Just as late blight poses an enhanced threat to potato growers, a unique opportunity exists for a real impact in the short term. First, a new awareness has taken place in the developed and developing worlds and in the public and private sectors that something has to be done. In a world where global markets are erasing artificial borders, common control measures, such as quarantines and cargo inspections, are not going to be as effective as in the past at stopping the spread of plant diseases.

The modern migration of Phytophthora infestans was possible because of world seed trade. Pathologists trace the new strains to a shipment of potatoes from Mexico to Europe in the mid-1970s. The fungus then spread through exports of infected potato seed. Even before the North American Free Trade Agreement (NAFTA) opened up the frontier between Mexico and the United States, the new strains of P. infestans had migrated northward. They first appeared in Texas, and then in Florida.

Today, it is unacceptable environmentally, legally, scientifically, and economically to rely primarily on fungicides for control. More chemicals are applied to potatoes than to any other food crop. Each year, farmers spray about US$1.8 billion worth of fungicides, says CIP breeder Juan Landeo, drawing on 1991 FAO estimates. Developing-world farmers spray an estimated $600 million worth. Because of heavy late blight pressure, some farmers in the highland tropics apply fungicides more than 15 times per growing season.

Many poor farmers in developing countries cannot control late blight adequately with fungicides. They simply do not have the money to buy agrochemicals nor can they assume the risk of having a crop wiped out; thus, they plant other crops during rainy weather favorable for late blight. Scientists and producers in the industrialized North and the developing South now admit that other alternatives have to be found. For instance, by adopting potato varieties that resist late blight, farmers can reduce their fungicide applications by half. Practically all commercial varieties are susceptible to the fungus.

Third, scientists have recently developed new technological tools to study the epidemiology of the disease in detail, tracking its spread around the world, monitoring its adaptation to new environments, and probing into its genetic makeup and evolution. Breeders are now beginning to use genetic mapping to identify desirable traits in wild species or native cultivated varieties. This will allow CIP and other institutions to speed up their breeding efforts to confront the late blight threat.

CIP’s fight against late blight

Over the coming years, CIP will seek to expand the genetic base for long-lasting resistance through the transfer of resistance genes from wild species and primitive cultivars to cultivated potatoes. This work will both broaden and increase the level of resistance to late blight. Rapid progress is expected because CIP scientists and CIP's colleagues have already overcome most of the incompatibility barriers between wild species and cultivated potatoes.

For the immediate future, CIP will attempt to introduce broader-based durable resistance into locally adapted potato varieties and advanced breeding lines. Scientists hope to develop resistant populations through a blend of molecular marker-assisted selection and limited chromosome transfer. In addition, genetic mapping will be done to locate resistance genes within the potato genome; the genes will then be cloned and inserted into existing potato cultivars. Emphasis will eventually be directed to promoting widespread use of new, late-blight-resistant varieties. These materials should provide the catalyst needed to allow farmers to control late blight through integrated disease management systems.

Breeding for late blight resistance

Can durable genetic resistance be developed within a short time frame of about six years? CIP believes that it can. The Center’s initial efforts to develop late-blight-resistant cultivars began with its "population A" that produced both late-blight-resistant varieties and cultivars used as progenitors for the development of other varieties.

Although population A contains major vertical resistance genes (R genes), it also possesses moderately high horizontal polygenic resistance. R genes are hypersensitivity genes that react specifically to a particular race of the fungus; they either mask durable resistance, or mimic it. Horizontal resistance is the result of many genes acting together in concert so that the plant can withstand an attack from all races of the fungus. Despite these limitations, population A has had substantial value and continues to show promise at several locations.

In the 1990s, CIP built upon the success of population A and developed a second generation of clones. By selectively intercrossing within population A, and by test-crossing potential progenitors to susceptible clones followed by progeny testing, CIP plant breeders and pathologists eliminated R genes from this population, thus creating population B. CIP geneticists have also produced a population from the primitive cultivated form Solanum tuberosum subsp. andigena, which after four cycles of recurrent selection exhibited high levels of late blight resistance. Subspecies andigena evolved in the high Andes of Peru, an area where late blight is not endemic. Consequently, it evolved independently of the fungus and does not possess R genes. Its resistance is believed to be polygenic and should prove durable over the long term. Because it is non-race-specific, it lends itself to rapid selection and development of varieties.

Population B and the andigena population represent unique sources of resistant, R-gene-free germplasm that eliminate for plant breeding purposes the difficulty of distinguishing between R-gene-mediated and horizontal resistance that is expected to be durable. It is believed that this will facilitate the incorporation of durable resistance into cultivars for developing countries. By applying modern molecular techniques such as bulk segregation analysis to identify quantitative trait loci (QTL), which are associated with this type of resistance, it should be possible to enhance selection procedures and gain greater insights into the genetic architecture of late blight resistance. Recent studies by CIP pathologists have also indicated that certain accessions of S. phureja carry very high levels of resistance. The resistance of S. phureja, a cultivated diploid potato species, can be more easily used to contribute new resistance genes and may prove useful in molecular marker studies involving crosses at the diploid level.