D. H. Lambert

Differences in tuber rot development for North American clones ofPhytophthora infestans

The relative aggressiveness ofPhytophthora infestons clones in potato tubers was compared in three trials using 7 to 24 isolates of 2 to 4 clones. Visible rot developed slowly at 13C with isolates of the US-1 genotype, the only significant clone found in North America prior to 1979, but substantially faster with most isolates of the newer clonal genotypes US-6, US-7 and US-8. Certain US-7 isolates were similar to US-1, and US-6 isolates also had a broad range of aggressiveness. Secondary infection byFusarium sp. increased rot development in many instances, but this effect was not clone-related. Differences in rot development may affect potato storage or late blight disease transmission.

Phenotypic stability of resistance to late blight in potato clones evaluated at eight sites in the United States

Changes in the fungal pathogenPhytophthora infestans in the United States pose a significant threat to potato production. Sources of resistance to these new genotypes of P.infestans need to be identified for potato breeders to have parental materials for crossing, and the phenotypic stability of late blight resistance in these potato clones needs to be determined. Sixteen potato clones which reportedly have some resistance to late blight were evaluated at eight locations: Florida (FL), Maine (ME), Michigan (MI), Minnesota (MN), North Dakota (ND), New York (NY), Pennsylvania (PA) and Wisconsin (WI) in 1996. Percent infected foliage was recorded at approximately weekly intervals following the onset of the disease at each location. Area under the disease progress curve (AUDPC) was calculated. Clones were ranked for mean AUDPC within location and the nonparametric stability statistics, mean absolute rank differences and variance of the ranks, were analyzed for phenotypic stability. Neither of these statistics was significant, indicating a lack of genotype x environment interaction on the rankings of these clones across locations in 1996. The four clones with lowest AUDPC scores were U.S. clones AWN86514-2, B0692-4, B0718-3 and B0767-2. These clones should be useful parental materials for breeders seeking to incorporate genes for late blight resistance into potatoes.

Transmission of Phytophthora infestans in Cut Potato Seed

One or two cycles of late blight transmission may occur in cut potato seed between cutting and emergence. If seed are not planted immediately, mycelium from diseased pieces can infect across adjoining cut surfaces within 8 hr. Sporangia production begins on freshly cut diseased tubers in 1–2 days, on mycelium-infected seed pieces within 4–5 days, and on spore-infected seed pieces in about 1 wk. These spores are further dispersed during handling and planting, and may infect contaminated seed in the ground, regardless of suberization. Sporangia can be transferred to healthy tubers on cutting knives, although this appears infrequent. The risk and degree of secondary spread increase with the length of time cut seed are held before planting. Such secondarily infected seed are more likely to survive and produce infected stems than the original infected tubers. Treatment of seed with appropriate fungicides immediately after cutting substantially reduces or prevents transmission of late blight to healthy seed. Certain seed treatments withoutPhytophthora-specific components may increase the risk of tuber blight.

