Controlling Diplodia SER on Grapefruit

Studies that show some potential for control of Diplodia stem-end rot (SER) on fresh grapefruit were discussed at the recent Citrus Packinghouse Day. University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) researcher Mark Ritenour made the presentation. Ritenour works as a professor of postharvest physiology and management at the Indian River Research and Education Center in Fort Pierce.

Ritenour said the diplodia fungus was consistently detected in the abscission zone and juice of HLB-affected fruit, and that a greater abundance of diplodia was positively correlated with lower fruit detachment force.

The researcher discussed the results of numerous products tested in 2020–2021 and 2021–2022 trials for preharvest diplodia SER control in multiple red grapefruit groves. Findings include the following:

• Topsin 4.5 FL demonstrated the best control but is not registered.
• Graduate A+ controlled diplodia SER but is not registered.
• Strobilurin-based fungicides such as Abound and Headline moderately reduced the disease.
• Miravis Prime, which is not yet registered for grapefruit, consistently showed significant and moderate control. Ritenour said it appears to be a good candidate for grapefruit registration for diplodia SER control.

Ritenour also presented results of studies using chlorine dioxide (CIO₂) for postharvest control on grapefruit.

CIO₂ treatment significantly reduced diplodia SER of both inoculated and naturally infected grapefruit without causing peel damage and/or altering other quality attributes.

The CIO₂ gas treatment shows potential to be included as a component of Florida’s fresh citrus decay control strategy. However, this system requires further testing under more commercial conditions.

See Ritenour’s full Packinghouse Day presentation, which also includes a discussion of maximum residue limits, here.

Diplodia SER substantially increased after HLB became prevalent in Florida citrus groves and is now observed on hanging fruit preharvest. Learn more here from Ritenour and other UF/IFAS scientists.

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