W. R. Stevenson

Somatic hybrids between Solanum bulbocastanum and potato : a new source of resistance to late blight

Abstract Solanum bulbocastanum, a wild, diploid (2n=2x=24) Mexican species, is highly resistant to Phytophthora infestans, the fungus that causes late blight of potato. However this 1 EBN species is virtually impossible to cross directly with potato. PEG-mediated fusion of leaf cells of S. bulbocastanum PI 245310 and the tetraploid potato line S. tuberosum PI 203900 (2n=4x=48) yielded hexaploid (2n= 6x=72) somatic hybrids that retained the high resistance of the S. bulbocastanum parent. RFLP and RAPD analyses confirmed the hybridity of the materials. Four of the somatic hybrids were crossed with potato cultivars Katahdin or Atlantic. The BC1 progeny segregated for resistance to the US8 genotype (A-2 mating type) of P. Infestans. Resistant BC1 lines crossed with susceptible cultivars again yielded populations that segregated for resistance to the fungus. In a 1996 field-plot in Wisconsin, to which no fungicide was applied, two of the BC1 lines, from two different somatic hybrids, yielded 1.36 and 1.32 kg/plant under a severe late-blight epidemic. In contrast, under these same conditions the cultivar Russet Burbank yielded only 0.86 kg/plant. These results indicate that effective resistance to the late-blight fungus in a sexually incompatible Solanum species can be transferred into potato breeding lines by somatic hybridization and that this resistance can then be further transmitted into potato breeding lines by sexual crossing.

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.

Transfer of tuber soft rot and early blight resistances from Solanum brevidens into cultivated potato

Tuber soft rot and early blight are serious potato diseases. Development of potato varieties resistant to these diseases has been hindered by the scarcity of resistant germplasm. A diploid wild species, Solanum brevidens, shows significant resistance to both diseases. Numerous potato breeding lines have been developed from a potato-S. brevidens somatic hybrid, A206. A BC3 clone, C75-5+297, derived from this somatic hybrid as well as its BC1 and BC2 parental lines showed resistance to both tuber soft rot and early blight. Clone C75-5+297 has consistently out-yielded common varieties under disease stress. Using both molecular and cytogenetic approaches we demonstrated that a single copy of chromosome 8 from S. brevidens replaced a potato chromosome 8 in C75-5+297. Thus, C75-5+297 represents a potato-S. brevidens chromosome substitution line. Our results suggest that the presence of a single chromosome from S. brevidens may significantly impact the resistance to multiple potato diseases. The high yield potential of C75-5+297 makes it an excellent parent for developing potato varieties with resistances to both tuber soft rot and early 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.

Water management, disease development, and potato production

ConclusionsThe increased use of high frequency, sprinkler irrigation programs, particularly those with early season requirements for maintenace of high soil moisture levels, can mean increased tuber yields and quality, and reduced incidences of common scab. However, such programs may lead to increased incidences of brown center (and hollow heart) and powdery scab. More importantly, with respect to most of the currently important diseases of potato in the United States (i.e., late blight, early blight, white mold, bacterial stem rot, early dying, bacterial ring rot), growers using such programs face serious and complicated disease management problems because of the effects of more favorable canopy and soil moisture and temperature regimes on pathogen infection, growth, reproduction, dispersal, and survival.This means that today’s potato grower needs to devote attention to better integration of disease and irrigation management strategies. For example, irrigations must be carefully timed, so that water is applied only when crop demands warrant while minimizing the duration of leaf wetness. This will require use of potato crop irrigation scheduling programs based on calculations of crop evapotranspiration. Such programs will have to have their crop coefficients (17) adjusted for reductions in leaf area caused by foliar diseases so as to reduce the likelihood of applying excessive water and ending up with additional problems of haulm or tuber rot.

