Lyndon D. Porter

Characterization of white mold disease avoidance in common bean

White mold, caused by Sclerotinia sclerotiorum, is a devastating fungal disease of common bean (Phaseolus vulgaris L.) worldwide. Physiological resistance and disease avoidance conferred by plant architecture-related traits contribute to white mold field resistance. Our objective was to further examine white mold disease avoidance in common bean. A comparative map composed of 79 quantitative trait loci (QTL) for white mold resistance (27), disease avoidance traits (36) and root traits (16) was generated. Thirteen white mold resistance QTL, six with strong and seven with weak associations with disease avoidance traits, were observed. Root length and lodging QTL co-located in three regions. Canopy porosity and height, and lodging were highly correlated with disease severity score in field screening trials conducted from 2000 to 2011. Resistance to lodging was extremely important for reducing disease severity in both dry and snap bean (r = 0.61 across 11 trials). Avoidance traits were less effective in reducing disease severity in trials with heavy disease pressure. Dry bean lines with physiological resistance in combination with disease avoidance traits did not require fungicide application to protect yield potential under moderate and heavy disease pressure. Given the complexity of disease resistance as evidenced by the comparative QTL map, marker-assisted breeding for disease avoidance is not recommended at this time. Instead, selecting for resistance to white mold in the field, in combination with high yield potential and acceptable maturity, is the recommended strategy for improving both disease avoidance and physiological resistance to white mold in cultivars with commercially acceptable agronomic traits.

Post-harvest applications of zoxamide and phosphite for control of potato tuber rots caused by oomycetes at harvest

Potato storage tuber rots caused by the late blight and pink rot pathogens at harvest can cause severe economic losses, warranting the need for effective post-harvest fungicide applications. The purpose of this study was to evaluate the efficacy of select post-harvest fungicides in reducing tuber infections by the late blight and pink rot pathogens when applied at various post-inoculation time intervals. ‘Russet Burbank’ potatoes were inoculated by submersion in an aqueous suspension ofPhytophthora infestans orPhytophthora erythroseptica zoospores at 0, 1, 2, 4, and 6 h prior to receiving a post-harvest treatment. Products evaluated were zoxamide (various rates and formulation), phosphite (335 g a.i./MT), and a hydrogen peroxide/peroxyacetic acid mixture (HPPA, 9 g a.i./MT), all applied at 2.08 L/MT of tubers as a low pressure spray prior to storage. Zoxamide and phosphite significantly reduced late blight and pink rot incidence and severity when applied immediately after inoculation. HPPA was less effective at controlling disease development. Phosphite was effective at reducing late blight development at all time intervals up to 6 h post-inoculation (7% vs 80% in untreated). Zoxamide appeared to have good post-harvest disease control if applied soon after inoculation. The maximum time intervals between inoculation and treatment where significant reductions in pink rot incidence were observed was 0 h for HPPA (28%), 2 h for zoxamide (55%; 64 g a.i./MT) and 6 h for phosphite (13%) compared to the untreated (73%). Phosphite provided consistent disease control even when applied several h after inoculation and has potential to be a reliable post-harvest fungicide for the potato industry.ResumenLas pudriciones de los tubérculos de papa en almacén, causados por los patógenos del tizón tardío y la pudrición rosada durante la cosecha, pueden causar pérdidas económicas cuantiosas, haciendo necesaria la aplicación de funguicidas después de la cosecha. El propósito de este estudio ha sido evaluar la eficacia de fungicidas selectos, aplicados después de la cosecha para reducir la infección de los patógenos antes mencionados a varios intervalos. Papa ‘Russet Burbank’ fue inoculada sumergiéndola en una suspensión acuosa de zoosporas dePhytophtora infestans o dePhytophthora erythroseptica por 0, 1, 2, 4, 6 horas antes de recibir el tratamiento de post-cosecha. Los productos evaluados fueron zoxamide (a varias dosis y formulaciones), phosphite (335g de i.a./T) y una mezcla de peroxido/ácido peroxiacético (HPPA, 9g de i.a./T) todo aplicado a 2.8 L/T de tubérculos como pulverización a baja presión antes del almacenamiento. Zoxamide y phosphite redujeron significativamente la incidencia de tizón tardío y de pudrición rosada cuando se aplicaron inmediatamente después de la inoculación. El HPPA fue menos efectivo en controlar el desarrollo de la enfermedad. El phosphite fue efectivo en reducir el desarrollo de tizón tardío en todos los tiempos de post-inoculación hasta las 6 horas (7% contra 80% de las no tratadas). Zoxamide parece ejercer buen control post-cosecha de la enfermedad si se aplica lo más pronto después de la inoculación. Los máximos tiempos de intervalo entre la inoculación y el tratamiento donde se observó reducción significativa en la incidencia de pudrición rosada fue 0 h para HPPA (28%), 2 h para zoxamide (55%; 64g de i.a./T) y 6 h para phosphite (13%) comparado con el testigo no tratado (73%). Phosphite proporcionó un consistente control aún cuando se aplicó varias horas después de la inoculación y tiene el potencial para ser un funguicida confiable para ser usado después de la cosecha en la industria de papa.

