William W. Kirk

Susceptibility of potato (Solanum tuberosum L.) foliage and tubers to the US8 genotype ofPhytophthora infestans

Late blight is an important disease of potato(Solanum tuberosum L.) worldwide, and therefore, many potato-breeding programs have prioritized the development of late blight-resistant potato cultivars. Although the emphasis has been to enhance foliar resistance, it is also necessary to evaluate tuber late blight resistance in new breeding lines and new sources of late blight resistance. We report here on the assessment of foliar and tuber resistance and the correlation between these aspects of resistance in a sample of Michigan State University potato breeding lines. Two MSU breeding lines had significantly less infected foliage than the susceptible check cultivars. Tuber susceptibility was significantly different (P < 0.05) only between the most susceptible and the least susceptible breeding lines/cultivars. Foliar and tuber susceptibility to potato late blight were not correlated as low tuber susceptibility was associated both with extremely low (e.g., MSG274-3) and high (e.g., MSE202-3Rus) foliar susceptibility.

Evaluation of wildSolanum species for resistance to the US-8 genotype ofPhytophthora infestans utilizing a fine-screening technique

Greenhouse assays were carried out to characterizeSolanum accessions previously reported to be late blight resistant and to identify individuals within the accessions with high levels of resistance to late blight. Foliage of wildSolanum species accessions or hybrid cultivated potato x wild species (diploid or tetraploid) from Mexico, Russia, or South America was inoculated withPhytophthora infestans (US-8) and was scored for severity of foliar symptoms at 7 and 14 days after inoculation (DAI). Mexican accessions (S. brachycarpum,S. pinnatisectum,S. guerreroense, andS.fendleri) were significantly more resistant (1.1% infection at 14 DAI) than either Russian (59.1% infection at 14 DAI) or South American (53.4% infection at 14 DAI) accessions. Moreover, the genotypes within the Mexican accessions tended to be more uniformly resistant whereas genotypes within the Russian and South American accessions tended to segregate for resistance. The more resistant genotypes of Russian and South American accessions (176/546) were retested, and 56 genotypes were identified as having potential for use in potato breeding programs for resistance to late blight. Fifty percent of the selected genotypes were fromS. microdontum accessions PI498124, PI595510, and PI595511. The potential of these species for incorporation into breeding programs for late blight resistance is discussed.ResumenUn experimente fue conducido en invernadero para la seleción de genoti.pos entre accesiones deSolanum previamente identificados como resistentes al tizón tardÌo de la papa (Phytophthora infestans). El foliaje de accesiones de especies silvestres o hibridos entre cultivares de papa y especies silvestres (diploide y tetraploide) de MÈxico, Rusia y America del Sur fue inoculado con el genotipo US-8 de P.infestans. El porcentaje del foliage infectado fue evaluado a los 7 y 14 dias despues de la inoculación. Las accesiones de MÈxico (S. brachycarpum,S. pinnatisectum,S. guerreroense yS. fendleri) fueron significativamente mas resistentes al tizón tardÌo a los 14 dias depues de la inoculación (1,1%) que las accesiones de Rusia (59,1% ) o de America der Sur (53,4% ). Tambien, la resistencia al tizón tardÌo fue uniformemente distribuida al nivel de genotipo en las accesiones de MÈxico, pero segregación para resistancia ocurrio en accesiones de Rusia y de America del Sur. Selecciones hechas en las accesiones de Rusia y de America del Sur (176/546) fueron reevaluadas en 1998 y resultarón en 56 genotipos que pueden ser utilizados en un programa de mejoramiento para resistencia ai tizón tardÌo de la papa. Un total de 50% de los genotipos seleccionados sonSolanum microdontum accesiones PI 498124, PI 595510 y PI 595511. Tambien se discuten las posibilidades del uso y las estrategias de mejoramiento para la utilizatión de estas especies en un programa de resistencia al tizón tardÌo de la papa.