The critical period for control of early blight (Alternaria solani) of potato

The purpose of this study was to identify when the critical period for control of early blight occurs and thus the need to initiate a spray schedule of protectant fungicides. The initiation of weekly fungicide spray schedules was staggered to identify the critical period. The accumulation of 400 P-Days, environmental conditions favorable for sporulation, spore dissemination and plant infection and airborne spores ofA. solani were detected prior to the critical period each year. Since early blight is a disease of old plants, host susceptibility appeared to be involved in the timing of the critical period in addition to favorable weather and the presence of the pathogen. A measure of potato plant development, and therefore susceptibility toA. solani, is needed to adequately predict the critical period. The accumulation of 400 P-Days may aid in the prediction of the critical period by estimating the time of plant susceptibility and therefore the timing of the first necessary fungicide spray.CompendioEl propósito de este estudio fue identificar el momento del período crítico para el control del tizón temprano y, por lo tanto, para iniciar un programa de aspersión de fungicidas protectores. La iniciación de programas semanales de aspersión de fungicidas se hizo en forma escalonada con el objeto de identificar el período crítico. La acumulación de 400 P-días, condiciones ambientales favorables para la esporulación, diseminación de las esporas e infección de las plantas y esporas aerotransportadas deA. solani se detectaron cada año con anterioridad al período crítico. Puesto que el tizón temprano es una enfermedad de plantas viejas, parece que la susceptibilidad tiene ingerencia en la definición del período crítico, además de las condiciones climáticas favorables y la presencia del patógeno. La medición del desarrollo de la planta y por lo tanto su susceptibilidad aA. solani son necesarios para predecir adecuadamente el período crítico. La acumulación de 400 P-días puede ayudar en la predicción del período crítico por estimación del momento de susceptibilidad de la planta y, por lo tanto, el momento oportuno para la necesaria primera aplicación de fungicida.

The potential impact of field resistance to early blight on fungicide inputs

ConclusionsEarly blight is clearly an important risk to potato crops around the world. In North America current control practices include the intensive use of fungicide sprays to protect susceptible cultivars from infection and premature defoliation. Estimated normal seasonal costs for fungicide applications to North American potato crops exceed at least

Developing a pesticide risk assessment tool to monitor progress in reducing reliance on high-risk pesticides

21 million and depending on the choice of material, costs can exceed

Pest management for potatoes in Wisconsin— A pilot program

44 million. During years when late blight is a widespread threat to major production areas, the cost for controlling foliar blights can be much higher due to an increased number of spray treatments and higher rates of fungicide use. Some would argue that these costs represent but a small investment to protect the grower investment and total crop value. Others, however, are demanding a reduction in pesticide use regardless of risk to the crop. The potato crop is a vulnerable target for future cuts in pesticide use since so many of the materials used in potato production are being scrutinized by government agencies. Cultivars with a high level of field resistance to early blight may become a necessity for the continued strength of the potato industry, especially if the industry loses the currently available fungicides that have provided insurance for the past several decades. The technology and genetic pool are available for making needed advances in field resistance to early blight. Perhaps the time is right for pathologists and breeders to declare this a priority project and move ahead together toward this common goal.

Megachip — a new potato variety for chipping

A methodology is presented for the development of a pesticide risk assessment tool that was used to monitor progress in reducing use of high-risk pesticides in Wisconsin potato production. Multi-attribute toxicity factors are calculated that reflect each pesticide’s acute and chronic toxicity to mammals, birds, fish and small aquatic organisms, and compatibility with biointensive Integrated Pest Management. These factors are then multiplied by the pounds of active ingredient of a given pesticide applied to estimate pesticide-specific toxicity units. Wisconsin potato industry baseline toxicity units by type of pesticide and for 11 targeted higher-risk pesticides are presented for 1995. Reductions in toxicity units from this baseline are reported for 1997 and 1999, as are reductions achieved in a commercial-scale field experiment in 2000.ResumenSe presenta una metodología para el desarrollo de una herramienta de evaluación de riesgos de los plaguicidas que fue usada para vigilar los progresos en la reducción del uso de plaguicidas de alto riesgo en la productión de papas de Wisconsin. Se calculan los atributos múltiples de los factores de toxicidad, que reflejan la toxicidad aguda y crónica de cada plaguicida para los mamíferos, aves, peces y organismos acuáticos pequeños, y la compatibilidad con un biointensivo Manejo Integrado de Piagas. Estos factores son luego multiplicados por las libras del principio activo del plaguicida aplicado, con el fin de calcular las unidades de toxicidad específicas del plaguicida. Se presenta el punto de comparación para 1995 de las unidades de toxicidad de la industria de papa de Wisconsin por tipo del plaguicida y para 11 plaguicidas seleccionados por ser de más alto riesgo. Se reportan las reducciones del punto de comparación en las unidades de toxicidad para 1997 y 1999, como reducciones logradas en un experimento de campo de escala comercial en el 2000.