Nutritional interactions influencing diseases of potato

Nutrient management can decrease the severity of a number of important potato diseases, and certain practices, such as maintaining a low pH for scab control, have been followed for that single objective. More commonly, growers have incorporated fertility modifications into their particular disease and farming situations. Unfortunately, disease minimization may or may not be consistent with optimal fertilization for yield, quality, and profitability. Optimization for one disease may not match that for another, and the exact mechanisms involved are often complex and poorly understood. Potato growers will continue to experience conflicting production constraints. These constraints include price-driven needs for yield improvement and expense reduction; customer-driven demands for quality improvement; varietal shifts driven by the above considerations rather than by disease reduction; increasing pressure to justify, shift and reduce pesticide use; continuing concerns about nitrogen and phosphorus movement in groundwater and runoff; and increased attention to management of rotation crops.On the positive side, nutrient management strategies for highly specific situations continue to improve and practices addressing nutrient and disease variability within fields are becoming more sophisticated. In this context, there are opportunities for cultural management practices that reduce disease pressure and reliance on chemical controls. To be effectively integrated into such specialized management systems, the mechanisms of these control measures and the conditions under which they are practical will need to be better understood. Likewise, disease responses to these tactics need to be better quantified to allow an adequate cost-benefit analysis. With mounting concerns about the effects of agricultural pesticides on food safety, farm workers, and the environment, management of plant-available nutrients may become practical approaches for disease suppression in the future.ResumenEl manejo de nutrientes puede disminuir la severidad de muchas enfermedades importantes de papa y ciertas prácticas, tal como el mantener un pH bajo para el control de la sarna, se ha seguido con este simple objectivo. Con frecuencia, los productores de papa han incorporado modificaciones de la fertilidad con respecto a ciertas enfermedades en particular y condiciones de cultivo. Desgraciadamente, la reducción de la enfermedad puede ser consistente con una fertilización óptima para rendimiento, calidad y rentabilidad. Lo que puede controlar una enfermedad puede no ser bueno para otra enfermedad y los mecanismos involucrados son a menudo complejos e insuficientemente comprendidos. Los productores de papa continuarán experimentando limitaciones conflictivas en la producción. Estas limitaciones incluyen la influencia del precio de los artículos para mejorar el rendimiento y la reducción de los gastos; influencia de las demandas del consumidor para el mejoramiento de la calidad; cambios de variedad debido a las consideraciones anteriores más que a la reducción por enfermedades; incremento en la presión para la justificación; cambios y reducción en el uso de pesticidas; preocupación continua acerca del movimiento del nitrógeno y fósforo en el agua del suelo y su pérdida y un aumento de atención en la rotación de cultivos. En el lado positivo, las estrategias en el manejo de nutrientes para situaciones altamente específicas continúan mejorando y las prácticas referentes a los nutrientes y la variabilidad de las enfermedades dentro del campo se están volviendo más sofisticadas. En este contexto, existen oportunidades para el manejo de practicas culturales que reducen la presión de la enfermedad y la confianza en el control químico. Para estar efectivamente integrado a tales sistemas especializados de manejo, el mecanismo de estas medidas de control y las condiciones bajo las cuales son practica, necesitarán ser mejor comprendidas. Asimismo, las respuestas de estas tácticas necesitan ser mejor cuantificadas para permitir un adecuado análisis costo-beneficio. Con referencia a los efectos de los pesticidas agrícolas en la seguridad alimentaria, los trabajadores y el medio ambiente, el manejo de los nutrientes disponibles para la planta pueden convertirse en una manera de enfocar la practica para la supresión de enfermedades en el futuro.

Foliar resistance to late blight in potato clones evaluated in national trials in 1997

Changes in the oomycetePhytophthora infestans in the United States and other parts of the world pose a significant threat to potato production. A continual evaluation of potato clones for resistance to late blight is necessary to identify clones with resistance and to monitor the stability of resistance in light of the emergence of new and more aggressive strains of this pathogen. Twentytwo potato clones (10 cultivars and 12 selections) were evaluated in 1997 for late blight resistance at seven U.S. locations. Seven late blight differentials (R1R2R3R4, R1R2R4, R1R3R4 R3, R8 R10, and Rmulti) were also included in the test at five of these locations. The US-8 strain of P.infestans was present at all locations. Percent infected foliage was recorded at approximately weekly intervals following the onset of disease. Area under the disease progress curve (AUDPC) was calculated. The nonparametric stability statistics mean absolute rank differences (Si(1)) and variances of the ranks (Si(2)) were used to analyze phenotypic stability. Although neither of these statistics was significant for individual clones, both of these statistics were significant when summed over clones, indicating the importance of genotype × environment interactions on the rankings of these clones across locations. The most late blight-resistant and susceptible clones were the most stable; clones in the intermediate ranges were most subject to rank changes due to genotype × environment interactions. The most late blight-resistant clones were AWN86514-2, B0692-4, B0718-3, and B0767-2. The most susceptible clones were B0811-13, B1004-8, Nor-Donna, and Krantz. AUDPC was very low for the late blight differentials R8 and Rmulti, moderately low for R10 and very high for the remaining differentials. This study is important in characterizing the reaction of potato clones to new strains of P.infestans.

