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UF/IFAS Research

Tomato Yellow Leaf Curl Resistant Varieties

The Tomato Magazine
August 2005

Tomato yellow leaf curl virus (TYLV) was first discovered in Florida in 1997. Although it has caused some serious losses on isolated farms, losses have been minimized because of the use of imidocloprid (Admire) insecticide drenches and judicious management by Florida tomato growers. However, more widespread losses were encountered in West Florida in spring 2004. This resulted from the ability of the whitefly to overwinter because the winter was mild and some tomato crops were grown through the winter period.

One of the most attractive strategies to prevent TYLCV is for growers to plant resistant varieties. Up to now, resistant varieties have not been widely grown. However, there are several varieties now available, and it is suggested that growers try these on adequate acreages in various cropping seasons to determine their acceptability. This is important should the TYLCV problem worsen in the future and threaten the crop production of susceptible varieties altogether.

Resistant Varieties
Two seed companies presently have resistant varieties available. Seminis recently released Tygress, which was tested as EX 1432427. Tygress is also resistant to Fusarium Wilt Races 1 and 2, Verticillium Wilt Race 1, Gray Leafspot and Tomato Mosaic Virus. Hazera has released HA-3073 which is resistant to the same diseases as Tygress. Hazera is also in advanced testing of HA-3074 which is resistant to the above diseases plus Nematodes, Spotted Wilt Virus and Bacterial Speck.

We had a severe outbreak of TYLCV at Gul Coast Research and Education Center (GCREC) during the summer of 2003, and the fall tomato trial was heavily infected. There were eight TYLCV resistant varieties from Hazera in the trial, and they performed very well. They had no TYLCV and yielded well with very large fruit. Growers should contact their seed company representatives for availability of these (and other new) varieties for testing.

Hazera also has a plum type, HA-3371, that has resistance to both TYLCV and Spotted Wilt for growers interested in plum production. Other seed companies may soon have TYLCV resistant varieties available, but I am not aware of them at present.

From a disease management standpoint, growers need to understand that resistant varieties can harbor the TYLCV virus, although at a much lower level than that of a susceptible variety. Thus, resistant varieties can be a source of inoculum for nearby susceptible varieties if whiteflies are not controlled.

Research by Lapidot et al. (2001) has shown that severely infected susceptible plants do not transmit the virus too well since whiteflies are not attracted to them as much as they are to a healthy plant. However, plants of a susceptible variety that get infected late in the season are attractive to the whitefly, have high virus titers, and, thus, are the most dangerous source of inoculum.

Regardless, the bottom line is that even if resistant varieties are grown, it is important that growers maintain good virus management practices. Besides the possible spread to susceptible varieties, whitefly management will prevent irregular ripening caused by the whitefly itself and reduce the chances that a virulent strain of the virus will emerge.

Resistance from the IFAS Breeding Program
At IFAS, Dave Schuster, Ph.D., and I have been working on geminivirus resistance since 1990 with the help of several post-doctoral scientists and a Ph.D. student. The resistance we are working with has primarily been derived from three accessions of Lycopersicon chilense, a wild tomato species. From this work, we have discovered four resistance genes; any two are required for resistance in a given breeding line or variety.

Each year, approximately 16,000 plants are screened for geminivirus resistance. Much progress has been made, but no varieties have been released. The genes are additive, which means a hybrid between a resistant and a susceptible parent (heterozygous resistance) has intermediate resistance (Table 2). Whereas this would be an improvement over a susceptible variety, it would be inadequate compared to the seed company-resistant varieties mentioned above.

Thus, testing of experimental hybrids with resistance from both parents began in 2004. Last spring, the resistance of such hybrids was good (Table 2 and data not shown), but generally the fruit size or other horticultural traits was not quite at commercial standards. A few of the better hybrids were tested again in the fall of 2004 as were new hybrids that were made in that spring.

Improved Inbreds on the Way
Improved inbreds are being developed and used in the new hybrids. Still more new hybrids were made in the fall of 2004. It is hoped that a resistant variety will be ready for release sometime in 2005 or 2006. Some of the largest fruited inbreds have partial resistance which, when combined with a high level of resistance from the other parent, may provide hybrids with good resistance.

Because two genes are needed for resistance, only one cross has been made every two years to insure that no resistance genes are lost. To speed up this process, we have conducted research to find molecular markers tightly linked to the resistance genes. Recently, my post-doctoral associate, Yuanfu Ji, has been making good progress in identifying the best markers. Once these markers are available, breeding progress should increase fourfold in that we will be able to make the four crosses every two years rather than one. Furthermore, little disease screening would be necessary as the resistance genes can be selected in the laboratory. The markers will be a great asset to our program and to other breeders.

Introgressing multiple genes from a distantly related species like L. chilense to tomato is a long, arduous process, and, as mentioned, there are no varieties available at present from the IFAS program. However, good progress has been made, and with the incorporation of molecular markers to speed up the selection process, the future outlook is promising. For instance, our ability to integrate TYLCV resistance with other desirable traits such as heat-tolerance and bacterial spot resistance (two other projects that have received considerable emphasis) should move rather quickly. Thus, I am confident that the IFAS TYLCV breeding project will make a significant impact on Florida tomato production in the future. In the meantime, it is fortunate that seed company varieties are available to our industry as well as other valuable control measures.

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