Survival of Phytophthora infestans in Surface Water

ABSTRACT Coverless petri dishes with water suspensions of sporangia and zoospores of Phytophthora infestans were embedded in sandy soil in eastern Washington in July and October 2001 and July 2002 to quantify longevity of spores in water under natural conditions. Effects of solar radiation intensity, presence of soil in petri dishes (15 g per dish), and a 2-h chill period on survival of isolates of clonal lineages US-8 and US-11 were investigated. Spores in water suspensions survived 0 to 16 days under nonshaded conditions and 2 to 20 days under shaded conditions. Mean spore survival significantly increased from 1.7 to 5.8 days when soil was added to the water. Maximum survival time of spores in water without soil exposed to direct sunlight was 2 to 3 days in July and 6 to 8 days in October. Mean duration of survival did not differ significantly between chilled and nonchilled sporangia, but significantly fewer chilled spores survived for extended periods than that of nonchilled spores. Spores of US-11 and US-8 isolates did not differ in mean duration of survival, but significantly greater numbers of sporangia of US-8 survived than did sporangia of US-11 in one of three trials.

Effects of tuber depth and soil moisture on infection of potato tubers in soil by Phytophthora infestans

The effects of tuber depth, soil type, and soil moisture on potato tuber infection due to Phytophthora infestans were assessed under greenhouse conditions in soil contained in large pots. Healthy tubers were used to assess infection and were either hand buried in soil at specific depths or naturally formed from potato plants growing in the soil. A spore suspension of P. infestans was chilled to induce zoospore formation and a suspension of resulting zoospores and sporangia were applied to the soil. Soil depth at which tubers became infected was used to determine the extent of spore movement in the soils. Tuber infection significantly decreased with increasing soil depth. Most infected tubers were found at the surface of soil; infection was rare on tubers at 5 cm or deeper in the soil. Amount of tuber infection varied among soil types. Significantly less tuber infection occurred in a Shano silt loam than in medium and fine sands. Only tubers on the soil surface were infected in the Shano silt loam. Depth in soil at which tubers became infected did not differ significantly among Quincy fine sand, Quincy loamy fine sand, and Quincy medium sand. Increased soil moisture did not significantly increase the soil depth at which tuber infection occurred, regardless of the soil type.

Identification and Characterization of Resistance to Phytophthora infestans in Leaves, Stems, Flowers, and Tubers of Potato Clones in the Pacific Northwest

Resistance to Phytophthora infestans in leaves, stems, flowers, and tubers of eight cultivars grown commercially in the Columbia Basin and 29 advanced clones from the Tri-State Potato Variety Development Program was quantified. Foliage of all eight cultivars was susceptible in field and greenhouse tests. Six advanced clones had high levels of resistance in stems and leaves in the field at Mount Vernon, WA, but only two of these six clones had high levels of resistance in leaves and stems in greenhouse tests. Flowers of most plants with high to moderate levels of foliar resistance were susceptible to infection in both the field and greenhouse. Tubers of Umatilla Russet, Russet Legend, Gem Russet, and nine advanced clones, including A90586-11, had high levels of resistance in most laboratory tests. Level of foliar resistance against US-8 and US-11 clonal lineages of P. infestans was variable among clones, but tuber resistance by pathogen clonal lineage varied little among clones. Clones with high resistance expressed quantitative differences compared with susceptible cultivars and likely possess a high level of partial resistance.

