Abstract in Spanish

About the Author

Hector Lozoya-Saldana, Agronomist, received a Masters degree in Botany from Colegio de Postgraduados of Chapingo, Mexico, in 1973, and his Ph.D. in Plant Pathology from the University of California, Riverside, in 1981. He joined the faculty of the departments of Plant Sciences and Agricultural Parasitology at the University of Chapingo in 1981 as Plant Physiologist and Pathologist, working on viruses and tissue culture of potatoes and ornamentals. He is presently the Technical Director of the International Cooperative Program on Potato Late blight (PICTEPAPA), Fondo Terra. Lozoya-Saldana has been closely linked to the potato crop in Mexico in the last 25 years, first, as a graduate student, working in the International Potato Program (IPP) in Toluca, later as a researcher of the National Potato Program (INIA); as a Ph.D. student in California; and finally as Professor at the University of Chapingo. His present assignment includes field tests for late blight resistance of international potato clones in collaboration with the Mexican and international potato programs in the Toluca Valley.

Traditional Potato Breeding

Hector Lozoya-Saldaña
Fondo Terra, Pictipapa, Metepec, México
and Debra A. Inglis
Washington State University
Mt. Vernon, WA

Introduction and Background
Classical genetic improvement involves identification of potential parents, followed by controlled pollination, collection of fruits and seeds, and several cycles of field selection. This traditional, directed, or controlled potato breeding was initiated in the 19th century, as a second procedure, after selecting clonal genotypes derived from naturally fertilized flowers.

The development of new varieties was encouraged by the late blight epidemics in the 1840’s. It is estimated that from 1851 to 1910, 380 varieties were developed or introduced to improve yield and quality in USA. In Ireland, 255 new varieties were exhibited in 1885. However, none of these was better than the traditional "Champion," which even in the 1950’s was still considered resistant to late blight in England (Akeley 1966; Dowley 1995; Burton 1966; Smith and Plaisted 1968).

Resistance in Wild Species
The sources of resistance to late blight were unknown to plant breeders until early in this century, when, in 1905, crosses between Solanum. tuberosum and S. demissum were made in United States and sent to Germany (Robertson 1991; Umaerus et al. 1983). Radcliff Salaman in 1906 realized the resistance to late blight in S. endinense and in 1909 he observed resistance in a hybrid between a Scotish variety of S. tuberosum and S. demissum (Jellis 1995). Reddick in the 1920’s also mentioned such characteristic in S. demissum. Later, S. bulbocastanum, S. cardiophyllum, S. oxycarpum, S. stoloniferum, S. ehrenbergii S. polytrichon, S. phureja, S. pinnatisectum, S. chacoense, S. hjertngii, S.papita, S. polyadenium, S. berthaultii, S. microdontum, S. sparsapilum, S. vernei, and S. verrucosum were also reported with foliar resistance to the fungus (Bradshaw et al. 1995; Colon 1988; Darsow 1995; Forbes and Jarvis 1994; Ross 1986; Smith and Plaisted 1966; Wastie 1991). In the 1996 field trials in Toluca, with materials from the Inter-Regional Potato Introduction Station, Sturgeon Bay, WI, kindly provided by John Bamberg, S. iopetalum was as resistant as S. demissum (7% infection), while S. bulbocastanum showed 12 to 15% foliar damage by late blight, followed by S. cardiophyllum with 27% and S. stoloniferum with 32 to 50% infection. S. tuberosum was 50 to 60% blighted at the end of the season.

Traditional Plant Breeding
There has been at least a 40-year breeding tradition in México for potato late blight resistance with parallel support by the Rockefeller Foundation (later the International Potato Program), the former National Agricultural Research Institute, INIA, currently INIFAP, and the Government of the State of México (CODAGEM, SEDAGRO). Most of the Mexican varieties with tuberosum, demissum and andigenum background were obtained with traditional direct breeding. The first cultivars could have vertical, monogenic resistance, with few genes conferring specific resistance to some pathogenic races of the fungus. Such resistance has been lost in time by the appearance of new, more aggressive races or combination of races of the pathogen. Recent cultivars are resistant to a wider range of pathogenic variability (horizontal resistance). Although the cultivars are well accepted for fresh market, most of them have a long growing season and many do not have enough quality for the industry. However, some Mexican materials have been adopted in many tropical countries under the original or different name (French 1996) and because they are short-day plants, they may not adapt to northern regions.

