By: Robin Koestoyo
FORT PIERCE, Fla. – Dipping fruit after harvest with hot water and essential oil dips may reduce postharvest development of citrus black spot (CBS) lesions per fruit by up to 50 percent, according to new University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) research.
The new management techniques are the result of Jiaqi Yan’s recently completed doctorate she earned at UF. Yan’s dissertation focused on citrus black spot and developed post-harvest treatments using hot water, fungicides and essential oils to significantly inhibit the development of citrus black spot lesions.
Citrus black spot is caused by a pathogen called Guignardia citricarpa, a fungal disease first detected in 2010 in an Immokalee grove. Similar to canker, citrus black spot forms dark lesions on fresh fruit skin and adversely impacts the crop’s marketability. The disease is currently believed to be confined to Hendry, Collier and Polk counties.
As with costly citrus diseases like canker and greening, pathogens often spread throughout the state’s production regions. Yan’s research identified methods to deter citrus black spot development after harvest and during storage.
“Using hot water and fungicides will restrain the pathogen on the fruit. But, essential oils are more effective against the pathogen,” said Yan, who conducted her research at the UF/IFAS Indian River Research and Education Center (IRREC).
In carrying out Yan’s research, a team of UF/IFAS scientists in three production regions collaborated to help resolve the black spot issue. Mark Ritenour, an associate professor of post-harvest technology, supervised her dissertation at IRREC.
“Her work is important because it gives us alternatives for citrus black spot control after harvest that may also be applicable to other important decay organisms,” said Ritenour. “Her work also demonstrated that these can be incorporated into a commercial wax for easy application to the fruit.”
Yan used essential oils from thyme for the experiments. Small portions of two essential oils, carvacrol and thymol, were mixed into food-grade wax that was then applied to fresh fruit on which citrus black spot lesions were present. The wax is typically used in packinghouses to protect fresh fruit during shipping.
Two additional experiments were conducted to control black spot: One measured the effectiveness of dipping fruit into hot water; a second looked at six commercial fungicides.
“We found the essential oils were more effective in inhibiting the pathogen by 50 percent,” said Yan. “But the essential oils are expensive, so packers may decide to use commercial fungicides that we identified in our study until the cost of essential oils are lower.” Of the six fungicides tested against the pathogen, thiabendazole, azoxystrobin and imazalil were the most effective in reducing lesions, Yan said.
Yan said essential oils are a new way to control post-harvest disease on fruit and vegetables, but more research is needed to identify the most efficient methods to apply the oils during post-harvest. The oils she used are natural essential oils, carvacrol and thymol, at a cost of $1 per 100 oranges.
“I used one synthetic carvacrol essential oil in the experiments and found it to be equally effective as the natural carvacrol,” said Yan. “I found that the synthetic carvacrol cost was only 20 cents per 100 oranges.”
For now, Yan recommends packinghouse leaders use a combination of hot water and fungicide dips to deter the lesions. More research is needed with the synthetic oils, she said.
“We found that dipping fruit into water at 56 degrees Celsius for 120 seconds significantly inhibited lesion development on fruit that had citrus black spot symptoms,” said Yan.
“Using hot water along with fungicides is the best treatment for CBS at this time because the fungicides are already commercially used,” said Yan. “We are considering more tests with synthetic essential oils because they are affordable.”
Yan completed her doctorate in horticultural sciences with a minor in plant pathology. She is continuing her work as a postdoctoral associate, looking at essential oils incorporated in commercial wax to control diplodia stem-end rot in fresh fruit, said Ritenour.
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