US: Texas citrus survey seeks exotic pests, diseases

For the past year, citrus researchers have fanned out across Texas looking for the proverbial needle in a haystack. They say that after much success, they're preparing to do it again to look for even more "needles."

In March 2006, personnel at the Texas A&M-Kingsville Citrus Center in Weslaco canvassed the state to identify where citrus is grown in order to find citrus psyllids, tiny insects that spread the potentially devastating citrus greening disease, said Dr. John da Graca, the center's deputy director.

Their findings, he said, surprised them. A second survey, to scout for other potentially damaging insects, will start soon.

"We just went out and contacted people who might know where citrus trees are growing," da Graca said. "In the end, we visited 89 of Texas' 254 counties and found citrus growing in 85 counties. In addition to the commercial groves of the Lower Rio Grande Valley, we found citrus in other parts of the state in nurseries, small fruit stand orchards and in dooryards, where homeowners plant a tree or two."

Citrus trees were found growing as far west as El Paso, as far north as Fort Worth, and more heavily than expected in East Texas, da Graca said.

"We found two large nurseries in Tyler that supply a huge retail market through stores like Home Depot and the like," he said, "We also found citrus growing throughout Houston and along the Louisiana border and that entire Gulf coast region where satsuma (tangerine-like) trees are grown."

The Texas citrus survey was commissioned by the U.S. Department of Agriculture soon after the discovery of citrus greening, da Graca said. This devastating bacterium is vectored from tree to tree by the citrus psyllid. The disease is not harmful to humans, but can ravage commercial citrus production regions.

"We knew we had psyllids in the Rio Grande Valley, but we wanted to know two things by conducting this survey," da Graca said. "We wanted to know how far the psyllid had spread and we wanted to know if we could detect any signs of greening."

Of the 89 counties with citrus, the survey found psyllids (pronounced SILL-ids) in 36 South Texas counties, he said. The area is roughly south of a line running from Del Rio to Houston, including Uvalde, San Antonio, Austin, and Brenham, but not in East Texas.

"We found no trees with typical greening symptoms," da Graca said, "but if we found leaves with some discoloration, some yellowing from nutritional disorders or some other disease, we collected them and sent them to a USDA lab for molecular testing. We sent 309 samples from all over Texas and they all came back negative."

Greening, an incurable disease of citrus trees, was first reported in India in 1750 and in China in 1880, da Graca said. In the next few decades, the disease spread throughout Southeast Asia. By 1920, the disease was widespread in China and was found in Africa.

"Greening is caused by a bacterium that lives in the phloem, or the food-conducting vessels in the leaves and stems of citrus trees," he said. "As psyllids insert their mouth parts into citrus leaves to feed on plant juices - and they prefer new growth - they pick up the bacterium; it regenerates in their bodies, which they then spread to another tree when they insert their mouth parts again."

Symptoms include yellowing of leaves and discoloration called blotchy mottle for its irregular shape.

"The fruit of an affected tree does not color up properly," he said. "The bottom of the fruit remains green, hence the name greening. The fruit develops an oblong shape and a very bitter, unpleasant taste. Fruit drops off and, over time, the tree starts declining and will eventually die."

The presence of psyllids does not immediately translate into greening, da Graca said. Brazil has had psyllids since the 1940s but did not have greening until 2004. The first psyllid in the U.S. was found in Florida in 1998, but greening was not detected there until 2004.

"Greening has now been confirmed in 13 counties in Florida," he said. "At about two years after infection, we think, we start seeing leaf symptoms, then fruit symptoms, then over the next two or three years will be the gradual decline of the tree."

While greening has not been found in Texas, the citrus industry can take steps to manage the disease, da Graca said.

"A rotation of insecticides can be used to suppress populations of psyllids, nurseries and budwood nurseries must remain under insect-proof screening, and the cycle of planting new trees in an orchard may have to be shortened to every 10 years or so," he said. "But for now, we'll maintain our vigilance and if any incidence of greening were to surface, the Texas Department of Agriculture will do all it can to keep it isolated."

In the coming days, a new citrus survey program in Texas will begin to identify the presence of other potentially harmful insects and diseases, da Graca said.

"The USDA's APHIS (Animal Plant Health Inspection Service) and PPQ (Plant Protection and Quarantine) has commissioned the Citrus Center to begin scouting Texas citrus for a wide range of pathogens and insects including greening, canker, citrus tristeza virus, several nematode species, scale insects, a thrips species, brown citrus aphids, and anything else harmful to citrus trees not presently known to exist in Texas," he said.

Source: bangkokpost.com