Texas Cooperative Extension,
Texas A&M University, College Station, Texas

November-December 2004

Texas' Virus-free Citrus Budwood Program

by Dr. John da Graca, Deputy Center Director
Texas A&M University-Kingsville Citrus Center, Weslaco, TX

'Changsha' citrus
'Changsha' citrus, home of Bill Welch

Citrus, which exists in many species and varieties, has its origins in Asia (south China, north east India, south east Asia). It has been cultivated for over 4,000 years, and over centuries it has been moved around the world as seed, rooted plants and budwood, so that nowadays it is grown in virtually every country which has a tropical or subtropical climate.

Any plant will in time acquire its own collection of pests and pathogens. Citrus would have been affected by several from the beginnings in Asia, and most of these moved around the world with citrus plants. In addition, when a plant is grown in a new area, some pests and pathogens there will jump species and become established on citrus. The result of this over many centuries is that there are around 40 graft and/or insect-transmitted diseases - these include ones caused by viruses, viroids, and some bacteria, but share many features such as general symptoms and transmission methods. Many of them can cause devastating diseases, and many examples of catastrophic losses have been recorded - take the death of 30 million trees in South America in the 1930s caused by tristeza virus, and an additional 70 million worldwide since. Greening, blight, variegated chlorosis (CVC), leprosis, psorosis, exocortis, witches' broom of lime (WBDL), sudden death (SD) and others have all taken a toll. Some of these are quite new - CVCand WBDL appeared less than 20 years ago, and SD in Brazil was unknown before 1999.

We have complicated the natural disease situation, not only by growing citrus in new areas, but also by our practice of grafting - many of the diseases are the result of grafting infected scion buds onto susceptible rootstocks.

What can we do to avoid losses? Once a tree is infected by a virus, it is impossible to cure it. Prevention thus becomes the only option. By ensuring that nurseries propagate only from disease-free source trees, one prevents those diseases which are only spread by grafting, and by implementing certain management practices one can reduce the chances of later infection either on contaminated cutting tools, or by insect transmission of the others.

California led the way in establishing a program to supply virus-free budwood. In 1948, they initiated their program, and have maintained it (and improved it) ever since. Texas started soon after, but it was a voluntary program and did not last; another voluntary program in the 1980's also failed. The new one is, however, mandatory. Other countries which have implemented successful programs include Spain, South Africa and Australia, and most other countries are now following suit.

Citrus in Texas has so far been spared major disease catastrophes. While some of the pathogens are present, their efficient vectors are absent, or susceptible varieties (especially rootstocks) are not grown. The voluntary programs of the past did help to reduce pathogen incidence, and freezes also eliminated many infected trees. We do have some weak spots - the biggest is our dependence on sour orange rootstock. Once the brown citrus aphid arrives (it is now in Florida and Mexico) it will start to spread tristeza virus and because it has a tendency to select for severe strains we will see death of trees on sour orange.

Research is on-going to find tristeza-tolerant rootstocks. In addition, we have vectors of other diseases (leprosis, greening, CVC), and must guard against illegal importation of citrus from other states and countries. As we change rootstocks, we have to ensure that other viruses which affect them are not present.

We now have a foundation block of citrus trees of both the main commercial varieties and many of the varieties favored by ornamental and home growers - over 80 in all. These have all gone through a virus clean-up program (either in Texas or in California), and are tested on a regular basis for the main pathogens. Shortly, those varieties for which we have a sufficient supply of budwood, and have been evaluated for fruit quality, will be declared mandatory. We also continue to introduce new varieties for which there are requests, or which we think are worth evaluating for Texas.

We are currently providing nurseries across Texas with nearly 100,000 virus-free buds and cuttings annually, thereby reducing the risk of disease losses. Our main concern is tristeza, since this disease could wipe out the commercial industry. It is essential that all citrus be tristeza-free so that the aphids have no source to spread the virus from; some varieties show no symptoms but could be 'typhoid Marys' for the rest. But we have to be alert for all the other diseases too; tristeza resistant and tolerant rootstocks can be severely affected by exocortis, cachexia, tatter leaf, etc. People who grow tolerant varieties, such as satsumas, on resistant rootstocks, such as trifoliate, bear the moral responsibility of the consequences of the BCA spreading severe tristeza to commercial trees on sour orange. Down the road, tristeza could have another impact; even if all grapefruit trees were on tolerant or resistant rootstocks, some strains of the virus cause stem pitting of the scion, a gradual decline with severe economic impacts over time.

The law forbids the uncontrolled importation of citrus plants, budwood, or seed. Varieties can be introduced by the Citrus Center only from certain areas under permits issued by the Texas Department of Agriculture (TDA) and with suitable certificates from the source state.

By everyone cooperating in this program, everyone benefits. Experience elsewhere ha shown that using only selected, disease-free budwood leads to better quality fruit for both commercial growers and homeowners.

Originally published in the TNLA Green magazine. Reprinted by permission of the author. The program is managed by John Watson, who can be contacted at 956-968-2132 or jw-watson@tamu.edu.