Assessment of Solanum hougasii in Washington and Mexico as a Source of Resistance to Late Blight

A segregating introgression population, established by crossing an accession ofSolanum hougasii from the central highlands of Mexico with two successive recurrent corky ringspot resistant parental lines, was screened againstPhytophthora infestans. Foliage and tuber reactions were compared under natural epidemics of US-8 and US-11P. infestans at Mount Vernon, Washington, natural epidemics at Toluca, Mexico, and by laboratory assays with US-8 and US-11P. infestans at Pullman, Washington. Relative area under disease progress curve (RAUDPC) values in the field ranged from 3 to 80 and 2 to 42 for the 1998 and 1999 populations at Mount Vernon, respectively, and from 5 to 63, 2 to 79, and 4 to 76 in 1998, 1999 and 2000 for the populations at Toluca, respectively. Of the progeny lines tested during 1998 and 1999 at Mount Vernon, 7% were resistant, 60% were intermediate, and 33% were susceptible. Of those tested during 1998, 1999, and 2000 at Toluca, 33, 31, and 36% were resistant, intermediate and susceptible, respectively. RAUDPC values in the field at Mount Vernon and Toluca were significantly (P = 0.0001) correlated. Late blight severity on detached leaflets inoculated with US-8 and US-11P. infestans in the laboratory ranged from 0 to 64% or 65%, respectively. Severity of infection on inoculated tubers ranged from 0 to 68% for US-8 and 0 to 80% for US-11. Disease severity on leaflets in laboratory tests was significantly correlated with field RAUDPC values, but tuber severity in laboratory tests was not, although some lines exhibited resistance in both the foliage and tubers. Foliar resistance in the field was characterized by leaf chlorosis, as well as limited lesion expansion and sporulation. Estimate of broad-sense heritability was relatively high. Only 15% of this BC1 population showed significant instability, giving little indication of specific interactions between genotypes and populations of theP. infestans pathogen that would be indicative of R-gene interactions. The introgression population (BC1) appears to be expressing highly heritable durable resistance. The high heritability estimate suggests that utilization of highly resistant and stable BC1 genotypes, such as 53.78, as a parental source of foliar late blight resistance will transmit substantial nonrace specific genetic resistance to future progenies, and that ultimately, after several cycles of backcrossing and selection, this genetic potential could be deployed in new potato cultivars.ResumenElSolanum hougasii de las zonas montañosas centrales de México con dos líneas de progenitores sucesivamente recurrentes, resistentes al anillo corchoso, fue tamizado contraPhytophthora infestans. Se compararon reacciones de follaje y tubérculos de US-8 y US-11 bajo epidemia natural deP. infestans en Mount Vernon, Washington, epidemia natural en Toluca, Méxicoy por pruebas de laboratorio conP. infestans de US-8 y US-11 en Pullman, Washington. Los valores del área relativa de bajo de la curva de progreso de la enfermedad (RAUDPC) en el campo variaron de 3 a 80 y de 2 a 42 para poblaciones de 1998 y 1999 en Mount Vernon, respectivamente y de 5 a 63, 2 a 79 y 4 a 76 en 1998,1999 y 2000 respectivamente para poblaciones de Toluca. De las líneas de progenie probadas durante 1998 y 1999 en Vernon, el 7% fueron resistente, el 60 intermedias y 33% susceptibles. Entre aquellas probadas en Toluca durante 1998,1999 y 2000,33,31 y 36% fueron resistentes, intermediasy susceptibles respectivamente. Los valores RAUDPC en el campo en Mount Vernon y Toluca fueron significativamente correlacionados (P=0.0001). La severidad del tizón tardío en foliolos separados inoculados conP. infestans de US-8 y US-11 en el laboratorio variaron de 0-64% o 65% respectivamente. La severidad de infección en tubérculos inoculados varió de 0-68% para US-8 y de 0 a 80 para US-11. La severidad de la enfermedad en foliolos en pruebas de laboratorio fue significativamente correlacionada con los valores RAUDPC de campo, pero la severidad en el tubérculo de las pruebas de laboratorio no lo fue, aunque algunas líneas mostraron resistencia en el follaje y tubérculos. La resistencia foliar en el campo se caracterizó por clorosis, así como lesiones de expansión y esporulación limitadas. Los estimados de heredabilidad en sentido amplio fueron relativamente altos. Solo el 15% de esta poblacion BC1 mostró una significativa inestabilidad, dando poca indicatión de las interacciones específicas entre genotipos y poblaciones del patógenoP. infestans que indicaría interacciones de genes R. La población introgresiva (BC1) parece estar expresando resistencia hereditaria altamente durable. El alto grado de heredabilidad estimada sugiere que la utilizatión de genotipos BC1 altamente resistentes y estables, por ejemplo el 53.78, como fuente de resistencia de los progenitores altizón tardío foliar transmitirä la resistencia genética de raza no específica a futuras progenies y finalmente después de varios ciclos de retrocruzamientos y selection, este potencial genético puede ser desplegado en cultivares nuevos de papa.

