Viral diseases can be as challenging for researchers as they are for farmers. Potato yellow vein disease, for example, has tormented potato farmers in Ecuador and Colombia for more than 50 years, causing tubers to wither and cutting yields by as much as half. But scientists are still not sure what causes it.

Luis F. Salazar, a CIP virologist and leader of the Center's Disease Management Program, says that yellow vein is transmitted by whiteflies (Trialeurodes vaporariorum) and is probably caused by a virus. "From a practical point of view," he says, "until we identify the pathogen and learn how it spreads, we will not be able to control it."

As with many virus diseases, yellow vein presents complex problems for researchers. Salazar thinks the disease originated in weeds or vegetables, most probably carrots. It seems that all potato tubers coming from infected plants carry the virus, although some tubers may not develop symptoms. Nevertheless, offspring propagated from those apparently healthy tubers carry the disease.

The disease is particularly damaging in those parts of Colombia where potatoes and beans are intercropped, and spreads most quickly where intensive insecticide use has wiped out the whitefly's natural predators. Salazar says that the increased popularity of intercropping has made the yellow vein problem more acute, an example of what can happen when new technologies are introduced without considering their impact on pests and diseases.

Breeding for virus resistance

Because virus-infected potato plants cannot be cured, as is the case for potatoes infected with fungi or bacteria, strategies for virus disease control must focus on preventive measures such as the use of resistant varieties, the control of insect vectors, and the detection and elimination of contaminated plants and seed. Potato farmers frequently buy seed grown at higher elevations or in isolated locations that are relatively free of viruses and the insects that transmit them. But poorer farmers cannot always purchase seed. Nor do companies that export seed to developing countries normally breed for virus resistance.

Breeders and virologists at CIP are working to incorporate virus resistance into new varieties. Indeed, some virus resistance is now available in about a fourth of CIP-bred genotypes. The goal is to combine resistance to the three most common of the more than 25 known potato viruses: potato leafroll virus (PLRV), potato virus X (PVX), and potato virus Y (PVY). PLRV and PVY are the most widespread and are responsible for significant crop losses.

Mapping virus X and Y nearly complete

From 1997 through 2000, CIP plans to spend US$8.7 million, or about 10 percent of its research budget, on potato virus control. Another $2.7 million will be devoted to sweetpotato virus research. "Virus research," says CIP Director General Hubert Zandstra, "is as sophisticated as it is costly. It is also an area where the Center is trying to fill a gap left by the private sector. Our experience is that seed companies are not eager to provide developing-country farmers with virus-resistant varieties."

Zandstra notes that case studies on the interaction between potato plants and potato leafroll virus, the most damaging of the potato viruses, are helping CIP researchers discover different types of genetic resistance mechanisms in potato species. The studies have shown that some genes act on virus multiplication, whereas others affect the transmission of the virus by aphids. By combining these genes, CIP breeders have developed genotypes that have resisted infection by PLRV through seven years of field trials.

CIP virologists are collaborating with scientists at the United Kingdom's Sainsbury Laboratory to locate and label major resistance genes for potato virus X and potato virus Y through the use of molecular markers. The virus X resistance gene has been mapped at the Sainsbury lab and is currently being cloned, while work toward the mapping and screening of the Y resistance gene continues at CIP and at Sainsbury. Maddalena Querci, a CIP molecular virologist who leads the Center's potato virus research project, expects the cloned genes to appear in CIP varieties and advanced breeding lines within three years.

CIP virologists are also studying the potato spindle tuber viroid (PSTVd), one of more than a dozen plant viroids that were originally believed to be viruses that carry unusual characteristics. PSTVd causes deformed, spindle-shaped tubers, and can significantly reduce yields in susceptible cultivars. PSTVd has also been found to interfere with the plant's resistance to leafroll virus. PSTVd is normally transmitted by physical contact between plants and through true potato seed, but CIP virologists recently learned that it can also be transmitted by aphids from plants infected with PLRV.

The sweetpotato challenge

Virus research at CIP is not restricted to work on potatoes, however. For example, sweetpotato virus disease (SPVD) is receiving increasing attention due to its impact on resource-poor farmers in sub-Saharan Africa. SPVD is caused by the synergism between the aphid-borne sweetpotato feathery mottle virus (SPFMV) and a whitefly-borne virus (WBV). It is currently the Center's top priority for virus research in sweetpotato.

In recent years, CIP researchers developed cultivars resistant to SPFMV, but the plants were found to be susceptible to sweetpotato virus disease. Resistance genes for WBV are being sought in gene bank accessions, but work on developing transgenic resistance using DNA recombination may be required should this effort fail to uncover a natural form of resistance.

Resistance breeding is not the only defense against viral diseases of the sweetpotato. Because planting materials quickly become infected in the field, stocks must be renewed frequently, thus increasing the cost of production. The addition of virus resistance, it is believed, will allow poor farmers to use their seed for longer periods of time.

Indeed, the use of healthy planting materials is a highly effective control measure for SPVD. In China, the world's largest producer of sweetpotato, field experiments demonstrated that virus-free planting materials yielded two to three times more than those infected with the virus. The Shandong Academy of Agricultural Sciences has estimated that over an area of more than 70,000 hectares, farmers' yields increased an average of 35% following the introduction of healthy planting materials.

"Our work with sweetpotato viruses is particularly exciting for us," notes Salazar, "as it involves some of our most upstream research on a crop that addresses the concerns of some of the world's poorest farmers and consumers. With some comparatively minor fine-tuning on the virology side, we should be able to help farmers realize more of the yield potential of this naturally high yielding commodity."

Detection kits made simple

Virus detection is the most important component of virus control. When CIP's virus research program began in 1972, scientists relied on indicator plants and visual observation of virus symptoms. CIP virologist Luis F. Salazar was part of a group of about 25 investigators who developed a serological virus detection technique in the mid-1970s. The enzyme-linked immuno-sorbent assay (ELISA) remains the most sensitive detection method in use, allowing scientists to determine both the presence of a particular virus and its concentration.

CIP has produced large numbers of user-friendly, low-cost ELISA kits, and distributed them throughout the developing world. Meanwhile, researchers continue to develop more powerful and less expensive kits. The use of these simple mobile laboratories in tuber seed production and research is now routine around the globe.

Potato and sweetpotato pest and disease guides

CIP recently published field guides for identifying the major pests, diseases, and abiotic stresses for potato and sweetpotato. The full-color guides, originally printed in English, are suitable for co-publication and are currently being translated into Spanish, Bahasa Indonesia, and Vietnamese. To purchase CIP field guides contact CIP Publications at P.O. Box 1558, Lima 12, Peru (or by telephone at 51-1-349-6017). Queries and purchase orders may also be sent to: CIP-CPAD@cgiar.org. For more information see the CIP Publications webpage.