Screening for Late Blight Susceptibility in Potato Tubers by Digital Analysis of Cut Tuber Surfaces

A method for quantification of late blight (Phytophthora infestans) in potato tuber tissue using a digital scanner and image analysis software is presented. The average reflective intensity of light reflected from the cut surface of sample tubers measures the darkened, diseased potato tuber tissue amid lighter, late blight-free tissue. In the absence of disease, potato variety, tuber shape, and tuber size do not influence the scan results. While digital quantification of late blight in tubers under controlled inoculation conditions is consistent, the digital assessments of late blight did not correspond exactly with those from a conventional subjective visual method. Used together, the methods can provide complementary information regarding varietal susceptibility to P. infestans development on the tuber surface and internal tuber tissue. The method of image analysis presented may be used to determine susceptibility of potato tubers to late blight in varietal development programs, storage research programs, or other tuber research programs.

Susceptibility of potato varieties and advanced breeding lines (Solanum tuberosum L.) to Phytophthora infestans (Mont.) de Bary in greenhouse screenings

Late blight (Phytophthora infestons (Mont.) de Bary) has re-emerged as an important pathogen of the cultivated potato (Solanum tuberosum subsp.tubero-sum L.) in North America. The purpose of this study was to evaluate the relative susceptibility of potato germplasm in the greenhouse in order to initiate a breeding program for resistance to the US-8/A2 mating type which is the more aggressive and prevalent strain of late blight. Whole plants of 147 cultivars and breeding lines were evaluated. Percent plant area infection was visually assessed. Seven days after inoculation, infection ranged from 0 to 100% and the overall mean was greater than 50%. Two-thirds of the cultivars and breeding lines tested were very susceptible to the US-8 genotype. The highest resistance was identified in the somatic hybrids betweenS. tuberosum andS. bulbocastanum and their backcross derivatives. Pike and Snowden were less susceptible than the other North American cultivars. Zarevo was most resistant among the European cultivars. Seven of the advanced breeding lines were equivalent to Zarevo in infection levels. The host plant resistance identified among the material tested in this study can be used by breeding programs to develop improved cultivars with resistance to US-8 genotypes of late blight.ResumenEl tizón tardío (Phytophthora infestons (Mont.) de Bary) ha reemergido como un patógeno importante de la papa cultivada (Solanum tuberosum subsp.tuberosum L.) en América del Norte. El genotipo predominante en América del Norte en 1995 fue el tipo sexual US-8/A2. El objetivo de este estudio fue evaluar la susceptibilidad relativa del germoplasma para empezar un programa de mejoramiento para resistencia al tizón tardío. Se evaluaron las plantas completas de 147 cultivares y líneas de mejoramiento. El porcentaje del area de infectión foliar fue determinado en forma visual. Siete días después de la inoculatión el porcentaje de infection estaba en el rango de 0 a 100% y la media global era mayor al 50%. Las dos terceras partes de los cultivares y líneas de mejoramiento probados fueron muy susceptibles al genotipo US-8. La resistencia más alta fue identificada en los híbridos somáticos entreS. tuberosumy S. bulbocastanum y los derivados de sus retrocruzamientos. Pike y Snowden fueron menos susceptibles que los otros cultivares norteamericanos. Zarevo fue el más resistente entre los cultivares europeos. Siete de las líneas de mejoramiento avanzadas mostraron niveles de infección equivalentes a los de Zarevo. La resistencia de las plantas hospedantes identificada en el material probado en este estudio puede ser usada por los programas de mejoramiento para desarrollar cultivares mejorados con resistencia a los genotipos US-8 del tizón tardío.

Genetic linkage mapping of economically important traits in cultivated tetraploid potato (Solanum tuberosum L.)

The objective of this study was to construct a single nucleotide polymorphism (SNP)-based genetic map at the cultivated tetraploid level to locate quantitative trait loci (QTL) contributing to economically important traits in potato (Solanum tuberosum L.). The 156 F1 progeny and parents of a cross (MSL603) between “Jacqueline Lee” and “MSG227-2” were genotyped using the Infinium 8303 Potato Array. Furthermore, the progeny and parents were evaluated for foliar late blight reaction to isolates of the US-8 genotype of Phytophthora infestans (Mont.) de Bary and vine maturity. Linkage analyses and QTL mapping were performed using a novel approach that incorporates allele dosage information. The resulting genetic maps contained 1972 SNP markers with an average density of 1.36 marker per cM. QTL mapping identified the major source of late blight resistance in “Jacqueline Lee.” The best SNP marker mapped ∼0.54 Mb from a resistance hotspot on the long arm of chromosome 9. For vine maturity, the major-effect QTL was located on chromosome 5 with allelic effects from both parents. A candidate SNP marker for this trait mapped ∼0.25 Mb from the StCDF1 gene, which is a candidate gene for the maturity trait. The identification of markers for P. infestans resistance will enable the introgression of multiple sources of resistance through marker-assisted selection. Moreover, the discovery of a QTL for late blight resistance not linked to the QTL for vine maturity provides the opportunity to use marker-assisted selection for resistance independent of the selection for vine maturity classifications.

