Héctor Lozoya-Saldaña

Population Structure of Phytophthora infestans in the Toluca Valley Region of Central Mexico

ABSTRACT We tested the hypothesis that the population of Phytophthora infestans in the Toluca valley region is genetically differentiated according to habitat. Isolates were sampled in three habitats from (i) wild Solanum spp. (WILD), (ii) land-race varieties in low-input production systems (RURAL), and (iii) modern cultivars in high-input agriculture (VALLEY). Isolates were sampled in 1988-89 (n= 179) and in 1997-98 (n= 389). In both sampling periods, the greatest genetic diversity was observed in RURAL and VALLEY habitats. Based on the Glucose-6-phosphate isomerase and Peptidase allozymes, the subpopulations from the three habitats were significantly differentiated in both sampling periods. In contrast to allozyme data for 1997-98, no differences were found among the three subpopulations for sensitivity to metalaxyl. Two groups of isolates identical for allozyme and mating type were further investigated by restriction fragment length polymorphism fingerprinting; 65% of one group and 85% of another group were demonstrated to be unique. The genetic diversity data and the chronology of disease occurrence during the season are consistent with the hypothesis that populations of P. infestans on wild Solanum populations are derived from populations on cultivated potatoes in the central highlands of Mexico near Toluca.

The Population Structure of Phytophthora infestans from the Toluca Valley of Central Mexico Suggests Genetic Differentiation Between Populations from Cultivated Potato and Wild Solanum spp.

ABSTRACT The population structure of Phytophthora infestans in the Toluca Valley of central Mexico was assessed using 170 isolates collected from cultivated potatoes and the native wild Solanum spp., S. demissum and S. xendinense. All isolates were analyzed for mitochondrial DNA (mtDNA) haplotype and amplified fragment length polymorphism (AFLP) multi-locus fingerprint genotype. Isolate samples were monomorphic for mtDNA haplotype because all isolates tested were of the Ia haplotype. A total of 158 multilocus AFLP genotypes were identified among the 170 P. infestans isolates included in this study. P. infestans populations sampled in the Toluca Valley in 1997 were highly variable and almost every single isolate represented a unique genotype based on the analysis of 165 AFLP marker loci. Populations of P. infestans collected from the commercial potato-growing region in the valley, the subsistence potato production area along the slopes of the Nevado de Toluca, and the native Solanum spp. on the forested slopes of the volcano showed a high degree of genetic diversity. The number of polymorphic loci varied from 20.0 to 62.4% for isolates collected from the field station and wild Solanum spp. On average, 81.8% (135) of the AFLP loci were polymorphic. Hetero-zygosity varied between 7.7 and 19.4%. Significant differentiation was found at the population level between strains originating from cultivated potatoes and wild Solanum spp. (P = 0.001 to 0.022). Private alleles were observed in individual isolates collected from all three populations, with numbers of unique dominant alleles varying from 9 to 16 for isolates collected from commercial potato crops and native Solanum spp., respectively. Four AFLP markers were exclusively found present in isolates collected from S. demissum. Indirect estimation of gene flow between populations indicated restricted gene flow between both P. infestans populations from cultivated potatoes and wild Solanum hosts. There was no evidence found for the presence of substructuring at the subpopulation (field) level. We hypothesize that population differentiation and genetic isolation of P. infestans in the Toluca Valley is driven by host-specific factors (i.e., R-genes) widely distributed in wild Solanum spp. and random genetic drift.

Phytophthora ipomoeae sp. nov., a new homothallic species causing leaf blight on Ipomoea longipedunculata in the Toluca Valley of central Mexico

A Phytophthora species was found on blighted foliage of Ipomoea longipedunculata, a morning glory native to the highlands of central Mexico. Based on host range, morphology, allozymes, mitochondrial DNA haplotype and rDNA sequences it is concluded that a new Phytophthora species, P. ipomoeae sp. nov., is the causal agent of leaf blight disease on I. longipedunculata.

