Knowing the exact date when a new disease is found in a particular location is unusual.

In downtown Charleston, five years ago, on April 2, 2009, two large lemon trees tested positive for the citrus greening bacterium. To try to stop the spread of one of the worst citrus diseases in the world, these trees had to be removed.

The name of the disease, citrus greening, comes from the diseased fruit. Fruits produced by diseased trees remain green and turn bitter instead of ripening normally.

In Chinese, the disease is huanglongbing, "yellow dragon disease," because the new leaves on diseased trees turn yellow. It is thought that the disease originated in China in the early 1900s.

The citrus greening bacterium is spread by a tiny gnat-size insect called the Asian citrus psyllid.

The United States was the last of the top five citrus-producing countries where citrus greening was found. The disease was discovered in Miami-Dade County, Fla., in September 2005 and spread throughout the state.

What has happened with citrus greening in the Lowcountry and the rest of the country in the past five years?

The news from the Lowcountry has been good. In 2009, both the insect and the disease were present in Charleston and Beaufort counties.

The Asian citrus psyllid, but not citrus greening, was found in Colleton County. Berkeley and Dorchester counties were free of the insect and the disease.

There have been no new findings of citrus greening or the Asian citrus psyllid in the Lowcountry.

The rest of the United States has not fared as well. In the past five years, inspectors found the Asian citrus psyllid in several new counties in southern California and Arizona.

On March 30, 2012, citrus greening was discovered in Los Angeles County in one backyard lemon tree. In September 2013, it also was found in the Rio Grande Valley of Texas in a backyard grapefruit tree. Both trees were destroyed.

To follow outbreaks of the disease and the insect, check the map at www.aphis.usda.gov/citrusgreening.

A quarantine is an official attempt to keep pests confined in geographic areas where they currently exist and to stop them from moving into new, uncontaminated areas. Charleston County, and all other counties in which the psyllid or the disease have been confirmed, is under a citrus quarantine. No citrus plants or seeds may be taken out of the county, because they might harbor the citrus greening bacterium or the Asian citrus psyllid.

Naturally, it is easier to quarantine the bacterium, which only lives in citrus plants, than the insect. This is why the quarantine map shows more states with the psyllid than the disease.

Oranges and mandarins (tangerines) are very susceptible to citrus greening. The disease also affects ornamental plants related to citrus, such as orange jasmine (Murraya paniculata) and Chinese box orange (Severinia buxifolia).

Movement of these and other plants in the citrus family also is restricted by the federal quarantine.

Citrus greening is rampant in Brazil, the largest producer of orange juice. The Brazilian government instituted a drastic eradication campaign to slow or stop the spread of citrus greening. Whenever a diseased tree is found, it is removed, as were the two trees in Charleston.

Millions of trees have been pulled up and burned in Brazil since 2004. A recent comparison of groves in which trees were removed and groves in which they were not removed proved that the disease spread slower when diseased trees were destroyed.

The most exciting discovery in the past five years happened in early 2012. Spinach, like all plants not related to citrus, is naturally resistant to citrus greening. A scientist at Texas A&M University found two spinach genes that trigger the resistance to bacteria.

Using genetic engineering techniques, he and his associates cut the genes out of spinach and inserted them into orange and grapefruit seedlings.

When the seedlings had recovered from "surgery," scientists injected the citrus greening bacterium into them, and the plants were able to fend off the disease.

Young, genetically modified trees remain healthy, even though they were planted in Florida in a confined grove next to diseased trees. These results look very promising.

Anthony Keinath is professor of plant pathology at the Clemson Coastal Research & Education Center in Charleston. His expertise is in diseases of vegetables. He also is an avid gardener. Contact him at tknth@clemson.edu.