Optimizing Fungicide Timing for the Control of Rhizoctonia Crown and Root Rot of Sugar Beet Using Soil Temperature and Plant Growth Stages

Azoxystrobin is applied early in the sugar beet growing season in north-central United States for control of Rhizoctonia damping-off and Rhizoctonia crown and root rot caused by Rhizoctonia solani anastomoses groups (AGs) 4 and 2-2, respectively. Fungicide application timings based on crop growth stage and soil temperature thresholds were evaluated in inoculated small-scale trials and in commercial fields with a history of Rhizoctonia crown and root rot. Soil temperature thresholds of 10, 15, and 20°C were selected for fungicide application timings and used to test whether soil temperature could be used to better time applications of azoxystrobin. In both small- and large-plot trials, timing applications after attainment of specific soil temperature thresholds did not improve efficacy of azoxystrobin in controlling damping-off or Rhizoctonia crown and root rot compared with application timings based on either planting date, seedling development, or leaf stage in a susceptible (E-17) and a resistant (RH-5) cultivar. Application rate and split application timings of azoxystrobin had no significant effect on severity of crown and root rot. Other environmental factors such as soil moisture may interact with soil temperature to influence disease development. Cv. RH-5 had higher sugar yield attributes than the susceptible cultivar (E-17) in seasons conducive and nonconducive to crown and root rot development. All isolates recovered from both small- and large-plot trials in all years were AG 2-2. R. solani AG 4 was not identified in any samples from any year.

Half-sib progeny evaluation and selection of potatoes resistant to the US8 genotype of Phytophthora infestans from crosses between resistant and susceptible parents

The objectives of this study were to evaluate the use of potato (Solanum tuberosum L.) late blight (Phytophthora infestans (Mont.) de Bary) resistant parents in cultivar development and identify superior clones possessing moderate to high late blight resistance combined with acceptable maturity and tuber quality. Ninety-five crosses were made between eight unadapted parents with reported late blight resistance (B0718-3, Bertita, Bzura, Greta, Libertas, Stobrawa, Tollocan and Zarevo) and susceptible parents (cultivars or advanced breeding clones) adapted to North American growing conditions. A total of 408 field selected clones were assessed for late blight resistance in the greenhouse and in the field using a mixture of US8 P. infestans isolates (A2 mating type, metalaxyl resistant) that overcame all known R-genes except R8 and R9. Clones with ≤ 10% infected foliar area in the greenhouse test or ≤ 0.30 RAUDPC (relative area under the disease progress curve) value in the field in 1998 were re-tested in 1999. A total of 118 (29% of 408) putative late blight resistant clones were selected. The eight late blight resistant parents differed in both the ability to transmit late blight resistance and in the level of resistance transmitted to the progeny. The Tollocan and B0718-3 families (half-sib progeny) had the greatest degree of resistance and frequency of resistant clones. Scott-Knott cluster analysis ranked 79 clones (67% of 118) in the high and moderate late blight resistant groups. Among these 79 clones, 19 clones had vine maturity equal to or earlier than mid-season combined with acceptable tuber quality. Further selection in 2000 resulted in eight advanced selected clones (six from Tollocan and two from B0718-3 families) with the same level of resistance as the parent combined with vine maturity and tuber quality equivalent to Atlantic, a standard cultivar for chip processing in North America. The results indicate that this breeding approach can be used to select parents for late blight resistance breeding and to identify superior clones with high levels of late blight resistance and marketable vine maturity and tuber quality.

