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Common bacterial pathogens affecting Florida watermelon and management options.

By: Mathews Paret

The University of Florida, North Florida Research and Education Center, Quincy and Live Oak, and Plant Pathology Department, Gainesville

Bacterial leaf spot and Angular leaf spot diseases are caused by strains in the genus Pseudomonas. The symptoms of the disease start with water-soaking of the leaves followed by spots with yellow chlorotic area around the lesion and then blighting. The disease is seen in early spring when cool and wet environmental conditions are present. Since a major outbreak of the disease almost a decade ago, we have not seen widespread disease occurrences in Florida. However, the disease continues to be present during spring season production and if not managed well has the potential to cause delayed growth of watermelon transplants. 

Symptoms of bacterial leaf spot of watermelon

Bacterial fruit blotch is caused by Acidovorax citrulli. The symptoms of the disease start with oily water-soaked areas on the underside of cotyledons or leaves that may turn dark brown or black. A yellow halo can be seen around the lesions. On the fruit, dark green stained sections can be seen which starts small that can expand into a larger area on the fruit. The disease is seen when contaminated seeds or transplants are used in production and is only rarely seen in Florida. The disease cycle can be seen in the linked article.


Chemical and biological options against bacterial diseases and how does it work?

Copper: It is a contact bactericide that does not penetrate leaf surfaces. Copper protects natural openings and damaged sections on leaves through which the bacteria can penetrate the plant. Copper has the ability to kill bacteria by affecting multiple sites in the bacterial cell. Adding Mancozeb to copper improves protection from bacterial diseases. This is due to the availability of more free Cu ions in the system that can directly affect bacteria. Copper-based products include Kocide, Nordox, and Mankocide (a pre-mix of copper with mancozeb).

Acibenzolar S-Methyl: It is a systemic material that directly does not kill bacteria, but instead activates the natural defense of plants against pathogens. Applying preventatively is critical for effective use. A product in the market is Actigard.

Biologicals (examples include beneficial bacteria in the genus Bacillus and Streptomyces). Biologicals have multiple modes of action that helps to suppress bacterial population through direct activity or by improving plants natural defense system. Biologicals should be applied preventatively. Biological products include Serenade and Actinovate.

Performance scale of Copper, Copper + Mancozeb, Actigard, Actinovate, and Serenade (As relevant to the management of bacterial leaf spot of watermelon)

This research is funded through a USDA-SCRI project “Integrated Management of Emerging Seedborne Bacterial Diseases of Cucurbits and Chenopods” (2019-51181-30019) funded by the University of Florida team of Mathews Paret, Jeff Jones, Josh Freeman, and Bob Hochmuth with support from the Florida Watermelon Association and National Watermelon Association serving as stakeholders in this project, and also funded by the University of Florida Hatch project FLA-NFC-005974. Please contact Mathews Paret,paret@ufl.edu for further information, and for reporting disease outbreaks this spring season on watermelon and other cucurbits. Research and extension activities in this project include contributions from Susannah Da Silva, Kiersten Bushong, Fanny Iriarte, Jeff Jones, Tatiana Sanchez, and Bob Hochmuth.

Florida Watermelon Association

PO Drawer Eight

Fort Myers, Florida 33902


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