The difference in chomosome numbers makes interspecific crosses difficult, for wild species have to be hybridized with S. tuberosum. The ploidy barrier may induce embryo abortion followed by endosperm collapse. This phenomenon is quantified in a concept called endosperm balance number (EBN), where the parental EBN’s have to be equalized through manipulation of ploidy levels (Bradshaw et al. 1995; Hanneman 1994). For instance, S. demissum, with 72 chomosomes, will cross with S. tuberosum (48 chromosomes) when the first is used as recipient female, not the opposite. Also, hybrids from parents with different ploidy can be obtained and self incompatibility overcome by crossing the desired species with the diploid S. phureja (Smith and Plaisted 1966). Crossing S. phureja x S. tuberosum could result in dihaploid hybrids, which are easily crossable with diploid species. This approach (diploids and dihaploids) facilitates selection due to the high incidence of extreme types among the offspring (Ross 1986).

Segregating selections may be backcrossed to dilute or minimize undesirable traits. A backcross is when the progeny is crossed back with the parent having the most desirable traits, especially when such traits are dominant and expressed in every backcross. The Mexican cultivar Michoacán is an example of a backcross (Fernandez-Elguezabal 1993). In the United States S. demissum hybrids were backcrossed with S. maglia and S. fendleri in the 1940’s (Ross 1986).

The pedigree method (recombination of parental genes with simply inherited characters) has been used to incorporate late blight resistance into many varieties. This procedure may confer vertical, unigenic, or specific resistance, although in some instances, backcrossing has been used to induce nonhypersensitive reactions.

Recurrent selection, in which the selected resistant progeny is intercrossed, was used to improve quality for the industry (Smith and Plaisted 1966). The potential of this method to accumulate resistance genes to provide general, horizontal resistance to late blight is mentioned by several authors (Henfling 1987; Simmonds 1966; Upadhya 1996). It proved effective in Mexican and South American varieties for four breeding cycles in the 1980’s (Mendoza 1987).

Mechanisms of Genetic Resistance
Table 1 summarizes the types of genetic resistance or mechanisms in the host to delay or prevent attack by the fungus. More than one mechanism may be involved in any specific clone or wild species, but still the resistance is considered to be inherited as a single genetic physiological unit (Forbes and Jarvis 1994; Landeo et al. 1995; Wastie 1991).

The study of tuber resistance is limited to the tuberosum and demissum-type mechanisms (either resistant and susceptible tubers, respectively, Table 1). In addition, correlation between foliage and tuber infection is not clear. F. A. Langton, cited by Wastie 1991, suggests that resistant foliage provides less inoculum than susceptible foliage. The opposite is reported in Mexican cultivars, although no explanation is given (Fernández-Elguezabal 1993). In addition, soil factors may be involved in tuber infection, as observed every year in Toluca where tuber rot is infrequent regardless the level of foliar infection in the susceptible controls.

Table 1. Mechanisms of host resistance to late blight ¹
I. Hypersensitive resistance
*Monogenic, vertical, major genes R1-R11, gene for gene relationship.
*Host cells dead by elicitors with antifungal consequences.
*Microscopic (local) lesions, sometimes partially expressed.

II. Non-hypersensitive resistance
*Polygenic, field resistance, horizontal.
*General resistance, additive effect, relatively stable over time.
*Loss of resistance due to:
---Pathogen environment interactions.
---Increased pathogenic fitness (shift in population structure).

III. tuberosum resistance
*Artificially constructed by selection on susceptible cultivars.
*Expressed in all parts of the plant (foliage, tubers, stems).
*Basically a short-day plant, resistance depends on day length and plant age.
*Correlation between resistance and late maturity.
*Adult (flowering) plant resistance.
*Resistance to penetration, retarded fungal growth, long latent period, reduced sporulation.
*Many components, behavior considered single genetic and physiological unit.

IV. demissum-type of resistance
*Impedance to leaf penetration, fewer lesions.
*No resistance is conferred on tubers.
*Retarded formation of appresoria.
*Unaffected by daylength or age of plant.

V. tuberosum spp andigena and phureja-type of resistance
*Non-hypersensitive resistance.
*High levels of resistance at all stages of plant growth.
*No major genes for resistance.
__________________________________
¹Forbes and Jarvis 1994; Landeo et al. 1995; Wastie 1991.

