acidovorax citrulli | Pest Rating Proposals and Final Ratings

California Plant Pest Rating for

Acidovorax citrulli (Schaad et al., 1978) Schaad et al., 2008

Pest Rating: A

PEST RATING PROFILE

Initiating Event:

In July 2013, the bacterial fruit blotch pathogen (BFB), Acidovorax citrulli, was detected in a melon field (Cucumis melo) in Yolo County. This detection marked the first official record of the bacterial fruit blotch disease in California. Three contiguous fields planted to cucumber (2 fields) and watermelon (1 field), were also tested, however only melon was found infected with BFB. Subsequent trace back investigations revealed that the source seed for the Yolo County melon site was grown in a seed lot in Sutter County in 2012 and later, was found positive for BFB (and Cucumber green mottle mosaic virus – which is not evaluated in this proposal). The 2013 trace back also revealed that in 2012, two sites in Sutter County produced a total of 6 melon and watermelon seed lots that were positive for BFB. Those two sites are currently planted to non-hosts, and volunteer plants of 2013 tested negative for BFB. Two foreign sources of melon and cucumber seed lots planted in the two Sutter County sites were identified as Chile and Romania. However, no (melon) seed remained for testing from the Chilean source, and the source cucumber seed from Romania was not tested for BFB as the progeny seed tested negative for the pathogen. As for the 2013 Yolo County BFB positive site, County and State approved abatement measures were implemented. Wheat, a non-host, was grown at the site and in 2014, volunteer melon plants tested negative for BFB. To date, additional monitoring and testing of volunteer plants continues and those in the field are treated with herbicide and biodegraded. Furthermore, in 2013, approximately 120 trace forward seed lots were evaluated for risk of potential infection with BFB. Pathways identified as possible risk links for potential BFB infection were source seed, shared irrigation, proximity to a positive detection, mechanical transmission (equipment and workers), and seed processing operational steps. Trace forward investigations revealed that 2012 Sutter County melon seeds had been sent to San Benito County and tested positive for BFB. Also, BFB infested watermelon seed had been sent to San Benito County as well as shipped overseas to Lebanon, Italy and Africa. To date, 34 trace forward and trace back seed lots were sampled and tested for BFB, of which 6 were positive for the pathogen. Any remaining seeds from positive seed lots are currently on hold in Sutter and San Benito Counties pending destruction or CDFA approved treatment and proven freedom from the bacterium. Given this current detection and subsequent actions taken, A. citrulli is presently considered a quarantine, actionable pathogen that is temporary, transitional and under eradication, and therefore, not established within California. A permanent pest rating for the pathogen is herein proposed.

History & Status:

Background: Acidovorax citrulli is the causal agent of bacterial fruit blotch and seedling blight of cucurbit plants. The disease is seed transmitted and is a serious pathogen particularly of watermelon and melon worldwide.

Bacterial fruit blotch of cucurbits was first reported in 1965 causing seedling blight symptoms on watermelon seedlings at the USDA regional plant introduction station in Griffin, Georgia, USA. The causal agent was determined to be seed-borne and seed transmissible with the capability of surviving up to 7.5 years in stored seed. Seedling blight was also observed in squash and melon that were artificially inoculated with the pathogen. In 1969, the disease was observed on watermelon at a research farm in Leesburg, Florida, USA, and in 1978 it was observed in Queensland, Australia. The causal agent was identified as Pseudomonas pseudoalcaligenes subsp. citrulli (Schaad et al., 1978). The first outbreak of BFB in commercial watermelon fields was observed in the Mariana Islands (Guam and Tinian). In 1989, the first outbreak of BFB in the continental USA was in major watermelon producing states including Florida, South Carolina and Indiana. Between 1990 and 1995 sporadic and devastating outbreaks occurred in watermelon production regions of the Midwest, Northeast and Southern USA. Despite the implementation of stringent BFB management guidelines developed by academic and industry professionals, BFB outbreaks were reported in cucurbits other than watermelon in 1999 (Walcott, 2005, 2008). In 2013, BFB was first detected in California and consequently treated as a state actionable pathogen (CDFA-PEA, 2013).

Taxonomically, the pathogen belongs to the family Comamonadaceae in the order Burkholderiales. As a result of further taxonomic studies in 1992, Pseudomonas pseudoalcaligenes subsp. citrulli was found to be molecularly closely related to the genus Acidovorax and was subsequently reassigned (Willems et al., 1992). In 2008, Acidovorax avenae subsp. citrulli was reclassified and elevated to species rank, namely, A. citrulli (Schaad et al., 2008).