Metalaxyl-M-Resistant Pythium Species in Potato Production Areas of the Pacific Northwest of the U.S.A.

Several Pythium species causing leak on potato are managed by the systemic fungicide metalaxyl-M. Metalaxyl-M-resistant (MR) isolates of Pythium spp. have been identified in potato production areas of the U.S.A., but information is lacking on the distribution of MR isolates in the Pacific Northwest. Soil samples from numerous fields (312) cropped to potatoes in Idaho (140), Oregon (59), and Washington (113) were assayed using metalaxyl-M-amended agar for the presence of MR isolates of Pythium in 2004 to 2006. Altogether, 1.4%, 42.4% and 32.7% of the fields from these states, respectively, were positive for MR Pythium. Isolates of Pythium ultimum that were highly resistant to metalaxyl were recovered from 53 fields representing ID, OR, and WA. Greater than 50% of the Pythium soil population consisted of MR isolates in ten of 64 fields from Oregon and Washington. Nine species of Pythium were recovered from soil samples, of which MR P. ultimum and P. spinosum were identified. Isolates of MR P. ultimum recovered from soil were pathogenic on potato tubers and may pose a serious threat to the management of Pythium leak and seed rot of diverse crops rotated with potato.ResumenVarias especies de Pythium que causan la pudrición acuosa en papa son controladas por el fungicida sistémico metalaxilo-M. Cepas de metalaxilo-M-resistente (MR) de Pythium spp. han sido identificadas en áreas de producción de papa de los Estados Unidos, pero no hay información de la distribución de cepas MR en el Pacifico Noroeste. Muestras de suelo de numerosos campos (312) cultivados con papa, en Idaho (140), Oregon (59), y Washington (113) fueron ensayadas utilizando agar metalaxilo-M-enmendado para la presencia de Pythium MR del 2004 al 2006. En total, 1.4%, 42.4% y 32.7% de los campos de estos estados, respectivamente, dieron positivo a Pythium MR. Cepas de Pythium ultimum altamente resistentes al metalaxilo fueron recuperadas de 53 campos representando a Idaho, Oregon y Washington. Más del 50% de la población de Pythium del suelo, consistió de cepas MR en 10 de los 64 campos de Oregon y Washington. Nueve especies de Pythium fueron recuperadas de muestras de suelo, de las cuales fueron identificadas P. ultimum y P. spinosum MR. Cepas de P. ultimum resistente al metalaxilo-M recuperadas del suelo fueron patogénicas en tubérculos de papa y pueden representar una seria amenaza en el manejo de la pudrición acuosa por Pythium y la pudrición de la semilla en diversos campos rotados con papa.