Selection for Fungicide Resistance Within a Growing Season in Field Populations of Phytophthora infestans at the Center of Origin

ABSTRACT The central highlands of Mexico should provide an optimal testing ground for evaluating the potential threat of selection for resistance to fungicides in the population of Phytophthora infestans. We evaluated the hypotheses that exposure to the fungicides azoxystrobin, cymoxanil, dimethomorph, fluazinam, mancozeb, metalaxyl, and propamocarb hydrochloride would lead to (i) a shift in the sensitivity distributions (i.e., selection) and (ii) a lower genotypic diversity of the population. We compared populations from unsprayed plots with populations that had been exposed to several applications of each of the fungicides within a single field season. This study provides novel baseline data and shows that the Toluca valley P. infestans population has a wide range of sensitivities to the fungicides fluazinam, cymoxanil, dimethomorph, metalaxyl, and propamocarb. Directional selection toward resistance combined with a reduction in genetic diversity of the P. infestans population was observed only for the fungicide metalaxyl. The results obtained provide direct experimental support for continuing vigilance regarding further introductions of exotic strains of P. infestans into the United States.

The use of constant and alternating temperature regimes and tissue culture to obtain PVS—Free potato plants

Growth of 6 potato virus S (PVS)-infected potato clones in tissue culture in temperature regimes alternating between supraoptimal (40°C–45°C) and optimal (25°C) temperatures was compared to incubation of etiolated shoots at constant moderate temperatures (37°C) to obtain virus-free plants by shoot tip culture. Both procedures were effective in obtaining PVS-free propagative material. Virus-free plants were obtained in 5 of 6 clones by the alternating temperatures procedure and in 4 of the 6 clones by the constant 37°C incubation prior to shoot tip isolation. Heat tolerance, virus inactivation, and development of pathogen-free buds from the heat-treated plants depended upon the potato cultivar and the type of culture media in which the tips grew, but these characteristics did not coincide in any clone. The variety Chieftain was the least tolerant to the high temperatures and no virus-free individuals were recovered. White Rose was the most heat resistant, but Russet Burbank resulted with the highest percentage of PVS-free plants. The virus was eliminated from the variety Kennebec only by the alternating temperature treatments. Exposing potato plantlets in the alternating temperature regimes prior to isolation and regeneration of shoot tips was slightly better than the traditional method of incubation of plants at constant moderate temperatures that the plant will withstand and offers a new option in freeing plants of more tenacious viruses.ResumenSe comparó el crecimiento de 6 clones de papa infectados con PVS en cultivo de tejidos en régimes de temperaturas alternando entre temperaturas supraoptima (40°C–45°C) y optima (25°C) con respecto a incubación de brotes etiolados en temperatura constante moderadamente alta (37°C), para obtener plantas libres de virus por medio del cultivo de ápices de tallo. Ambos procedimientos fueron efectivos en la obtención de material propagativo libre de PVS. Se obtuvieron plantas libres de virus en 5 de 6 clones mediante el procedimiento de temperaturas alternadas, y en 4 de los 6 clones por medio de la incubación constante a 37°C previa al aislamiento de ápices. La tolerancia al calor, la inactivación del virus y el desarrollo de yemas libres del patógeno de las plantas tratadas con calor dependió de la variedad y del tipo del medio de cultivo en que los ápices crecieron, pero estas caracteristicas no coincidieron en ningun clon. La variedad Chieftain fue la menos tolerante a las altas temperaturas y no se recuperaron individuos libres de virus. White Rose fue la mas resistente al calor, pero Russet Burbank resultó con el porcentaje menos alto de plantas libres de PVS. El virus se eliminó de la variedad Kennebec unicamente de los tratamientos con fluctuación de temperaturas. Exponiendo las plantulas de papa a los regímenes de temperaturas alternadas antes del aislamiento y regeneración de los ápices fue ligeramente mejor que el método tradicional de incubación de las plantas a temperaturas mas moderadas que la planta puede soportar, y ofrece una nueva opción para liberar plantas de virus mas tenaces.