Jacqueline Lee: A late-blight-resistant tablestock variety

Jacqueline Lee has a bright golden skin, yellow flesh, attractive oval shape, and excellent cooking qualities that make it suitable for tablestock use. In addition, it has been determined to have a high level of foliar resistance to the US-8 genotype ofPhytophthora infestans (Mont.) de Bary under Michigan field and greenhouse conditions. It is a full-season variety with vine maturity similar to Snowden. Jacqueline Lee has high yield potential. It has a high set (14–16 tubers/hill) of medium sized tubers that leads to 65%–80% marketable yield. Tubers are low in internal and external defects. It has a scab reaction similar to Atlantic, and the tubers have been observed to have a long dormancy.ResumenJacqueline Lee es una variedad de papa de piel amarilla brillante para consumo fresco, tubérculos de forma oval muy atractivos y de excelente calidad de cocimiento. Esta variedad tiene altos niveles de resistencia foliar al genotypo US-8 de Phytophthora infestans (Mont.) de Bary, tanto en invernadero como en condiciones de campo, esta evaluacion fue hecha en Michigan, USA. Jacqueline Lee es una variedad con alto potencial de rendimiento y maduración media tardia similar a Snowden. Produce un gran número de tubérculos (14-16 tubérculos/semilla) de tamaño mediano, de los cuales 65-80% son tubérculos comercializables. Los tubérculos tienen resistencia a defectos internos y externos. Tambien tienen resistencia a la sarna (Streptomyces scabies Thaxter) similar a la variedad Atlantic y dormancia larga.

Heterokaryotic nuclear conditions and a heterogeneous nuclear population are observed by flow cytometry in Phytophthora infestans

A simple and reliable method for preparation of whole nuclei of a common oomycete, Phytophthora infestans, is described for laser flow cytometry. The ease of preparation, the absence of detectable debris and aggregates, and the precision in determinations of DNA content per nucleus improve interpretation and understanding of the genetics of the organism. Phytophthora infestans is the pathogen that causes potato and tomato late blight. The genetic flexibility of P. infestans and other oomycete pathogens has complicated understanding of the mechanisms of variation contributing to shifts in race structure and virulence profiles on important agricultural crops. Significant phenotypic and genotypic changes are being reported in the apparent absence of sexual recombination in the field. Laser flow cytometry with propidium iodide is useful in investigating the nuclear condition of the somatic colony of field strains of P. infestans. The majority of the studied strains contain a single population of nuclei in nonreplicated diplophase. However, mean DNA content per nucleus varies considerably among isolates confirming the heterogeneity of the nuclear population in regard to C‐value, for field isolates. Nuclear DNA content varies from 1.75× to 0.75× that of nuclei in a standard strain from central Mexico. Some strains contain two to three populations of nuclei with differing DNA contents in the mycelium and are heterokaryons. Such a range in DNA content suggests DNA‐aneuploidy, but direct confirmation of aneuploidy will require microscopy of chromosomes. Heterokaryosis and populations of nuclei of differing DNA content necessarily confound standardized assays used worldwide in crop breeding programs for determination of race profiles and virulence phenotypes of this important pathogen.

Field optimization of dimethomorph for the control of potato late blight Phytophthora infestans: application rate, interval, and mixtures

Aspects concerning the field use of dimethomorph for the control of foliar and tuber potato late blight were examined. When application rate and interval manipulation for a season-long dimethomorph and mancozeb mixture was performed,the rate increasing through the season and 80% of full rate programs had equal final foliar blight control as the full rate program,regardless of interval. The minimum application rate for control equivalent to the full rate program was 1.34 kg ha � 1 week � 1 . When dimethomorph was tank-mixed with one of three protectant fungicides and integrated into a chlorothalonil-based late blight control program,all programs were as effective as the season-long chlorothalonil program. None of the mixtures were more effective than the others. When tank-mixed with pyraclostrobin and alternated with chlorothalonil applications,rate reduction to 50% of full rate gave foliar blight control equivalent to a full rate,for a dimethomorph and pyraclostrobin mixture and pyraclostrobin alone. r 2003 Elsevier Science Ltd. All rights reserved.