Application of a PCR-based test for detection of potato late blight and pink rot in tubers

A polymerase chain reaction (PCR)-based test for potato late blight (Phytophthora infestans) and pink rot (P.erythroseptica, P. nicotianae) diseases has been developed for use with potato tuber tissue. Primers based on sequence analysis of the ITS2 region of ribosomal DNA of late blight and pink rot pathogens were utilized in PCR assays of inoculated tubers and tubers harvested from plots known to have late blight and/or pink rot. Assays of artificially inoculated Kennebec and Russet Burbank tubers revealed thatP. infestans was detected by PCR as early as 72 h after inoculation and in the absence of visible symptoms. Much higher detection frequencies were obtained by PCR compared with plating on selective medium or placement of tissue in moist chambers. Tubers from plots known to have late blight and/or pink rot were tested using the PCR assay. Assay of late blight lesions showed ca. 80% recovery for late blight-infected tubers from the field. Results indicate that the PCR assay provides a rapid and accurate test for diagnosis of late blight and pink rot in potato tubers.

Effects of pest and soil management systems on potato diseases

Long-term cropping systems research is important in order to reduce production costs, to control crop pests, and to optimize the sustainability of agro-ecosystems. Soil amendment use, improved disease management practices, and careful cultivar choice are some of the potential components for improving potato production systems. This research was conducted in long-term cropping systems plots in order to evaluate the impact of soil amendments, pest management practices, and cultivar on foliar and soil-borne potato diseases and to assess the relationships of soil and pest management practices to disease levels and soil microbial activity. Fungicide applications for management of foliar diseases varied between the pest management systems (e.g., biological, reduced input, and conventional). Incidence of potato foliar diseases was quantified five times during the cropping season. The impact of soil amendment and pest management practices on soil microbial activity and tuber-borne diseases was also investigated. Low incidences of foliar and selected soil-borne diseases were recorded. Disease levels varied between years, cultivars, pest management, and soil amendments. Significant differences between cultivars were detected for early blight, white mold, and black dot. The cultivar Superior had higher incidence of white mold and black dot, while cv Atlantic had higher early blight incidence. Pest management system significantly affected foliar early blight incidence in 1998, but not in 1997. Pest management system did not affect late blight, white mold, or black dot incidence, or tuber disease incidence in either year. The addition of soil amendments significantly impacted tuber black dot incidences. Microbial activity responded to increasing temperature as the season progressed and was significantly enhanced by the addition of manure and compost soil amendments; however, lower disease incidence was not associated with increased microbial activity. While pest management practices were not major determinants of disease levels in these experiments, the results show that soil amendments can increase incidence of selected tuber diseases and microbial activity in soils.ResumenLa investigación sobre sistemas de cultivo a largo plazo es importante para reducir costos de producción, controlar enfermedades y optimizar la sostenibilidad de los sistemas agro ecológicos. Las enmiendas de suelo, prácticas mejoradas de manejo de enfermedades y cuidadosa elección del cultivar, son algunos de los componentes potenciales para mejorar los sistemas de producción de papa. Esta investigación fue realizada en parcelas de cultivo a largo plazo, con el objeto de evaluar el impacto de enmiendas de suelo, manejo de enfermedades y cultivar empleado, sobre las enfermedades foliares y las transmitidas por el suelo y de evaluar las relaciones del suelo y prácticas de manejo de plagas con los niveles de enfermedad y actividad microbiana del suelo. Las aplicaciones de funguicidas para el control de enfermedades foliares, varió con los sistemas de manejo sanitario (biológico, gasto reducido y convencional). La incidencia de enfermedades foliares se evaluó cinco veces durante el desarrollo del cultivo. También se investigó el efecto de enmiendas, prácticas de control sobre la actividad microbiana del suelo y las enfermedades transmitidas por tubérculos. Se registró baja incidencia de enfermedades foliares y transmitidas por el suelo. Los niveles de enfermedad variaron entre años, cultivares, manejo sanitario y enmiendas de suelo. Se detectaron diferencias significativas entre cultivares para tizón temprano, moho blanco y mancha negra. La variedad Superior tuvo la más alta incidencia de moho blanco y mancha negra, mientras que el cultivar Atlantic tuvo mayor incidencia de tizón temprano. El sistema de manejo sanitario afectó significativamente la incidencia de tizón temprano, moho blanco o mancha negra en 1998, pero no en 1997. El sistema de manejo sanitario no afectó la incidencia de tizón tardío, moho blanco, mancha negra o enfermedades al tubérculo en ambos años. La aplicación de enmiendas al suelo influenció significativamente la incidencia de mancha negra en el tubérculo. La actividad microbiana respondió al aumento de temperatura a medida del avance de la estación y fue significativamente incrementada cuando se agregó al suelo enmiendas de estiércol y compost; sin embargo, la baja incidencia de enfermedad no estuvo asociada con el incremento de la actividad microbiana. Aunque las prácticas de manejo sanitario no fueron un determinante del nivel de enfermedades en estos experimentos, los resultados demuestran que las enmiendas pueden incrementar la incidencia de determinadas enfermedades al tubérculo y la actividad microbiana del suelo.

