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United States Department of Agriculture

Agricultural Research Service

Title: Population Substructuring of Phytophthora Infestans on American Potato Clones in the Toluca Valley, Mexico

item Lozoya-Saldana, Hector - UNIV CHAPINGO, MEXICO
item Grunwald, Niklaus
item Garay-Serrano, Edith - PICTIPAPA, MEXICO
item Sturbaum-Abud, Anne - PICTIPAPA, MEXICO
item Brown, Charles
item Belmar, Claudia - PICTIPAPA, MEXICO

Submitted to: Acta Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 5, 2003
Publication Date: November 25, 2003

Interpretive Summary: Late blight, a severe disease of potato, caused by a water mold, is important throughout the world. It costs $100 million a year in the United States to control this disease. It results in foliage death and can cause extensive tuber rot in storage if the tubers become infected during the growing season. Effective use of fungicides is the current method of control, but current cultivars differ slightly in susceptibility and this can help a lot. The breeding research in Mexico on resistance is located in the probable birthplace of the disease organism, the Toluca Valley. The pathogen is extremely variable there and tends to break down most types of resistance except so-called durable resistance. Durable resistance consists of many components that work together to slow down the disease development no matter what the genetic variation in the pathogen is like. Typically in the US a few recurring clones of the pathogen appear every year. This is in stark contrast to the variability in the Toluca Valley. There are so many different genetic variants that it is impossible to identify a predominant type. This has the advantage of screening in concentrated, time-saving and cost-effective way for all the variation in the organism that is likely to appear worldwide into the distant future. In a sense, by screening the potato in the Toluca Valley we are selecting potato that will be resistant to future epidemics, the origin of which will be the Toluca Valley late blight fungus. The cooperation between the USDA and PICTIPAPA is akin to the use of a time machine to access future conditions. Breeding with materials selected in Toluca will protect future US potato production from its most serious enemy, the Irish Potato Famine water mold, and protect the American population from the type of food shortage that affected millions of people in Ireland more than 150 years ago.

Technical Abstract: Phytophthora infestans is a highly variable pathogen of potato. This disease organism shows broad genetic variability in the Toluca Valley of Mexico which has created the opportunity to pre-screen North American breeding clones at this location. During a five-year period breeding materials derived from highly diverse breeding populations have been tested in successive years. One question was how to define durable resistance, resistance that does not select the pathogen population to become highly virulent resulting in the sudden change of clone from being classified as resistant to susceptible. This has occurred in the past when the primary source of resistance was R-genes. The late blight oomycete co-evolved with Solanum hosts with R-genes. Pathogen avirulence genes capable of defeating every host R-gene had evolved. This was apparently a stable system in sparsely occurring wild habitats where wild potatoes occurred sparsely in a complex ecosystem. When the potato was introduced as a monoculture with R-genes, there equilibrium between host and pathogen was lost and the result was the decimation of cultivated potato harboring R-genes. Durable resistance is based on many quantitative aspects of resistance that add up to a sufficient reduction of the rate of disease with the positive outcome that the clone can produce an economic yield. In Toluca it was noted that breeding clones from the US did not become more susceptible over time. Furthermore, isolates of the sprorulating oomycete taken from the leaves of the resistant clones did not show greater genetic variability or new genotypes which would have indicated directional selection. The high degree of genetic variability was typical for sexually reproducing late blight populations in the Toluca Valley, and did not differ from the late blight isolates selected from locally bred Mexican varieties, Rosita and Nortena. The lack of selection for increased virulence, and the lack of increase in percent of foliage afflicted is a predictor that the resistance will be long-lasting in the United States and elsewhere.

Last Modified: 3/4/2015
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