Late Blight Epidemics in the Columbia Basin

Late blight was not originally expected to be a serious threat to potato in the semi arid environment of the Columbia Basin. However, the disease has occurred every year at various severities since 1990. Migration of Phytophthora infestans into the Columbia Basin in the 1990’s is well documented. The US-1 strain predominated in 1992 and several unique isolates were discovered in 1993, which were likely the result of genetic recombination. The recombinants were ephemeral and were not found in 1994. The US-8 stain was first observed in 1994 and came to predominate in 1995 and in subsequent years. Epidemics in the Columbia Basin have been traced to infected seed tubers, refuse tubers and volunteers. Late blight spreads in fields by foci with foci enlarging in size, producing daughter foci, and coalescing. The process continued as favored by the environment. Sporangia of P. infestans have the capability of surviving in water for extended periods of time after detachment from sporangiophores on potato tissue. Late blight has been successfully forecasted and managed regionally in the Columbia Basin. Early season rain is an effective indicator of late blight outbreaks because moisture is important for the build-up of inoculum in fields during the early stage of epidemics. Early in epidemics, moisture promotes transmission of P. infestans from infected seed tubers to emerged shoots in fields. Method of fungicide application affects fungicide distribution and cost. The alternate use of air application and chemigation provides good protection at a reduced cost compared to only air or ground application methods. Application of phosphorous acid to tubers after harvest and prior to storage can result in a reduction in post-harvest infection by P. infestans.

Exploring the genetics of lesion and nodal resistance in pea (Pisum sativum L.) to Sclerotinia sclerotiorum using genome-wide association studies and RNA-Seq

Abstract The disease white mold caused by the fungus Sclerotinia sclerotiorum is a significant threat to pea production, and improved resistance to this disease is needed. Nodal resistance in plants is a phenomenon where a fungal infection is prevented from passing through a node, and the infection is limited to an internode region. Nodal resistance has been observed in some pathosystems such as the pea (Pisum sativum L.)‐S. sclerotiorum pathosystem. In addition to nodal resistance, different pea lines display different levels of stem lesion size restriction, referred to as lesion resistance. It is unclear whether the genetics of lesion resistance and nodal resistance are identical or different. This study applied genome‐wide association studies (GWAS) and RNA‐Seq to understand the genetic makeup of these two types of resistance. The time series RNA‐Seq experiment consisted of two pea lines (the susceptible ‘Lifter’ and the partially resistant PI 240515), two treatments (mock inoculated samples and S. sclerotiorum‐inoculated samples), and three time points (12, 24, and 48 hr post inoculation). Integrated results from GWAS and RNA‐Seq analyses identified different redox‐related transcripts for lesion and nodal resistances. A transcript encoding a glutathione S‐transferase was the only shared resistance variant for both phenotypes. There were more leucine rich‐repeat containing transcripts found for lesion resistance, while different candidate resistance transcripts such as a VQ motif‐containing protein and a myo‐inositol oxygenase were found for nodal resistance. This study demonstrated the robustness of combining GWAS and RNA‐Seq for identifying white mold resistance in pea, and results suggest different genetics underlying lesion and nodal resistance.

Molecular and phenotypic characterization of variation related to pea enation mosaic virus resistance in lentil (Lens culinaris Medik.)

Jain, S., Porter, L. D., Kumar, A., Mir, R. R., Eigenbrode, S. D. and McPhee, K. E. 2014. Molecular and phenotypic characterization of variation related to pea enation mosaic virus resistance in lentil (Lens culinaris Medik.). Can. J. Plant Sci. 94: 1333-1344. Identification of genetically diverse lentil germplasm with resistance to pea enation mosaic virus (PEMV) through the combined approach of molecular marker analysis and phenotyping could prove useful in breeding programs. A total of 44 lentil (Lens culinaris Medik.) accessions, were screened for resistance to PEMV. Two accessions (PI 431663 and PI 432028) were identified with resistance to PEMV in field tests while several accessions were found resistant in greenhouse screenings. Thirty-six polymorphic simple sequence repeat (SSR) markers which produced 43 loci with 2 to 12 alleles per locus were used for genetic diversity analysis. The polymorphic information content (PIC) values for these markers ranged from 0.22-0.85 with a mean of 0.55 per marker. Using allelic data of 36 SSR primer pairs, dissimilarity ranging from 0.12 to 0.74 was calculated. Cluster analysis performed using the unweighted pair group method with arithmetic mean (UPGMA) determined that most of PEMV-resistant accessions were grouped in one cluster along with other accessions from Iran, Chile, Ethiopia, India, Pakistan, Turkey, Afghanistan and Lebanon. All the adapted cultivars originating from North and South America were grouped in another cluster along with some European accessions. The 44 accessions were classified into 4 subpopulations using Structure 2.2 software complimenting the results of UPGMA analysis and indicated the effect of geographical origin on the grouping of accessions. The results of this study can be used to select genetically diverse PEMV-resistant accessions for lentil improvement programs.