Herbicides and Fruit Drop

Weed scientist Ramdas Kanissery’s research has not shown evidence that herbicides, including glyphosate, increase citrus fruit drop or decrease yields. But in studies using different glyphosate rates, he did find that there is a relatively strong negative correlation with the fruit detachment force and the glyphosate amount applied.

“That means there may be something happening with the higher glyphosate rate” that ultimately impacts fruit drop and yield, Kanissery said.

Kanissery, a University of Florida Institute of Food and Agricultural Sciences researcher at the Southwest Florida Research and Education Center, announced his findings in an April 21 seminar hosted by citrus Extension agent Mongi Zekri. Glyphosate is the herbicide most often used in citrus, Kanissery said, adding that Florida citrus growers typically spend about 12% to 15% of their production budgets on weed control.

The scientist said he wanted to ensure that weed management practices aren’t contributing to fruit drop, which has caused huge yield reductions in Florida citrus in recent years. The fruit drop has been significant since HLB was discovered in Florida in 2005.

Kanissery offered suggestions to help ensure growers maintain yield when using herbicides:

• “First and foremost … avoid spray contact with the fruit on the foliage,” Kanissery said. He urged growers to be especially cautious when applying herbicide while trees are bearing fruit.
• Using adjuvants with herbicide sprays will help improve the yield safety of the herbicide program.
• Maintain optimal spray pressure of 20 to 30 pounds per square inch.
• Avoid spraying during very high temperatures.
• Consider rotating herbicides. “If you use glyphosate over and over again, there can be a lot of tolerance issues,” the researcher said.
• Give new plantings special care. Install protective wraps around the trunks of young trees and don’t use high herbicide rates on new plantings. “Go with the lower end of the rate,” Kanissery advised.

He also offered other suggestions, such as maintaining the right spray boom height, having the correct off-center nozzle angle and appropriately sized spray droplets.

View Kanissery’s seminar presentation here.

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