Update on the Citrus Virus-free Budwood Program, and the New Threat from Citrus Greening Disease

by John da Graca and John Watson,
Texas A&M University - Kingsville Citrus Center

The Texas virus-free budwood program for citrus managed by the Texas A & M University-Kingsville Citrus Center in Weslaco, designed to ensure that graft and arthropod borne diseases are not spread around the state in infected budwood, has continued to grow, and more and more nurseries are ordering budwood from the Center. We have introduced more varieties from the California program and now have over 100 different varieties. Last year we supplied over 250,000 buds to nurseries. The purpose of this article is to inform readers of some recent developments.

'Meyer' lemons and 'Eustis' limequats are popular home citrus in Texas

Experience in Texas in the past and in other places is that when budwood programs are entirely voluntary, they do not persist for long, and eventually people return to propagating trees from sources of unknown virus content, and may inadvertently contribute to the spread of serious diseases; this is known to have occurred in Brazil with citrus variegated chlorosis, a very serious disease related to Pierce’s disease of grapevine. The Texas program is covered by state law, but during its initial phase, it was not a legal requirement for budwood to be obtained from the program. We have concentrated on building up sources of the main commercial varieties, and from September 1, 2006, the Texas Department of Agriculture mandated that nurseries propagating these varieties, whether they are for commercial orchards or growing in private yards, must obtain the budwood from the Citrus Center. The varieties are:

• Rio Red grapefruit
• Marrs sweet orange
• Standard Valencia orange
• Olinda Valencia
• Pineapple sweet orange
• N-33 navel orange
• Meyer lemon
• Mexican lime (thorny and thornless)

In addition to the Texas Department of Agriculture charge of 10 cents a bud, there is also a $1/budstick handling fee. This has been charged for the past two years to those ordering small quantities of budwood, but has now extended to all orders to help cover the costs of running the program. The program has so far been funded largely by the Texas Citrus Producers Board and the Citrus Center, and these new charges are intended to spread the cost more evenly across our clientele, and to move the program towards financial self-sufficiency.

For other citrus varieties, it is not yet mandatory for nurseries to obtain budwood from Weslaco, but we strongly urge them to do so. If the variety is not available, we will attempt to acquire it from the California program. If a nursery has a source of its own, for a fee we will test it for the major pathogens. We also encourage homeowners who wish to propagate trees themselves to obtain their budwood from the Citrus Center. If we do not have the variety, and the nursery or homeowner knows of a tree, we will test the tree for tristeza and other diseases for a fee.

Many of the major citrus diseases are transmitted by insects such as aphids, psyllids and sharpshooters. We are therefore constructing insect-proof screenhouses to maintain budwood sources to protect the plants from insects which may be carrying pathogens. In the future, it is possible that nurseries will be required to produce trees under insect-proof screen.

Another recent development was the discovery in Florida in August 2005 of the devastating disease called citrus greening (known officially by its Chinese name Huanglongbing which means ‘yellow shoot disease’). This disease is caused by a bacterium which is spread by an insect called the Asian citrus psyllid. The disease causes the tree to produce yellow shoots (hence the Chinese name) whose leaves have an irregular blotchy mottle. Infected trees drop many fruit, and the fruits that do remain on the tree are lop-sided, remain green towards the bottom end (hence the name ‘greening’), and are acid making them inedible. The tree produces smaller leaves, twigs die back and eventually the tree can die. It is widespread in Asia and Africa and has killed millions of trees, and it was also recently discovered in Brazil. In both Brazil and Florida, nurseries are required by law to be under insect resistant screen.

Asian Citrus Psyllids, Candidatus Liberibacter asiaticus (Image courtesy of Michael Rogers, Extension Entomologist, IFAS, University of Florida, Lake Alfred).

The psyllid was first detected in Texas in 2001 in the lower Rio Grande Valley. With the finding of greening in Florida, the USDA contracted the Citrus Center to determine how widespread the psyllid has become in Texas, and whether citrus greening has been introduced into Texas. So far, we have found the psyllid in 32 counties, from Del Rio (Val Verde county) in the west to Corpus Christi and Port Aransas (Nueces county) in the east, and in the San Antonio, Houston and Brenham areas. Psyllids have not been found so far in counties near the Louisiana border. We have had hundreds of leaf samples from across Texas analyzed for the disease, and all have been negative. The Texas Department of Agriculture now requires that young trees can only be moved out of these 32 counties if they have been treated to ensure they are psyllid-free – this is to prevent spread of the insect to new areas, and to prevent spread of the disease if it is present but has not been detected.

To see what citrus greening and the psyllid look like, please visit the following website:
https://aggie-horticulture.tamu.edu/citrus/diagnostics/diagnostics.htm
State law prohibits the importation of citrus plants or budwood from any other state or country. With the confirmed presence of greening in Florida now, I hope it is clear to everyone why Texas has such a law. Please do not be tempted to bring such plant material into Texas; this law also covers plants related to citrus such as the orange jasmine (Murraya paniculata/ Murraya exotica), curry leaf (Bergera koenigii), and box orange (Severinia buxifolia), common plants in south Texas which are hosts the psyllid – orange jasmine is also a host of greening. Not only would an infected citrus tree serve as a source of infection which could devastate the citrus fruit industry, you are not going to get any edible fruit off the tree. Together we can keep the citrus trees of Texas, both in the commercial orchards and in our front and back yards, healthy by buying trees from registered nurseries in Texas who obtain budwood from disease-free sources, and keeping them psyllid-free. There are some registered pesticides, but there are also natural predators and parasites such as ladybeetles and parasitic wasp species which can effectively keep psyllid populations low.

If you have any questions about the program, please contact us : (956) 447 3362; j-dagraca@tamu.edu; jw-watson@tamu.edu John da Graca is the Deputy Center Director, and John Watson is the Budwood Program Manager.