Disease cycle: The disease cycle begins with contaminated seed. Seedborne inoculum is the most important source of primary inoculum for BFB development in cucurbit seed and fruit production fields as well as in transplant production houses. Acidovorax citrulli has been detected on the external seed surface as well as internally on the papery perisperm-endosperm layer (Kurowski, 2014). In seed and fruit production fields, other sources of inoculum include infected seedlings, the previous season’s plant debris, and volunteer watermelon seedlings or cucurbitaceous weeds. In the USA watermelon production is mostly from dry seeded plants, although transplants are increasing in popularity (CABI, 2014). Contaminated seed result in infected seedlings. Warm temperatures and high relative humidity are favorable for the development of the disease. Under such conditions, bacteria from infested seed infect emerging and developing cotyledons. Little is known about the epidemiology of BFB in seed production environments. Under cool temperatures and low moisture conditions typical BFB fruit and foliar symptoms may not develop however, even with the lack of symptoms infested seeds are produced (Walcott, 2008).

Under field conditions, high relative humidity and rainfall are critical for the spread of bacterial cells through splash dispersal by wind-driven rain or overhead irrigation to uninfected plants. Bacterial cells are also spread by contact with cultivation equipment, irrigation and human contact. Once deposited on uninfected plants, bacterial cells swim through open stomata and leaf wounds and penetrate intercellular sub-epidermal spaces where they rapidly multiply causing the production of water soaked lesions and secondary inoculum. Acidovorax citrulli does not move systemically through the plant. Under favorable conditions, a few primary infections sites can result in widespread infection of all plants by the time of harvest. Leaf lesions that develop are reservoirs of bacteria for fruit infection. At anthesis (fruit set), bacteria are splashed dispersed by wind-driven rain or irrigation onto the surfaces of immature ovaries which they penetrate through open stomata thereby initiating fruit infection (Walcott, 2008). Watermelon fruit are most vulnerable to bacterial infection 2-3 weeks after anthesis. After this brief infection window a wax layer is deposited over the surface of fruit covering stomata and preventing bacteria from penetrating (Frankle et al., 1993). Once wax is deposited, bacteria are only able to enter mature fruit through wounds. Symptoms of fruit infection do not usually develop until about 2 weeks before harvest maturity. The exact reason for this latency period is unknown but may be influenced by fruit physiology (ASTA, 2009). Debris or seed from infested and rotten fruit, cucurbitaceous weeds, and volunteer cucurbit plants may allow the pathogen to over-season and perpetuate the disease cycle in the next cropping cycle.

Typical transplant house conditions favor development of Bacterial fruit blotch. Those conditions include high watermelon seedling populations, high temperatures high relative humidity and overhead irrigation. Infected seeds serve as the source of primary inoculum and bacterial cells from a few infected seedlings can rapidly spread throughout a greenhouse by splash dispersal and production of aerosols. As in field conditions, dispersed bacterial cells are deposited on uninfected seedlings which are penetrated through open stomata to intercellular spaces eventually leading to numerous secondary infection cycles. Asymptomatic plants with epiphytic A. citrulli cells can cause disease outbreaks under field conditions. Infected seedlings produced in transplant greenhouses can result in introduction and widespread development of BFB in the field.

Hosts: Acidovorax citrulli are pathogenic to various species of the Cucurbitaceae family. The most susceptible hosts are watermelon (Citrullus lanatus) and melons (Cucumis melo) including cantaloupe and honeydew melons which develop symptoms on fruit and leaves. Other cucurbits include cucumber (Cucumis sativus), squash (Cucurbita pepo), and Cucurbita moschata which only develop symptoms on foliage and not on fruit. Wild cucurbits are also hosts, such as, wild Citron (Citrullus lanatus var. citroides) that develops symptoms on leaves and fruit with seed transmission. Host range studies using artificial inoculations of the bacteria have resulted in symptoms on several solanaceous hosts including tomato, eggplant and pepper foliage, but not on fruit. Bacteria have been isolated from tomato seeds and foliage of betel pepper (Piper betle).

Symptoms: Acidovorax citrulli can attack all stages of cucurbit plants. Initial symptoms are water-soaked areas along the edges or the midveins on the undersides of cotyledons. In time the water-soaked lesions dry and become reddish to dark brown along the length of the midrib. In severe situations lesions coalesce on cotyledons, hypocotyls and stems and cause seedling collapse. On true leaves water-soaked lesions and reddish-brown to dark brown water soaked lesions are present along the leaf veins. Leaf symptoms may become inconspicuous over time.