Relative susceptibility of potato varieties toStreptomyces scabiei andS. acidiscabies

In 2000 and 2004, 19 potato varieties were grown in separate plots inoculated with the scab pathogensStreptomyces scabiei andS. acidiscabies. Reaction of the varieties to these two species were highly correlated in both years, with no host variety-pathogen species interaction. These results are consistent with the central role of the bacterial toxin thaxtomin in scab development, and indicate that this and other mechanisms involved in pathogen infection and establishment are not expressed differentially with regard to variety. Likewise, there is no apparent differential response of the pathogens to those host factors determining the degrees of resistance and symptoms expressed in different varieties.

Occurrence and cross-infection of Phytophthora infestans on hairy nightshade (Solanum sarrachoides) and potato (Solanum tuberosum) in Maine

Late blight, caused by Phytophthora infestans, is a significant disease in all potato-growing regions of the world. Identification of additional hosts is critical for understanding potential sources of inoculum. In 2004, late blight was observed on leaves and stems of hairy nightshade (Solanum sarrachoides), a common weed in many potato fields in Maine. Diseased nightshade plants were detected in 2 of 18 locations having potato late blight. Lesions from diseased nightshade leaves ranged from 0.25 to 9 cm2 and, on diseased stems, the range of lesion length was 4.5 to 13.8 cm. Isolations from diseased samples and microscopic observations confirmed the presence of P. infestans. Based on allozyme analysis, 27 representative isolates of P. infestans from hairy nightshade were identified as the US 8 genotype. Pathogenicity tests and Kochs postulates were satisfied by inoculating intact plant and detached nightshade leaves with P. infestans isolated from hairy nightshade. Cross-infection of nightshade, potato leaves (‘Shepody’), and potato tubers (‘Yukon Gold’) occurred when the inoculum source was either hairy nightshade or potato. When the inoculum source was hairy nightshade, the mean infection frequency at 18 °C was 50% and 67% on hairy nightshade and potato leaves, respectively. Inoculation with Phytophthora infestans isolated from potato resulted in average infection frequencies of 73% and 83% on nightshade and potato, respectively. These results suggest that hairy nightshade may be an inoculum source for late blight in potato fields. Management practices for controlling hairy nightshade would benefit potato production.

Effectiveness of SIMBLIGHT1 and SIMPHYT1 models for predicting Phytophthora infestans in north-eastern United States

Accurate prediction of Phytophthora infestans outbreak in a cropping season is crucial for the effective management of late blight on potato. The SIMBLIGHT1, SIMPHYT1 and modified SIMPHYT1 models were assessed for predicting late blight outbreaks relative to NOBLIGHT model by using climatic and crop data from field experiments at Presque Isle, ME, from 2004 to 2009. The dynamics of late blight infection pressures were computed by SIMPHYT3 model to assess the conduciveness of environmental conditions for late blight infection and potential for disease development. Infection pressure indicated conditions were moderately conducive for late blight development but varied across years. The SIMPHYT1 model recommended fungicide applications to commence on 11 July, 21 July, 8 July, 10 July, 7 July and 7 July for 2004, 2005, 2006, 2007, 2008 and 2009, respectively. The modified SIMPHYT1 model (US-version) recommended similar fungicide application dates for the same years with the exception of 2007. Model simulations of disease outbreak differed from actual recorded observations in untreated plots by 24–65 days. Simulation of SIMBLIGHT1 based on high and low soil moisture conditions in the field resulted in vast differences in dates of first fungicide application. Validation of the models (differences in the number of days between recommendation of fungicide treatment and late blight outbreak) indicated a better goodness of fit for the models (intervals of 6–20 days from the start of fungicide application to first disease outbreak). The NOBLIGHT model was accurate in forecasting the timing of first fungicide applications for late blight control. Significant improvements in late blight predictions could result if these models were modified to account for external sources of inoculum, by combining weather-based forecasts with spore traps, disease detection methods or complimentary systems.