Final Remarks
The current incidence of the A2 mating type of late blight all over the world favors the development of a wide range of pathogenic races. However, the expression of such pathogenicity will be conditioned by environmental factors. A breeding program should not rely on any specific location. Instead, the exposure of the progeny to the most severe selection pressure, that is, to races or isolates containing all virulence genes, under an optimum environment for the pathogen, is a must (Henfling 1987; Umaerus et al. 1983; Wastie 1991).The Toluca Valley is considered one of the best places to start field screenings for foliage resistance. This is so because two crucial factors are reliably present there every year, 1) a wide range of races of the pathogen due to the large number of R-genes present in the wild Solanum species of the area. This variability is further enhanced by the presence of both mating types of P. infestans, hence, R-gene interactions can be identified in the field, and 2) an ideal climate for the development of the disease during the growing season.

Nevertheless, for a better understanding and identification of a nonhypersensitive, durable resistance, new approaches imply multisite, regional or worldwide testing as well as knowledge of nature and sources of resistance, including resistance to foliage and tuber infections.

Literature Cited

Akeley, R. V. 1966. Current status of potato breeding in the United States. Proc. 3rd Triennial Conference, European Association of Potato Research. 3:113-126.

Bradshaw, J. E., R. L. Wastie, H. E. Stewart, and G. R. Mackay. 1995. Breeding for resistance to late blight in Scotland. In: L.J. Dowley, E. Bannon, L.R. Cooke, T. Keane, and E. O’sullivan (Eds.), Phytophthora infestans 150, EAPR Pathology Section Conference. Boole Press Ltd. and Teagasc, Ireland. 246-254.

Burton, W. G. 1966. The Potato. A survey of its history, and factors influenceing its yield, nutritive value, quality and storage. H. Veenman & Zonen N.V. Wageningen, The Netherlands. 382 pp.

Colon, L. T., and Budding , D. J. 1988. Resistance to late blight (Phytophthora infestans) in ten wild Solanum species. Euphytica 5: 77-86.

Darsow, U. 1995. Using wild species in breeding of basic potato material with high resistance to late blight. In: L. J. Dowley, E. Bannon, L. R. Kook, T. Keane, and E. O’Sullivan (Eds.), Phytophthora infestans 150, EAPR Pathology Section Conference. Boole Press Ltd. and Teagasc, Ireland. 275-281.

Dowley, L. J. 1995. Research on Phytophthora infestans in Ireland. A short historical review. In: L. J. Dowley, E. Bannon, L. R. Cooke, T. Keane, and E. O’Sullivan, eds. Phytophthora infestans 150, EAPR Pathology Section Conference. Boole Press Ltd. and Teagasc, Ireland. 12-29.

Fernández-Elguezabal, J.1993. Influencia del genotipo de papa en la producción de oosporas de Phytophthora infestans (Mont.) de Bary. M.Sc. Thesis, Centro de Fitopatología, Colegio de Postgraduados, Montecillo, Méx. 67 pp.

Forbes, G. A., and M. C. Jarvis. 1994. Host resistance for management of potato late blight. In: G. W. Zehnder, M. L. Powelson, R. K. Jansson, and K. V. Raman, eds. Advances in potato pest biology and management. APS Press, St. Paul, Minn. 439-457.

French, E. 1996. Standard International Field Trials. Oral presentation in the Project Design Meeting for the Global Initiative on Late Blight (GILB). CIP, Lima, Perú. March, 1996.

Hanneman, R. E., Jr. 1994. The testing and release of transgenic potatoes in the North American center of diversity. In: A. F. Krattiger and A. Rosemarin (Eds.) Biosafety for Sustainable Agriculture. SEI, ISAAA, Ithaca and Stockholm Environment Institute (SEI). Stockholm. 47-67.

Henfling, J. W. 1987. Late blight of potato: Phytophthora infestans. Technical Information Bulletin 4. International Potato Center, Lima, Perú. 25 pp. (Second edition, revised).

Jellis, G. J. 1995. Breeding for resistance to late blight in Cambridge - the early years (Abstract). In: Dowley, L. J., E. Bannon, L. R. Cooke, T. Keane, and E. O’sullivan. Phytophthora infestans 150, EAPR Pathology Section Conference, Boole Press Ltd., and Teagasc. Ireland. 367.

Landeo, J. A., M. Gastelo, H. Pinedo and F. Flores. 1995. Breeding for horizontal resistance to late blight in potato free of R genes. In: Dowley, L. J., E. Bannon, L. R. Cooke, T. Keane, and E. O’Sullivan. Phytophthora infestans 150, EAPR Pathology Section Conference, Boole Press Ltd., and Teagasc. Ireland. 268-274.

