouse grown cucumber (Sitterly & Keinath, 1996).  The disease is most common in the southern United States, and in California, gummy stem blight was first reported in greenhouse-produced transplants of watermelon in the Salinas Valley (Koike, 1997).  Since 1997, there have not been any further reports of the pathogen in California.

Didymella bryoniae is the sexual (teleomorph) stage of the fungus that produces ascospores, while Phoma cucurbitacearum represents the asexual stage (anamorph) that produces conidia.  Didymella bryoniae was originally described as Sphaeria bryoniae, but since then has undergone several classifications that resulted in synonymizations of many different species.

Hosts: Members of the family Cucurbitaceae including wax gourd (Benincasa hispida), watermelon (Citrullus lanatus), melon (Cucumis melo), cucumber (C. sativus), pumpkin (Cucurbita sp.), giant pumpkin (C. maxima), ornamental gourd (C. pepo), buffalo gourd (C. foetidissima), bottle gourd (Lagenaria siceraria), bitter gourd (Momordica charantia), loofah (Luffa cylindrica), white bryony (Bryonia alba), red bryony (B. dioica), chayote (Sechium edule), and burcucumber (Sicyos angulatus) Occasionally, the pathogen has been found in members of Solanaceae, Caricaceae, and Primulaceae (CABI, 2015; Farr & Rossman, 2015).

Symptoms:  Didymella bryoniae invades the leaves and stems of watermelon, cucumber, and muskmelon (cantaloupe) and gummy stem blight diseased plants may exhibit a variety of symptoms which are referred to as leaf spot, stem canker, vine wilt and black fruit rot (Ferreira & Boley, 1992).  Initial spots on leaves, petioles and stems usually become pale brown or gray. On stems, spots usually start at the joints, frequently elongate into streaks and exude an amber-colored gummy liquid.  Lesions on leaves and fruit initiate as spreading water-soaked areas which in leaves may have a chorotic halo, become light brown and irregular in outline, and necrotic.  Leaves wilt and collapse; affected plants wilt and eventually die. On fruit, faded and irregular spots first form on the surface and eventually turn dark and may have a hardened droplet of exudates in the center; cracked sunken lesions form with internal rotting – especially in storage fruit.  In certain kinds of squash, lesions are superficial and spread almost over the entire fruit surface.  When seed-transmitted, the pathogen causes damping-off thereby, killing seedlings.  In the field, initial symptoms include plant collapse with sunken girdling cankers that result in total loss of plants.  Vine cankers are common near the crown of the plant.  (Agrios, 2005; CABI, 2015; Ferreira & Boley, 1992; Koike, 1997; Sitterly & Keinath, 1996.)

The main diagnostic symptoms are the gummy exudates on stem and fruit lesions, and the presence of abundant closely spaced groups of pale-colored pycnidia (asexual fruiting body) and dark brown to black-colored perithecia (sexual fruiting body) on fruit, stem or leaves.  During rainy seasons lesions can become water-soaked and spread resulting in severe defoliation.  Gummy substances may exude from cracks, and severe infections can result in death of plants (CABI, 2015).

Damage Potential: Didymella bryoniae has the potential of damaging plant growth causing reductions in plant growth, death of infected plants, fruit rot, and seedling death resulting in significant crop losses.

Disease Cycle:   The pathogen usually overwinters in diseased plant debris as chlamydospores and possibly in or on seeds.  Subsequently, spores or infected seed result in primary infection of plants. Cucurbit plants are predisposed to infection by wounds and bruises although uninjured plants have also been shown to become infected when exogenous nutrients are present (Ferreira & Boley, 1992).  The disease thrives in cool moist climates with an optimum temperature of 20-28 C for development.  Moisture, especially extended periods of wetness, is necessary for infection (at least 1 hour) and disease development.  Leaves are penetrated directly through the cuticle or through intercellular spaces around the bases of trichomes. Stems are penetrated through wounds and fruit are penetrated through wounds or flower scars at the time of pollination.  Fruit rot initiates approximately 3 days following infection.  Following penetration and development, the fungus produces numerous pycnidia and perithecia.  Pycnidia are filled with conidia (spores) that protrude from the fruiting body in a gelatinous substance appearing as long cream to pink tendrils.  Water dissolves this gelatinous substance and the conidia are dispersed usually by wind and rain.  Perithecia are also produced along with pycnidia and filled with ascospores. Both types of spores serve as inoculum for infection.  Neither type survives long after dispersal (Agrios, 2005; Ferreira & Boley, 1992; Sitterly & Keinath, 1996).

Transmission:  Infected planting material (transplants), infected fruit, plant debris, weeds, and soil.  In addition, conidia can be transmitted by air transport and water splashing (Ferreira & Boley, 1992).  Seed transmission has only been demonstrated experimentally (Brown et al., 1970; CABI, 2015; Lee et al., 1984).  Reports of seed transmission are conflicting and there is no evidence that seed transmission occurs naturally although fruiting bodies (pycnidia and perithecia) of the pathogen have been found on naturally infected cucumber and pumpkin seeds (CABI, 2015).  The striped cucumber beetles (Diabrotica undecimpunctata howardii and Acalymma vittatum) are believed to transmit D. bryoniae in a nonpersistent manner by providing wounds in plants as avenues for fungal infections. It has also been shown experimentally that cucumber plants infested with melon aphids were susceptible to D. bryoniae and powdery mildew (Ferreira & Boley, 1992).

