Study tracks plant pathogens in cicadas from natural areas

Phytoplasmas are bacteria that invade the vascular tissues of plants and can cause many different plant diseases. While most studies of phytoplasmas begin by examining plants showing symptoms of disease, a new analysis focuses on the tiny insects that carry the infectious bacteria from plant to plant. By extracting and testing DNA from archival cicada samples collected in natural areas, the study identified new phytoplasmic strains and found new associations between cicada and phytoplasma known to harm crops.

Reported in the journal biology, the study is the first to look for phytoplasma in insects from natural areas, said Valeria Trivellone, an Illinois Natural History Survey postdoctoral researcher who led the research with INHS State Entomologist Christopher Dietrich. It is also the first to use a variety of molecular approaches to detect and identify phytoplasma in leafhoppers.

“We compared traditional molecular techniques to next-generation sequencing approaches and found that the newer techniques outperformed the traditional ones,” said Trivellone. These methods will allow researchers to target more regions of the phytoplasma genomes to get a clearer picture of different strains of bacteria and how they harm plants, she said.

“One thing that’s really new about this study is that we focused on the disease vectors, on the leafhoppers and not on the plants,” Dietrich said. The standard approach to looking for phytoplasma in plants is much more labor-intensive, requiring scientists to extract the DNA from a plant that appears to be diseased and look for phytoplasma, he said.

“But even if you identify the phytoplasm, you don’t know which leafhoppers or other vectors transferred it to the plant,” Dietrich said. “So the researchers have to go back to the field to collect all the potential insect vectors. Then they do transmission experiments, where they let the planthopper feed on an infected plant and then put it on an uninfected plant to see if it catches the disease.”

Because this research is tedious and slow, “we still don’t have a clear idea of ​​which insects spread the most phytoplasma between plants,” Dietrich said. “It really limits your ability to develop an effective management strategy.”

For the new study, the researchers turned to leafhoppers from the INHS insect collection. Dietrich had collected many of these insects over a 25-year period as part of his work to classify their genetic relationships and evolution. Researchers studied 407 species of cicada collected from areas around the world less disrupted by human development. The specimens came from North and South America, Africa, Europe, Asia and Australia.

The team extracted all of the DNA from the samples and processed each one, using both traditional and newer sequencing approaches. The latter are less expensive and more informative than conventional methods, the researchers report. Of the insects sampled, 41 tested positive for phytoplasma, and the researchers obtained usable phytoplasma sequence data from 23 planthoppers. Phytoplasmas included those that cause a disease known as aster yellow, which inhibits photosynthesis and reduces the productivity of several different crops. These phytoplasmas have been found in several new planthopper species that had never been identified as vectors of the disease.

“These cicadas can transfer the phytoplasma to wild plants in natural areas,” Trivellone said.

The study found phytoplasma in regions of the world where such diseases were not reported and identified several new strains of bacteria. It also found previously unreported associations between some phytoplasma and cicada species.

Scientists don’t have the tools to attack the bacteria in asymptomatic plants to prevent disease outbreaks, so controlling phytoplasma requires the use of pesticides to kill the insect vectors.

“Because the insecticides are only partially specific to the target insects, they also kill a variety of beneficial insects, which is unsustainable,” Trivellone said.

“We’re finding that there are a lot of new phytoplasma out there in nature that no one has ever seen before,” Dietrich said. “They do not cause disease symptoms in the native plants with which they have been associated for perhaps millions of years. They only start causing disease when they jump onto a new host that has not previously been exposed to the phytoplasm.”

The new findings are consistent with those seen in emerging human infectious diseases of wildlife origin, Dietrich said. “So we need to look at nature more broadly and see what’s out there.”