Taxonomy

Domain	Bacteria
Phylum	Proteobacteria
Class	Betaproteobacteria
Order	Burkholderiales
Family	Comamonadaceae
Genus	Acidovorax
Species	Acidovorax avenae

Scientific Name

Acidovorax avenae ssp. citrulli

Common Name

bacterial fruit blotch

Author: John Bonkowski, Bacheline Joseph, Deanna Bayo, University of Florida

Reviewed by:Name, Organization

Pathogen

Acidovorax avenae pv. citrulli is a gram-negative, rod-shaped, motile bacterium that causes bacterial fruit blotch in cucurbits. The bacterium’s dimensions are on average 0.5 x 1.7 µm [2]. Infection can result in the complete crop loss of several commercial species of cucurbits such as watermelons, cantaloupes, honeydews, pumpkins, and squashes.

Symptoms and Signs

Seedling transplants, mature leaves, and fruits can exhibit symptoms. The stems, roots, and petioles are not infected and do not show symptoms. Primary symptoms include dark olive green blotches on the upper surface of the fruit. Blotches first form as an oily, water-soaked area. It starts as a small lesion less than 1 cm in diameter but can expand over the course of 7 to10 days until it covers a significant portion of the fruit’s surface. Late in disease development, the rind will rupture, emitting dark amber-colored ooze. The infected area does not normally extend below the rind into the flesh, but seeds can become infected when this occurs. Seed infection starts with water-soaked areas and then become necrotic [3, 1].

Acidovorax avenae pv. citrulli causes small, dark brown, and somewhat angular leaf lesions. Margins of the leaf lesions look water-soaked when humidity is high. Early embryonic leaves may appear water-soaked with oily areas surrounded by yellow halos. Infected areas develop into elongate, angled, black, necrotic patches. Infection can sometimes result in immediate death the seedling. In other cases, bacterial infection is retained and symptoms appear after fruit sets. Developing fruit can become infected from bacteria spread from leaf lesions. "Damping off' symptoms can be observed in severe field infections [4, 1].

Ecology and Spread

Acidovorax avenae pv. citrulli is a seedborne pathogen and is mainly transmitted by contaminated seeds. Infection and disease development is favored by temperatures over 90 °F and high humidity. Management practices in greenhouses facilitate the spread of bacterial fruit blotch. Overhead irrigation disperses the pathogen. A plant host is necessary for Acidovorax avenae pv. citrulli to survive during the summer months. Not all watermelon cultivars are equally susceptible to this pathogen. For example, light green-rind cultivars are documented to be more susceptible to bacterial fruit blotch than dark green rind cultivars. Also, diploid seeded watermelons are more susceptible to bacterial fruit blotch than triploid seedless melons. However, high levels of leaf infection in seedless melons may still result in low levels of fruit infection [3].

Geographic Distribution

The first outbreaks of Acidovorax avenae pv. citrulli were reported in 1989 in Florida, South Carolina, and Indiana. It is currently present in the following countries and states:

Europe

Greece, Hungary, Israel, Italy, Turkey, Adana Province.

Asia

China (Fujian, Hainan, Jilin, Neimenggu, Xinjiang), Japan (Honshu), Taiwan, Thailand, Iran.

North America

United States of America (Alabama, Arkansas, California, Delaware, Florida, Georgia, Iowa, Illinois, Indiana, Maryland, Mississippi, Missouri, North Carolina, Oklahoma, Oregon, South Carolina, Texas).

Central America and Caribbean

Costa Rica, Nicaragua.

South America

Brazil (Ceará, Pernambuco, Minais Gerais, Rio Grande do Norte, Rio Grande do Sul, Roraima).

Oceania

Australia (Queensland), Guam, Northern Mariana Islands.

Above info can be found at: https://www.eppo.int/QUARANTINE/Alert_List/bacteria/Acidovorax_citrulli.htm

Management

Management of any pathogen is often dependent upon both cultural and chemical options. Consult your local extension specialist or agent for recommendations relevant to your particular host and state. Remember: the label is the law.

Diagnostic Procedures

Procedures/Media	Result for Acidovorax avenae pv. citrulli
Urease	Positive; Positive; color change to pink (Laboratory Guide to Identifying Plant Pathogens)
Kings medium B	Negative; nonfluorescent
Nutrient agar	Clear to white colonies
Yeast extract-dextrose CaCO3 medium	Creamy white colonies
Hypersensitive Response	Tomato: Positive; necrotic area
	Tobacco: Positive; necrotic area
Miller-Schroth medium	No growth
Crystal violet pectate medium	Negative; no pitting is observed

Immunostrips and ELISA are two important and effective ways to identify this pathogen.

Immunostrips

https://orders.agdia.com/InventoryD.asp?loc=IN&collection=ISK%2014800&attribute_Size=25

Selected References

1. ELISA https://orders.agdia.com/InventoryD.asp?loc=IN&collection=SRA 14800&attribute_Size=1000Latin, R. X. 1996. Compendium of Cucurbit Diseases. The American Phytopathological Society, St. Paul, MN. 34.

2. Burdman, S., and Walcott, R. 2012. Acidovorax citrulli: generating basic and applied knowledge to tackle a global threat to the cucurbit industry. Molecular Plant Pathology 13:805-815.

3. Jett, L. W., Baker, T. P., and Corwin, B. Watermelon Bacterial Fruit Blotch. Mu Extension. University of Missouri-Columbia. Online: http://extension.missouri.edu/p/IPM1011.

4. Walcott, R.R. 2005. Bacterial fruit blotch of cucurbits. The Plant Health Instructor. Online publication. doi: 10.1094/PHI-I-2005-1025-02.

Resources and References

Acknowledgements

National Institute of Food and Agriculture, U.S. Department of Agriculture, under Agreement No. 2011-41530-30708 as part of "Diagnostic Image Series Development for Supporting IPM in the Southern Region" (USDA-NIFA-RIPM-003351)

Acidovorax avenae pv. citrulli