January 18, 2012

By Amanda Garris

With Florida’s $9 billion citrus industry threatened by a deadly bacterial disease, Rick Kress ’73 asked scientists at Cornell’s New York State Agricultural Experiment Station (NYSAES) in Geneva for help. Three years later, a grove of pocket-sized genetically engineered orange trees offers hope for a long-term solution.

Rick Kress

Huonglongbing impedes the distribution of nutrients within the tree, resulting in fruit that are abnormally small and frequently unmarketable due to off-flavors.

Provided by the USDA

The Asian citrus psyllid spreads the bacteria that cause huonglongbing, also known as citrus greening.

Provided by the USDA

Research support specialist Ewa Borejsza-Wysocka transferring genes for insect resistance into the "Hamlin" orange variety, growing in tissue culture.

“The Florida citrus industry is facing a serious threat from the bacterial disease Huanglongbing (HLB)—also known as citrus greening,” said Kress, president of Southern Gardens Citrus, Inc., one of the largest citrus growers and processors of not-from-concentrate orange juice in Florida. “HLB was first confirmed in Florida in 2005, and within three years it had spread to all of our citrus growing counties.”

The HLB bacteria interferes with nutrient delivery throughout the tree, resulting in perpetually immature fruit that tastes bitter, medicinal and sour, and slowly kills the tree. As of today, an infected tree cannot be cured, and a ubiquitous insect—the Asian citrus psyllid—spreads the infection from tree to tree.

“Because these insects feed by piercing the leaves and transmitting the bacterium, they are tiny factories for disease transmission into the tree,” explained entomology senior research associate Charles Linn. “The bacteria just hitch a ride.”

Growers currently face a regime of cutting out dying trees and spraying insecticides to reduce the psyllid populations, but Kress envisions a longer-term solution.

“If Cornell’s expertise in genetic engineering of fruit trees could be used to develop trees with natural resistance to the insect, we could arrest the spread of the disease,” Kress said. “It would also have a positive environmental impact by reducing insecticide sprays, because in Florida, trying to eliminate the Asian citrus psyllid is as daunting as trying to get rid of mosquitos.”

Plant pathology and plant-microbe biology faculty Kerik Cox and Herb Aldwinckle first identified a handful of naturally occurring insecticides produced by bacteria, fungi, and plants known to fend off other types of insects.

“We are testing more than a dozen genes and gene combinations,” said Cox. “Many of them produce proteins that deter insect feeding on other plants by interfering with insect digestion.”

Research support specialist Ewa Borejsza-Wysocka and technicians Peggy Abbott and Shirley Kuehne used genetic engineering to insert candidate genes individually and in groups into tomato plants. This allowed them to fast-track their evaluation of candidate genes in a model system—tomato plants and tomato psyllids—over a thousand miles from the nearest citrus grove.

“To be effective, the transgene must make the leaf surface so instantly unappealing to the insect that they do not probe to feed,” said Aldwinckle. “In feeding tests with tomato psyllid, run by technician Callie Musto and research associate Dong Cha, we saw striking differences among the transgenic lines, and some lines were quite unattractive to the insects.”

Borejsza-Wysocka then inserted the most promising genes from the tomato tests into the Hamlin orange variety.

Kress will now continue the research in Florida and evaluate the disease resistance on a commercial basis. Although genetically engineered varieties are not used widely in fruit production in the United States, he believes they could avoid the problems associated with insecticide-based control strategies, given that the agricultural practices used currently by growers to keep the trees alive are expensive and unproven.

“We know that in coming decades we are expected to double the food supply using less land,” he said. “Disease outbreaks like HLB force us to rethink how we will get there, leveraging new technologies and weighing environmental impacts.” Amanda Garris is a freelance writer in Geneva, N.Y.

Amanda Garris is a freelance writer in Geneva, N.Y.

A shorter version of this story originally appeared in the Cornell Chronicle, January 16, 2012.