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Rock Doc: Genomics might help defeat citrus disease

Did you have a glass of orange juice this morning? If so, you may want to know that the simple pleasures brought to us by citrus fruit are under attack from a disease called citrus greening, or yellow dragon disease. It’s caused by bacteria that cripple citrus trees by choking off their internal circulation system.

Citrus greening evidently originated in China in the early 20th century. Once a tree is infected by the disease, there is no cure. Fruit doesn’t ripen, staying green and misshapen.

Citrus greening is attacking orange groves in Florida. It is also moving to groves in Texas and California and threatening those in Arizona. If it isn’t successfully combated, citrus greening could wipe out the U.S. citrus industry.

The bacteria behind the disease is spread to the trees by an insect similar to aphids and whiteflies called the Asian citrus psyllid (pronounced “SILL-id”). The insects feed on the trees and transmit the bacteria to the plant. In time the bacteria in the citrus trees multiply and spread, causing the plant grave harm.

Citrus greening is believed to have reached the U.S. from China in the early 2000s. The insects that spread the disease are tough to control. Pesticides have been used with some success, but scientists are concerned that the psyllids will develop resistance to the chemicals. Another approach is to introduce into orchards “good” insects that prey on the psyllids. Using such biocontrols, however, has so far not been met with much success because the psyllids reproduce faster than the predatory insects. You might think that researching around the globe for disease-resistant trees might help, but so far no such trees have been found.

Enter more sophisticated approaches to interrupting the disease cycle.

“We are using genomics to see what genes are being ‘expressed’ in the psyllids as they feed on the citrus tree,” professor David Gang explains. Gang is on the faculty at Washington State University and is a member of a large team of researchers at several institutions researching new responses to citrus greening. The multifaceted effort is funded by the U.S. Department of Agriculture.

“If we know the genes and proteins involved in infection, we can try to interrupt the transmission of the disease,” Gang said.

Gang and others working with him have the goal of isolating and sequencing the genes expressed in the insects as they feed on citrus plants. Other scientists collaborating on the project can use the gene expression data in their work.

“We hope to ‘knock out’ genes that function in the transmission of the bacteria inside the psyllid,” Gang told me. “Then the insect won’t transmit the disease.”

Responding to new threats to food crops is a never-ending task for agricultural scientists. Their work is complex, spans years, and is sometimes expensive. But it keeps us fed – and free to drink our orange juice in the morning.

Dr. E. Kirsten Peters, a native of the rural Northwest, was trained as a geologist at Princeton and Harvard universities. This column is a service of the College of Agricultural, Human and Natural Resource Sciences at Washington State University.


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