For the last couple of decades, a tiny insect called the Asian citrus psyllid has fed on the stems and leaves of the orange trees in Florida, infecting them with bacteria that cause a lethal disease called citrus greening. The bacterial disease, huanglongbing (HLB), originated in China and has destroyed 90 percent of the state’s groves, devastating its $9 billion citrus industry.
After years of seeking remedies—everything from antibiotics to GMOs to psyllid-sniffing dogs—with little success, Florida’s embattled citrus growers have discovered a new tool, thanks to the work of researchers at the University of Florida: planting cover crops amidst the orange groves. These crops, which can include legumes, brassicas, or clovers, are not grown for commerce, but instead to improve soil by adding nutrients, helping with water retention, deterring weeds and certain pests, and often attracting beneficial insects.
In the case of Florida citrus, greening affects the soil microbial community and nutrient uptake, decreasing many soil microbes important for the nitrogen cycling necessary for plants’ survival. When the psyllid attacks the trees’ roots, they can no longer absorb the water and nutrients the trees need to thrive.
Juanita Popenoe, agricultural extension agent for commercial fruit production at the University of Florida Institute of Food and Agriculture Science (UF/IFAS), is working with a few Florida growers who are using cover crops to combat citrus greening. She and her colleagues at UF/IFAS are cautiously optimistic.
“Cover crops show remarkable promise,” Popenoe says. “Right now, there’s not enough data available to recommend [cover crops] as a viable remedy for citrus greening, but it looks good.” If trees are healthy, they can still produce good fruit even if they have HLB, she added.
Danielle Treadwell agrees. A UF/IFAS advisor, Treadwell has done landmark studies on cover crops, testing a variety of these plants for use with citrus. Treadwell said cover crops work with other crops in the Corn Belt to conserve water and prevent soil loss.
“Cover crops are near and dear to my heart,” says Treadwell. “They’ve been a thing in the Midwest for a while, but we need more data to set policy for citrus growers.”
Especially when used in concert with conservation tillage—low-till or no-till methods—cover crops improve soil, she says, “because a partial crop remains, causing less soil disturbance.” Remaining biomass contributes organic material and suppresses weeds, offering farmers considerable savings, she says. However, “farmers change [practices] at a cost,” Treadwell adds, “so we must create programs that county agricultural agents can easily implement.”
Success with Cover Crops for Tree Recovery
Ed James isn’t waiting. One of a few citrus growers who have already adopted cover crops, James has no doubts about their efficacy improving the health of his trees. In fact, he’s become the go-to expert on their selection and use in Florida citrus and regularly shares his knowledge with others. In November, he hosted a UF/IFAS gathering at his J&R Groves in Leesburg, Florida, to show his results improving soil with cover crops.
James has been in the citrus business since he landed his first job at a packinghouse in ninth grade. He studied citrus at Florida Southern College, and for years followed conventional farming methods, using commercial fertilizers and pesticides. Like the other citrus growers, HLB nearly put him out of business.
“I was ready to give up, switch to other crops,” James says. “I pushed over most of my trees, and decided to plant cover crops to get the soil ready for row crops.”
Then he noticed that as the cover crops grew, his remaining trees perked up, so he began to investigate cover crops.
As many citrus growers are selling out to developers or switching to alternative crops, James has recovered much of his 45 acres that succumbed to greening. Despite the presence of HLB, he says his trees look healthy and are producing normal fruit.
“A plant’s immune system is in the soil,” he says as he drives his tractor around his groves on a chilly Sunday. “Lots of my trees were sickly, and looked dead, but they’re now transformed.”
He notes that cover crops were popular prior to World War II, but that the rise of chemical fertilizers brought negative impacts. “Everything I’ve learned is about soil,” he said. “Chemical fertilizers leave salts behind and kill soil microorganisms. Combined with a lack of organic matter in the sandy soil, it’s depleted.”
Low- or no-till methods are vital to the method’s success, James has found. “There’s not enough organic matter in Florida’s sandy soil, and if there’s lots of tillage, it weakens the soil,” he says.
He recommends roller crimping rather than tilling for weed control because it doesn’t disturb organic matter built up in soil by cover crops.
“Tilling soil kills the roots of beneficial weeds, and bare soil can blow away,” he says. “We learned that in the Dust Bowl.”
Better Soil, Healthy Trees
Most citrus groves are in central Florida along the Lake Wales Ridge, where crops require extra organic matter to compensate for fine, sandy soil. For citrus, adding biomass through techniques such as cover cropping both increases the nutrients in the soil and also lowers irrigation needs by reducing percolation or drainage losses.
Sarah Strauss is a soil microbiologist for UF/IFAS based in Immokalee, in southwest Florida. While she doesn’t work directly with HLB or trees, Strauss works with a variety of cover crop mixes to improve nutrient uptake in different soils.
“Soil is a microbiome, much like the gut, and different varieties of plants supply different nutrients,” she says, noting all soils are different. “There are many beneficial bacteria that enhance plant health.”
“Anything we can do to improve the trees’ health is good, because they’re very stressed,” says Treadwell. “Being proactive and improving the soil theoretically makes trees stronger.”
The University of Florida’s Popenoe says the polyculture created by adding cover crops beats the monoculture because different plants have different characteristics. “Daikon radish discourages root weevils, which lay eggs producing larvae that eat citrus tree roots,” she says. “Many [other] cover crops attract beneficial insects.”
“It’s not ‘one size fits all’ with cover crops,” confirms James, who uses at least five different crops in his groves: sunn hemp, hairy indigo, alys clover, and several brassicas, including daikon radish, weed radish, and mustard.
The micronutrients that cover crops contribute, adds Popenoe, can help combat citrus greening. “Some may be antibiotic if they have a higher manganese count, which decreases the titer [concentration] of the bacteria,” she says.
Strauss and her colleagues are conducting field trials showing some cover crops, such as sunn hemp, create more biomass and increase organic matter in soil. These trials also found the bacterial community differed significantly between soils planted with different cover crops and soils containing a no-treatment control group.
Strauss has looked at James’s work with cover crops and said he’s had “fantastic results.” In fact, she recommended growers check out their cover crop options by attending the field day at his grove last November.
Going Organic with Cover Crops
It’s 9 a.m. on a chilly December morning, and Ben McLean has already been out in his citrus groves to check the ground for frost. Most of his trees, in the rolling hills near Clermont in central Florida, succumbed to greening, but those remaining are doing better since he introduced organic soil amendments and cover crops.
“I had to see if they needed irrigation this morning, but it was about 34 degrees, so they were okay,” he says.
McLean knows citrus. His father (who they call “Ben I”) owned groves in Central Florida, and now two of his sons, Ben III and Matt, carry on the tradition. Matt founded “Uncle Matt’s Organic Orange Juice,” which is now a profitable family business, though much of their fruit now originates at organic groves in Mexico, where the orchards have not been as widely affected by citrus greening.
Like most orange grove owners, McLean has been dogged by HLB. Because his farm is certified organic, they use no synthetic inputs, and McLean uses the two varieties of cover crop he can control without herbicides. Ed James, who is not strictly organic, uses pesticides occasionally on a target basis, and only on trees not near harvest. The McLeans also work with UF/IFAS researchers studying the success of cover crops.
“We grow cow peas and hairy indigo as cover crops between tree rows to fix nitrogen,” he says. “That cuts back on [nitrogen] fertilizer, but we still add manganese, which is expensive.”
McLean monitors the soil annually for zinc, boron, iron, and copper, important minerals needed in Florida’s sandy soil. He’s seeing positive results and stronger trees. So, while cover crops are still in a trial period among researchers, for McLean and others, the results are already in.