Earlier this spring, Florida watermelon growers reported mysterious lesions appearing on plants in their fields.

And University of Florida plant pathologists now believe it is a disease that has hit Georgia and California watermelons in recent years, but never before has it been reported in Florida.

“We began to receive calls and e-mails this spring from growers and Extension agents in Florida counties with commercial watermelon acreage,” said Mathews Paret, plant pathologist at the UF North Florida Research and Education Center in Quincy and the investigation leader.

“All the reports described an unknown disease which destroyed the leaves on these plants.”

The key symptom is circular, black-edged lesions with white to tan centers. Under the microscope a cut section of the lesions indicated heavy bacterial streaming.

Preliminary identification of the pathogen indicated Pseudomonas, a genus of gram-negative aerobic gammaproteobacteria. It is capable of colonizing a wide range of plants.

Reports from Georgia in 2003 and from California in 2006 and 2011 have identified a Pseudomonas species associated with watermelon disease as the cause of similar outbreaks. The spring 2013 occurrence is the first in Florida.

Paret, Nicholas Dufault and Carrie Harmon at the UF Plant Disease Clinic have collectively confirmed about 40 bacterial strains to date. Further sample collection and identification of the bacterium is in progress.

“Our objectives are three-fold,” said Paret. “First, we want to characterize the bacterial causal agent of this new watermelon disease, then test the bacterial strains for sensitivity to copper, and lastly to test the integrated use of copper, and Systemic Acquired Resistance inducers to manage this bacterial disease.”

Their research is funded by a grant from the Florida Watermelon Growers Association.

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