Researchers Dig Deep To Flush Out Citrus Greening

It is accepted knowledge by many that root health considerations are equally important to foliar health measures taken to mitigate the effects of HLB. Dr. Jim Graham, a UF/IFAS soil microbiologist, and research assistant Dr. Evan Johnson have been on the cutting edge of understanding how citrus roots are impacted by the disease.

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Out Of Cycle

One of Graham and Johnson’s newer observations is the manner in which root systems flush in trees infected with HLB. The flushes above and below ground have become more erratic as the disease spreads. In a sense, the trees go into survival mode and place a priority on root growth, which can be at the expense of fruit production and retention.

“We are drilling down on the root-flushing activity not being normal with this disease,” Graham says. “Root growth is sporadic and out of cycle as are the flushes, bloom, and fruit set. Later on in the season, fruit are smaller because they were set later, or larger fruit because they were set earlier, and variable sizes of fruit experience different levels of drop. This is all set up by what is happening with the roots leading to above ground responses.”Graphic illustrating how new root growth is not inhibited by HLB

Graham notes when HLB-infected trees lose roots, they are almost immediately replaced at the level of healthy or relatively healthy trees, despite overall root density continuing to decline. The priority placed on making new roots comes at a cost of carbohydrate investment in regrowing the lost roots. The energy that was focused on filling out and holding fruit is diverted and could ultimately result in the tree producing smaller fruit with less brix, or shedding the fruit altogether.

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“In normal cycles, the trees make an investment of carbohydrates in roots to sustain spring flush and the last fall flush of foliage,” Graham says. “So, we need to try to sustain those roots as long as possible and avoid root turnovers, which will come at a cost (to other parts) of the tree.”

Stress Relief

To avoid root turnovers, Graham continues to teach the cultural practices aimed at sustaining root health like mitigation of soil and water bicarbonates, which exacerbate HLB symptoms, spoon feeding nutrients to trees through fertigation, and managing soilborne pathogens like Phytophthora and pests like nematodes.

The acidification of water and/or soil in the root zone reduces pH and promotes the release of calcium and magnesium. Irrigation water conditioning is faster acting, but does not work during the rainy season. Soil conditioning takes longer to establish, but works full time. In trials, where 300 pounds of sulfur was applied per acre, it lowered pH in the soil after nine months. The trial also showed a significant increase in root density when compared to no sulfur treatment.

“In groves where bicarbonate levels are being managed well, we are seeing less fruit drop,” Graham says. “You can’t predict where the final crop will end up, but these actions make a difference compared to where nothing is being done.”

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