Oak Leaf Extract and Mulch for HLB

Researchers on the Indian River have proved “there is something in the oak leaf that can work” against HLB, said Lorenzo Rossi. “I’m not saying that I found the cure, and I’m not saying that oak extract will save the citrus industry.”

He described oak extract, and possibly oak mulch, as another tool against HLB. Rossi, a University of Florida Institute of Food and Agricultural Sciences researcher, discussed the issue at this year’s virtual Citrus Expo.  

Rossi said some Indian River growers several years ago noticed that citrus growing under nearby laurel oak trees “looked really good.” He explained that the citrus trees got under the nearby oaks after wild animals ate citrus in groves and excreted the seeds into nearby hammocks.

“So we started investigating this phenomenon” and saw reduced amounts of HLB-causing bacteria in the oak-shaded citrus trees, Rossi said. Researchers assumed rain was releasing compounds from the oak leaves, and that the citrus trees were uptaking the compounds from their leaves and through the soil.

The researchers soaked oak leaves in water overnight to obtain oak leaf extract. They applied the extract to HLB-affected citrus trees via soil drench. They noticed the plants treated with the extract “were looking really, really nice” and had bigger roots and greener leaves than control trees, Rossi said. The treated trees were also bigger. “The bacteria were declining” and some trees were “pretty much HLB-free,” he said. “A lot of physiological parameters were actually improved.” Researchers don’t yet know why the oak leaf extract is working but they are continuing to search for the reason.

The researchers are also investigating the use of oak mulch in groves. Rossi said the mulch is increasing the availability of phosphorus and potassium in the soil. “There is a major change in soil micronutrient content” after six months, he added.

So far, the trees with oak mulch haven’t shown any significant change in HLB bacteria content. The researchers expect to learn much more about the effects of mulch over the study’s remaining 18 months.

See Rossi’s full Citrus Expo presentation here. The presentation, and the continuing education units available to those who watch it, will remain online through the end of 2020.