Symptoms on fruit may be conspicuous or inconspicuous. Symptoms on watermelon fruit characteristically appear as small, dark olive, greasy irregularly shaped, blotch-like water-soaked spots on the rind of the upper surface of infected fruit. The lesions may rapidly enlarge from a few millimeters in diameter to cover the entire upper surface of the fruit leaving only the ground spot symptomless. In advance stages, the infected site becomes necrotic, cracks develop from which sticky, amber-colored ooze may exude. Internally, the lesions may penetrate into the flesh of the fruit causing rot. On fruit with netting, such as melon, symptoms initiate as small water-soaked spots. In muskmelons, netting does not form over lesions and the lesions do not expand on the rind surface but appears a small inconspicuous pits which extend into the flesh of the fruit forming rotten cavities (Walcott, 2008). In honeydew, initial water-soaked spots over time become brown and cracked with a water-soaked margin.

Damage Potential: Bacterial fruit blotch is the most economically important bacterial disease of cucurbits worldwide (Walcott, 2008). With its outbreak in 1989 in commercial watermelon fields in the continental USA, losses of more than 90% if the total marketable fruit occurred in some affected fields. Since then, although in most years BFB has occurred in relatively few fields, it has continued to be a threat to the watermelon industry and cucurbit seed, transplant and fruit producers, resulting in marketable watermelon fruit losses of over $100,000 to individual growers. In 1992, in Georgia thousands of hectares were lost to the disease (CABI, 2014).

Under favorable environmental conditions fruit losses of 90-100% have been observed in commercial watermelon fields in the USA. In 2000 estimated losses of 40-50% to 100% occurred in melon fields. A survey of 18 melon fields resulted in the detection of 4% to 47% of the disease in all fields (EPPO, 2014).

Transmission: Infested seed is the main mode of transmission of Acidovorax citrulli. The bacteria is also spread through infested seedlings, host crop debris, cucurbitaceous weeds, volunteer cucurbit plants, overhead irrigation, wind driven rain, contaminated cultivation equipment and tools, and human contact.

In field experiments, honey bees were shown to transmit the pathogen to female watermelon blossoms resulting in infection of 25% of seedlots although no fruit symptoms were observed (Fessehaie et al., 2005). Honey bees are not used for pollination in commercial seed production.

Worldwide Distribution: The geographical origin of Acidovorax citrulli is not known, however, the disease has spread to many regions of the world mainly through the passage of contaminated seeds.

Bacterial fruit blotch is distributed in Asia (China, Japan, Republic of Korea, Taiwan, Thailand, Turkey, Iran, Israel, and Malaysia), Europe (Greece, Hungary, Italy), Oceania (Queensland: Australia, Guam, Northern Mariana Islands), South America (Brazil), Central America and Caribbean (Costa Rica, Trinidad and Tobago), and North America (Canada, USA).

Within USA it is distributed in Alabama, Arkansas, Delaware, Florida, Georgia, Iowa, Illinois, Indiana, Maryland, Mississippi, Missouri, North Carolina, Oklahoma, Oregon, South Carolina, and Texas. The disease was detected in 1989 in Delaware, Iowa and Maryland but has not been reported since then (CABI, 2014; EPPO, 2014). The detection in California is considered transient, actionable and under eradication.

Official Control: Presently, 16 countries have included A. avenae subsp. citrulli (syn. of A. citrulli) on their ‘Harmful Organism Lists’. Those countries are; Chile, China, Colombia, Costa Rica, Ecuador, Guatemala, Honduras, India, Israel, Japan, Mexico, Nicaragua, Panama, Peru, Taiwan, and Turkey. Acidovorax citrulli is a quarantine pathogen in California, USA.

California Distribution: There are no official reports of BFB being established in field or natural environments within California – that would indicate an established distribution. The 2013 field detection in Yolo County, California is considered transient, actionable and under eradication.

California Interceptions: There are no state reports of Acidovorax citrulli detections in plant materials intercepted within or at points of entry in California.

The risk Bacterial fruit blotch 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): Although California climate is not considered conducive for the establishment (development and increase) of A. citrulli, the presence of suitable microenvironments and macro/micro climate changes enable establishment and perpetuation of pathogenic strains within California as evidence by the 2013 field outbreak in Yolo County, California.

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 to warrant of establishment and spread of this pathogen.

