Researchers report root bacteria could help defeat fatal citrus disease

A UC Riverside-led team is looking at tiny underground microorganisms for a way to prevent a huge problem - Huanglongbing, a disease that has decimated citrus orchards worldwide, and for which there is no cure.

The disease, also known as HLB or citrus greening, has multiple names but the same ultimate result: bitter and worthless citrus fruits. By some estimates, the end of citrus orchards in California and Florida could amount to $14 billion in lost commercial revenue.

"Often times, it is thought of as an above-ground disease of the fruits, leaves, and stems," said Caroline Roper, plant pathology professor and director of the new research effort. "However, we have seen the roots of trees decline with infection, and we want to understand why."

The National Institute of Food and Agriculture has awarded the UCR-led team $10 million over the next five years to investigate the role of soil and root microbes in the disease. 

Roper said data from previous studies shows the microbiome of the infected tree - which includes bacteria and fungi as well as protozoa and viruses - plays a role in the disease.

"We have seen a shift in the root microbiome as trees get sicker," she said.

The microbiomes shift to contain more potentially parasitic organisms that may act as secondary invaders to a tree that is suffering from HLB, Roper explained. The invasion of these root pathogens may be causing trees to die faster when they have HLB.

Part of this new research effort will test whether soil amendments like manure and compost might suppress parasitic microorganisms in the roots as well as the soil, and give the trees more strength to combat diseases including HLB.

In addition, the research team will try to determine the molecular basis of HLB resistance shown by citrus root stocks developed in Florida. They'll then see how those rootstocks perform in California, which has different soil and climate conditions.

It will be important to note how well root stocks from Florida, where there has been a heavy infestation of HLB, perform in California, where much less of the disease has been detected.