Electrotherapy and shoot tip culture eliminate potato virus X in potatoes

Potato stems infected with potato virus X (PVX) were exposed to either 5, 10, or 15 miliampers (mA), for 5 or 10 minutes, followed by immediate planting the axillary buds tipin vitro. Temperature increased from 4 to 10 C in the tissues during the exposure to the electricity. After a 60 days growing period, therapy efficiency (TE, TE = % plant regeneration X% virusfree resulting plants) was influenced by the severity of treatment, since organogenesis and virus elimination were both stimulated by the electricity. The highest TE values were obtained at 15 mA for 5 minutes. Under these conditions, 40% to 80% of the buds regenerated, and 60% to 100% of the regenerated plantlets tested virus negative.CompendioA tallos de papa infectados con el virus X de la papa (PVX) se les expuso a 5, 10 o 15 miliamperes (mA) por 5 o 10 minutos, e inmediatamente después se sembraron in vitro sus yemas axilares. La temperatura de los tejidos aumentó de 4 a 10 C durante la exposición a la electricidad. Después de 60 días de crecimiento, la Eficiencia de la Terapia (TE, TE = % de regeneración de plantas X% de plantulas resultantes libres de virus) fué influenciada por la severidad de los tratamientos, ya que tanto la organogénesis como la eliminación del virus fueron estimuladas por la electricidad. Los valores más altos de TE se obtuvieron con 15 mA por 5 minutos. Bajo estas condiciones, de 40% a 80% de las yemas se regeneraron, y de éstas, 60% al 100% resultaron negativas para la presencia del virus.

Potato minitubers: Technology validation in Mexico

The effects of four previously reportedin vitro tuber-inducing modified culture media on the potato cultivars Atzimba and Juanita were studied under three illumination regimes. Tuberization from stem segments started 7–18 days after transfer for all the cultivar-light-medium combinations, but this early start was not related to tuber yield. Atzimba tuberized more than Juanita (1.0 and 0.8 tubers/plant respectively), and 8 h light was optimal. The medium by Wang Hu (WH) promoted the greatest tuberization for cultivar Atzimba (1,198 mg/plant) and that by Palmer-Smith (PS) for Juanita (290 mg/plant). The modified Stallknecht’s (ST), on the contrary, slightly stimulated tuberization under all illumination conditions. The presence of cytokinins and light favored the tuberization process. Dormancy was not observed in any of the microtubers, regardless of the tuber-inducing treatment.CompendioSe estudiaron los efectos de cuatro medios de cultivo inductores de tuberización en los cultivares Atzimba y Juanitain vitro, bajo tres diferentes regímenes de iluminación. El proceso de tuberización se inició a los 7–18 días después de la siembra para todas las combinaciones de cultivar-luzmedio, pero esta estimulación temprana no estaba relacionada con el rendimiento de tubérculo. Atzimba tuberizó más que Juanita y un fotoperíodo de 8 hr diarias de luz fue mejor que los otros tratamientos con menos luz. Específicamente, el medio de Wang-Hu (WH) favoreció la más alta tuberización para Atzimba (1,198 mg/planta) y el de Palmer-Smith (PS) para Juanita (290 mg/planta). No obstante, se obtuvo una respuesta más general y uniforme para ambas variedades en el medio PS ó en el de Wattimena,et al. (WA). El medio de cultivo reportado por Stallknecht (ST), por el contrario, sólo estimuló ligeramente la tuberización bajo cualquier tratamiento de iluminación. La presencia de citocininas y de luz favoreció al proceso. No se observó latencia en los tubérculos, independientemente de los tratamientos inductores. En nuestro programa adoptamos como rutina los medios PS y WA con incubación de 8 hr diarias de luz para estimular la tuberizaciónin vitro.