Worldwide Distribution:  Didymella bryoniae is distributed worldwide in several countries in Asia, Africa, North America, Central America and Caribbean, South America, Europe, and Oceania (CABI, 2015).

In the USA, it has been reported from Florida, Georgia, New York, North Carolina, and South Carolina.

Official ControlDidymella bryoniae is included on the ‘Harmful Organism Lists’ for nine countries namely: Bangladesh, Ecuador, Egypt, Indonesia, Iran, Israel, Panama, Syrian Arab Republic and Timor-Leste (PCIT, 20115).  It is a quarantine pest in Jordan (EPPO, 2015).   Currently, it is an actionable, temporary ‘Q’-rated pathogen in California.

California Distribution: Didymella bryoniae has only been reported once in 1997 in Salinas in greenhouse watermelon transplants.  The diseased plants would have been destroyed following detection.  The pathogen has not been reported since then and is not known to be established in California.

California Interceptions:  None reported.

The risk Didymella bryoniae 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 Medium (2) Didymella bryoniae requires cool and moist conditions to infect cucurbit hosts and cause gummy stem blight disease.  At least 1 hour of wetness is required for infection and extended periods for disease development.  This may limit the establishment of the disease in cucurbit productions in California and may also be why this disease has not been observed outside greenhouse production in California since the early 1990s.  Therefore, a ‘medium’ rating is given to this category.

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

– 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 mainly limited to various species of the Cucurbitaceae family.  Nevertheless, Cucurbitaceous hosts, including watermelon, melon, squash and cucumber, are widely grown commercially within California.

3) Pest Dispersal Potential: Evaluate the natural and artificial dispersal potential of the pest. Score:

– 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 (3) – Under favorable environmental conditions Didymella bryoniae has a high reproductive rate and depends on wind and moisture or rain for its short distance dispersal.  Infected planting material, infected fruit, plant residues, weeds, soil, and possibly seed provide the means for long distance dispersal, also in fields and greenhouses. In addition, the striped cucumber beetles are believed to transmit D. bryoniae in a nonpersistent manner by providing wounds in plants as avenues for fungal infections.  

4) Economic Impact: Evaluate the economic impact of the pest to California using the criteria below. Score:

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) –  The cucurbit gummy stem blight pathogen could lower crop value and yield, cause increases in production costs for disease management, and negatively change normal cultural practices to mitigate potential damages.  There is also the possibility for the pathogen to be vectored by pestiferous striped cucumber beetles.  Therefore, a ‘high’ rating is given to this category.   

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

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. Score:

– 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 Medium (2) – Cucurbitaceous plants grown in home/urban gardens could be negatively impacted if infected with Didymella bryoniae.

Consequences of Introduction to California for Didymella bryoniae:

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 of Didymella bryoniae to California = Medium (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 is not established in California (0)

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 =12 (Medium).


Infested seeds and transplants are a ready source of introduction of the pathogen to CA fields.  The future status of Didymella bryoniae, gummy stem blight of cucurbits disease, can be known through periodic surveys, diligent monitoring and testing of seed/plants in greenhouses and will be necessary in order to mitigate risk of field introduction and potential establishment of the pathogen in California soils.

Conclusion and Rating Justification:

Based on the evidence provided above the proposed rating for the anthracnose pathogen, Didymella bryoniae is B.  


Agrios, G. N.  2005.  Plant Pathology Fifth Edition.  Elsevier Academic Press.  USA. 922 p.

Brown, M. E., E. M. Howard, and B. C. Knight. 1970. Seedborne Mycosphaerella melonis on cucumber. Plant Pathology, 19:198.

CABI.  2015.  Didymella bryoniae (gummy stem blight of cucurbits) datasheet (full) report.  Crop Protection Compendium.  www.cabi.org/cpc/

EPPO.  2015.  Stagonosporopsis cucurbitacearum (DIDYBR).  European and Mediterranean Plant Protection Organization PQR database.  http://www.eppo.int/DATABASES/pqr/pqr.htm .

Farr, D. F., and A. Y. Rossman.  2015.  Fungal Databases, Systematic Mycology and Microbiology Laboratory, ARS, USDA. Retrieved July 14, 2015, from http://nt.ars-grin.gov/fungaldatabases/

Ferreira, S. A., and R. A. Boley.  1992.  Didymella bryoniae Gummy stem blight, black rot, canker (Plant Disease Pathogen).  http://www.extento.hawaii.edu/kbase/crop/Type/d_bryon.htm

Koike, S. T.  1997.  First report of gummy stem blight, caused by Didymella bryoniae, on watermelon transplants in California.  Plant Disease, 81:1331. http://dx.doi.org/10.1094/PDIS.1997.81.11.1331B.

Lee, D. H., S. B. Mathur, and P. Neergaard. 1984. Detection and location of seed-borne inoculum of Didymella bryoniae and its transmission in seedlings of cucumber and pumpkin. Phytopathologische Zeitschrift, 109(4):301-308.

PCIT.  2015.  USDA Phytosanitary Certificate Issuance & Tracking System. July 21, 2015.  https://pcit.aphis.usda.gov/PExD/faces/ReportHarmOrgs.jsp .

Sitterly, W. R., and A. P. Keinath.  Gummy stem blight.  1996.  In, Compendium of cucurbit diseases.  Compendium of cucurbit diseases ed. T. A. Zitter, D. L. Hopkins, and C. E.  Thomas, APS Press  pg 27-28. http://www.apsnet.org/publications/apsnetfeatures/Pages/GummyStemBlight.aspx .

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.

 Pest Rating:  B

Posted by ls