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Abstract in Spanish

MEJORAMIENTO TRADICIONAL DE LA PAPA

HECTOR LOZOYA-SALDANA

El desarrollo de la genetica clasica implica la identificacion de parientes potenciales, seguido de una polinizacion controlada, recoleccion de frutos y semilla, y muchos ciclos de seleccion de campo. Este metodo tradicional, directo y controlado de mejoramiento de papas fue iniciado en el siglo 19 como un segundo procedimiento despues de una selecion clonal de genotipos derivados de flores fertilizadas naturalmente.

El desarrollo de nuevas variedades fue alentado por la epidemia de tizon tardio de 1840. Es una estimacion que de 1815 a 1910, 380 variedades fueron desarrollados para un mejor rendimiento y calidad en los EEUU. En Irlanda 255 nuevas variedades fueron exhibidos en 1885. Sin embargo, ninguna de esas fue mejor que la tradicional "Chapingo", que aun en los ano de 1950 fue todavia considerado resistente a tizon tardio en Inglaterra.

Las fuentes de resistencia a tizon tardio fueron desconocidos por los mejoradoeres hasta 1920 cuando Radcliff Salaman cruzo Solanum demissum con variedades cultivadas. Despues, Reddick en 1920 menciono que estas caracteristicas estan en la misma especie. Solanum bulbocastamon, S. cardiophyllum, S. oxycarpum, S. stoloniferum, S. ehrenbergii, S. polytrichon, S. plureja, S. pinnatisectum, S. chacoense, S. hjertngii, S. papita, S. polyadenium, S. berthautii, S. microdontum, S. sparsapilum, S. vernei, y S. verrucosum han sido reportados con resistencia al hongo. En nuestros experiemntos de Toluca en 1996, . iopetalum fue tan resistente como S. demissum (7% de infeccion), mientras S. bulbocastanum tuvo de 12 a 15% de dano por tizon tardio, seguido por S. cardiophyllum con un 27% y S. stoloniferum con 32 a 50% de infeccion.

La mayoria de las variedes de Mexico con resistencia a esta enfermedad, con antecedentes de tuberosum, demissum, y andigenumm, fueron obtenidos a traves del metodo tradicional de cruzamientos directos. Aunque muy bien aceptado para mercados frescos, todos ellos son tardias y no tienen calidad sufiente para la industria. Sin embargo, algunos materiales de Mexico han estado siendo adoptados en muchos paises en desarrollo con los mismos o diferentes nombres.

La diferencia en el numero de cromosomas puede causar dificultades en los cruzamientos interespecificos, pero esto depende de las especies involucradas. Por ejemplo, S. demissum, con 72 cromosomas, puede cruzar con S. tuberosum (48 cromosomas) cuando el primero es usado como hembra recipiente, pero no lo opuesto. Material segregante puede volver a cruzarse mas despues para eliminar o minimisar las caracteristicas indeseables. Tambien pueden ser obtenidos hibridos de parientes con diferentes proles, y la incompatibilidad puede ser vencido por cruzamientos con especies desables , como S. phureja. Una vez que un clon adecuado es alcanzado, estas caracteristicas son reservados para reproducciones sexulaes continuas.

El metodo de pedigre (recombinacion de genes parentales con una simple caracteristica heredable) con algunas variantes, que han estado siendo usados ampliamente para incorporar resistencia a tizon tardio en muchas variedades. Este procedimiento puede conferir una resitencia vertical, de un solo gen o especifica, aunque en algunas instancias retrocruzamientos han estado envueltos. En este caso, la progenie es cruzado otra vez con los parientes con las caracteristicas mas deseable, especialmente cuando estas caracteristicas son dominantes y pueden expresarse en retrocruzas. La seleccion actual, en el cual es selecionado la progenie reistente es intercruzado, esto ha probado que es efectivo para mejorar la calidad industrial. Sin embargo, todavia no es un metodo registrado de mejoramiento par obtener cualquier de las variedades liberads en Mexico.

La columna vertebral de mejoramiento es la exposicion de la progenie a una severa presion de seleccion. El valle de Toluca es considerado como uno de los lugares del mundo para la selecion de campo de los materiales con resistencia a tizon tardio. Esto es debido a que dos factores cruciales coinciden cada ano. A) La incidencia de un amplio rango de razas del patogeno, desde que los dos grupos de compatiblidad so siempre presentes, y B) la presencia de un clima ideal para el desarrollo de la enfermedad durante todo el tiempo del cultivo. Los clones resistentes en Toluca pueden anticipar dicha resistencia bajo un severo ataque del patogeno en cualquier parte del mundo. Muchos mejoradores de papa de los EEUU, Polonia, Peru, Irlanda tienen como estrategia de procedimiento de seleccion probar sus materiales bajo las condiciones de Toluca.