Late blight resistant potato germplasm release AWN86514-2

Potato breeding selection AWN86514-2, Solanum tuberosum gp tuberosum, is being released as germplasm that is highly resistant to prevalent North American strains of Phytophthora infestans. This selection has been tested under field conditions in Mount Vernon, Washington (P. infestans US11 and US8 with complex virulence pathotypes), as well as Corvallis, Oregon, and eight other locations in North America (predominantly P. infestans US8) between 1994 and 1997. Both foliage and tubers show partial resistance. Although AWN86514-2 is pollen sterile, it can be successfully used as a female parent. An average of 34% of the progeny from crosses between AWN86514-2 and four susceptible clones were resistant to late blight when tested at Toluca, Mexico, in 1996. AWN86514-2 also has high resistance to Verticillium wilt and potato virus Y. AWN86514-2 is late maturing, with medium yields of smooth, longoblong, buffskinned tubers. Specific gravity is high and french fry color from 7 C (45 F) storage is excellent. The male parent of AWN86514-2 was Ranger Russet, a dual purpose french fry and fresh market variety, and the female parent was KSA195-96, a selection made at Aberdeen, Idaho, from Polish germplasm received as true seed from the Polish Plant Breeding and Acclimatization Institute. Possible sources of the late blight resistance in this clone include S. acaule, S. demissum, S. phureja, S. simiplicifolium, S. stoloniferum, and S. tuberosum gp andigena which are in the lineage of KSA195-96. This germplasm was developed and released by USDA-ARS in cooperation with the Agricultural Experiment Stations of Idaho, Oregon, and Washington.

Growth regulators, photoperiod, and flowering in potatoes

The effect of 25 ppm gibberellic acid (GA3), 7,500 ppm Gapol(R) (0.4% molybdate; 0.42% cobalt; 0.56% tungstate; 2.84% borax; 4 ppm indoleacetic acid; 4 ppm naphthyltalamic acid; and 2 ppm ethylenediaminetetraacetic acid) and of two photoperiods (12 and 18 h daily) on flowering of potato cv Marijke was studied under greenhouse conditions. GA3 specifically reduced the number of days required for flowering, and Gapol(R), or long days, favored bud retention. When combined, only a slight additive effect was observed. In most cases spraying the products on plants subjected to long days resulted in an increase in the number of floral buds, but their attachment to the plant was mainly influenced by the presence of Gapol(R).

Thermotherapy and tissue culture for elimination of potato virus X (PVX) in Mexican potato cultivars resistant to late blight

Plants from 6 potato cultivars uniformly infected with potato virus X (PVX) were exposed to darkness and gradual temperature increases from 37 to 40 C, during a two-week period. This was followed by shoot tip culture to obtain virus free individuals. The procedure was effective in obtaining PVX-free propagation material from 4 of the 6 varieties. Differences were observed in the response of the cultivars to the heat and to thein vitro culture. At least one plant from every cultivar showed repeated negative serological reaction against PVX after the tissue culture regeneration, except Atzimba and Montsama. Tollocan and Murca were the most resistant to heat, developed better duringin vitro culture and resulted in the largest number of PVX-negative plants. Rosita was intermediate, and Juanita did not adapt to heat stress.ResumenSe expusieron plantas de papa de 6 cultivares que estaban uniformemente infectadas con virus X de la papa (PVX), a oscuridad e incremento gradual de temperatura de 37 a 40 C, durante un periodo de dos semanas, procediendo posteriormente al cultivo de ápices para obtener individuos libres de virus. El procedimiento fué efectivo en la obtención de material propagativo libre de PVX en 4 de las 6 variedades. Se observaron diferencias en la respuesta de los cultivares al calor y al cultivoin vitro. Por lo menos un individuo mostró repetida reactión serológica negativa contra PVX después de la regeneración en cultivo de tejidos de todas las variedades, excepto Atzimba y Montsama. Tollocan y Murca fueron las más resistentes al calor, las de mejor desarrollo durante el cultivoin vitro y las que resultaron con el mayor número de plantas negativas a PVX. Rosita fué intermedia y Juanita no se adaptó a las condiciones a las que fué expuesta.