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 BFB has a high reproductive rate and depends on artificial means for its dispersal. Infested seeds are the main means of long distance dispersal, and in fields and greenhouses, the bacterium is capable of widespread transmission through contaminated plant materials and debris, equipment, water-splash, human contact and possibly insects.

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): Incidence of the BFB pathogen could result in lowered crop yields, crop value, increased production costs, loss of markets due to the imposition of quarantine rules and regulations, alteration of cultural practices and possible interference with the delivery of irrigation water where a supply is shared between fields.

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 High (3): incidence of A. citrulli could result in additional official or private treatment programs as well as significantly impact cultural practices and home/urban gardening.

Consequences of Introduction to California for Acidovorax citrulli:

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 = 13 (High).

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: Acidovorax citrulli is not established in California (0). Actions taken subsequent to the detection of the pathogen in Yolo County and detailed in the “Initiating Event” section above, provide evidence of the current ‘non-established’ status of the pathogen.

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 = 13 (High).

Uncertainty:

Information of the source of the Yolo County BFB detection is not known. Infested seeds and transplant are a ready source of introduction of the pathogen to CA fields. Diligent monitoring and testing of seed/plants in greenhouses and fields must be followed to mitigate risk of further 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 Acidovorax citrulli is A.

References:

ASTA. 2009. “Bacterial fruit blotch – a commercial grower’s guide” brochure. June 2009.

Burdman S. and R. Walcott. 2012. “Acidovorax citrulli: generating basic and applied knowledge to tackle a global threat to the cucurbit industry.” Molecular Plant Pathology 13(8): 805-15.

CABI. 2014. “Acidovorax citrulli.” Crop Protection Compendium. Wallingford, UK: CAB International. www.cabi.org/cpc.

CDFA-PEA. 2013. Cucumber Green Mottle Mosaic Virus and Bacterial fruit blotch detections in California. California Department of Food and Agriculture, Pest Exclusion Advisory no. 29-2013.

EPPO. 2014. “Acidovorax citrulli Bacterial fruit blotch of cucurbits.” Panel review date 2013-03. http://www.eppo.int/QUARANTINE/Alert_List/bacteria/Acidovorax_citrulli.htm.

Frankle, W. G., D. L. Hopkins and R. E. Stall. 1993. “Ingress of the watermelon fruit blotch bacterium into fruit.” Plant Disease, 77(11):1090-1092.

Kurowski, C. “CGMMV – BFB Training.” Presentation at the ‘Cucumber green mottle mosaic virus and bacterial fruit blotch disease training” California Department of Food and Agriculture, Sacramento, CA on May 1and 2, 2014. Monsanto Vegetables, Woodland, CA.

Schaad, N. W., G. Sowell Jr, R. W. Goth, R. R. Colwell and R. E. Webb. 1978. “Pseudomonas pseudoalcaligenes subsp. citrulli subsp. nov.” International Journal of Systematic Bacteriology, 28(1):117-125.

Schaad, N. W., E. Postnikova, A. Sechler, L. E Claflin, A. K. Vidaver, J. B. Jones, I. Agarkova, A. Ignatov, E. Dickstein and B. A. Ramundo. 2008. “Reclassification of subspecies of Acidovorax avenae as A. avenae (Manns 1905) emend., A. cattleyae (Pavarino, 1911) comb. nov., A. citrulli Schaad et al., 1978) comb. nov., and proposal of A. oryzae sp. nov.” Systematics and Applied Microbiology 31: 434–446.

Walcott, R. R. 2005. “Bacterial fruit blotch of cucurbits.” The Plant Health Instructor. DOI: 10.1094/PHI-I-2005-1025-02.

Walcott, Ron R. 2008. “Integrated pest management of bacterial fruit blotch of cucurbits.” In Integrated Management of Diseases caused by fungi, phytoplasma and bacteria, Eds. A. Ciancio & K. G. Mukerji. 187-205 p.

Willems A., M. Goor, S. Thielemans, M. Gillis, K. Kersters K and Jde Ley. 1992. “Transfer of several phytopathogenic Pseudomonas species to Acidovorax as Acidovorax avenae subsp. avenae subsp. nov., comb. nov., Acidovorax avenae subsp. citrulli, Acidovorax avenae subsp. cattleyae, and Acidovorax konjaci.” International Journal of Systematic Bacteriology, 42(1):107-119.

Zhao, T., J. Feng, A. Sechler, P. Randhawa, J. Li and N. W. Schaad. 2009. “An imporved assay for detection of Acidovorax citrulli in watermelon and melon seed.” Seed Science & Technology, 37: 337-349.

Responsible Party:

Dr. 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.