Know Your Florida Watermelon Disease and Pathogen Series
Bacterial and angular leaf spot diseases of watermelon and other cucurbits caused by Pseudomonas syringae: Disease cycle and comparative assessment of control options
Kiersten Bushong, Mathews Paret, Susannah Da Silva, Josh Freeman, Jeff Jones and Bob Hochmuth, University of Florida
Question: Is bacterial leaf spot a different disease than angular leaf spot of cucurbits?
Answer: Angular leaf spot of cucurbits is caused by Pseudomonas syringae pv. lachrymans which is genetically different than the Pseudomonas syringae strains causing bacterial leaf spot of watermelon and other cucurbits.
A field picture from Florida watermelon affected by bacterial leaf spot
Question: Can these diseases be seed-borne?
Answer: Yes, this is especially noted in studies around the world for Angular leaf spot. Using seed produced in geographical locations with minimum risk of disease occurrence, good field management practices, seed testing, and seed treatments are paramount for ensuring production of good quality seed free of pathogens.
Question: Can the disease spread from a small number of infected plants in a transplant house?
Answer: A typical house produces thousands of transplants at the same time and uses an overhead irrigation system. The humid conditions in the house can favor localized spread of the bacterium.
Answer: What conditions are ideal for field occurrence and spread of the bacterium in Florida?
Answer: Contaminated seed/transplants, and cool and wet conditions favor disease occurrence and spread during early stages of spring watermelon production in Florida. Typically, the disease impact is due to significant leaf damage under these ideal environmental conditions that could delay plant and fruit maturity. In some cases, resetting new transplants may be necessary.
Question: Is the bacterium copper-tolerant? Would copper bactericides be effective? Are there other effective management options?
Answer: All strains isolated and characterized from Florida since 2012 were sensitive to copper. Hence copper fungicides have been very effective. Ideally copper should be used with ethylene bis-dithiocarbamate fungicide (a commonly known product is mancozeb) for effective and broad-spectrum control during cool and wet conditions of early spring. However, copper should be used at half of the maximum labeled rate to minimize risks of phytotoxicity on watermelon. The plant defense inducer, Acibenzolar S-methyl (Actigard), is an option, but comparatively less effective than copper+mancozeb based on findings from field trials in Florida.
Question: How does the bacterium survive and spread in watermelon and other cucurbit production systems?
Answer: Please see the cartoon below which shows the disease cycle:
Bacterial leaf spot and angular leaf spot disease cycle (causative agent Pseudomonas syringae strains). Note that we have not observed fruit symptoms in Florida watermelon and squash.
The cartoon was created using Information from: “Compendium of Cucurbit Diseases and Pests, 2nd ed., APS Press, 2017, pp. 69-70, and from “Epidemiology and management of bacterial leaf spot on watermelon caused by Pseudomonas syringae” in Plant Disease 2017 101:7, 1222-1229
Acknowledgement: This publication is funded through a USDA-SCRI project “Integrated Management of Emerging Seedborne Bacterial Diseases of Cucurbits and Chenopods” (2019-51181-30019) funded to the University of Florida team of Mathews Paret, Jeff Jones, Josh Freeman and Bob Hochmuth with support from the Florida Watermelon Association serving as a key stakeholder in this project. Please contact Mathews Paret, firstname.lastname@example.org for further information and reporting disease outbreaks this spring.