D. A. Inglis

Relative resistances of potato clones in response to new and old populations of Phytophthora infestans

Potato cultivars and clones were evaluated in Washington and New York in 1993 and 1994 for field reaction to recent immigrant genotypes of Phytophthora infestans. Plants were visually evaluated at regular intervals for percent blighted foliage. Relative cultivar susceptibilities were compared by ranking the values obtained for areas under disease progress curves (AUDPC) of each line tested. Cultivar rankings in response to infection by new, immigrant isolates of P infestans were nearly identical to rankings obtained previously with isolates prevalent prior to 1990. The cultivars Norchip, Hilite, Russet Norkotah, Goldrush, Superior, and Shepody were more susceptible than Russet Burbank. White Rose and Ranger Russet were similar in susceptibility to Russet Burbank. C008-3008-1, ND-2438-7R, Kennebec, and Elba were less susceptible than Russet Burbank. Comparisons of the number of blighted tubers at harvest showed that foliage and tuber susceptibilities differed among cultivars. Shepody and Russet Norkotah tubers were most susceptible to tuber blight of those tested.

Testing for resistance to metalaxyl inPhytophthora infestans isolates from northwestern Washington

Outbreaks of late blight occurred in commercial potato fields in northwestern Washington in 1989 and reached epidemic proportions in 1990. Since most blighted fields reportedly had received 2–4 late-season applications of metalaxyl, the possibility existed that pathogenic strains ofPhytophthora infestans had developed resistance to this systemic fungicide. In testing this hypothesis,P. infestans was isolated consistently from diseased leaves, stems, and tubers of potato when small tissue pieces were surface-disinfected in 0.5% NaOCl for 3 min. Virulence of isolates was assessed on detached leaflets of eight differential (R-gene) potato genotypes. Although simple races (single and double) were recovered, there was also a high frequency of complex race combinations. When tested in metalaxyl-amended media, 81% of the 73 isolates recovered from diseased tissues were highly resistant to 10 μg metalaxyl/ml and 19% yielded intermediate resistance. Sensitive isolates were not recovered. Fifty-five percent of 40 isolates sporulated on potato tuber disks in the presence of ≥10 ug metalaxyl/ml. On leaf tissues, 85% of 20 isolates sporulated in the presence of 10 μg metalaxyl/ml, and 50% sporulated in the presence of 100 μg/ml. This is the first report ofP. infestans resistance to metalaxyl in the U.S.CompendioEn 1989 se presentaron brotes de tizòn tardío en campos comerciales de papa del noroeste de Washington los que alcanzaron niveles de epidemia en 1990. Desde que la mayoría de los campos afectados, según se informó, habían recibido 2–4 aplicaciones de metalaxyl, al final de la temporada, existía la posibilidad que las razas patogénicas dePhytophthora infestons hubieran desarrollado resistencia a este fungicida sistémico. Al probar esta hipótesis, se aisló consistentementeP. infestans de hojas, tallos y tubérculos de papa cuando pequeños trozos de tejido fueron desinfectados en NaOCl al 0.5% por 3 minutos. Se determinó la virulencia de los aislamientos sobre foliolos desprendidos de ocho genotipos diferenciales (gen-R) de papa. Si bien se recobraron sólo razas simples (solas y dobles), hubo también una alta frecuencia de combinaciones complejas de razas. Cuando se probaron en medios con adición de metalaxyl, 81%, de los 73 aislamientos obtenidos de los tejidos afectados, fueron altamente resistentes a 10 g metalaxyl/cm3 y 19% mostraron resistencia intermedia. No se obtuvieron aislamientos sensibles. Cincuenta y cinco por ciento de 40 aislamientos esporularon sobre discos de tubérculos de papa en presencia de 10 g metalaxyl/cm3. Sobre tejidos de hojas, 85% de 20 aislamientos esporularon en presencia de 10 g metalaxyl/cm3, y 50% esporularon en presencia de 100 g/cm3. Este es el primer informe de resistencia delP. infestons al metalaxyl en los EE.UU.

Assessment of Greenhouse and Laboratory Screening Methods for Evaluating Potato Foliage for Resistance to Late Blight

Greenhouse and laboratory screening methods for assessing potato foliage for resistance to late blight were compared using 15 cultivars and advanced breeding selections with known field response to late blight. Screening methods included greenhouse inoculation of plants in several age classes, and laboratory assays of detached leaflets, leaf disks, and stem cuttings. Greenhouse inoculation of plants 7 to 11 weeks after planting, near the time of flowering, corresponded best to results obtained in field evaluations, but there were significant differences in disease severity between separate greenhouse tests. This is consistent with variation in late blight severity on a year-to-year basis when cultivars are compared in the field. The greenhouse inoculation method allowed for testing of several components of partial resistance, such as infection efficiency and lesion growth rate, which may exist for each cultivar. Laboratory assays proved less reliable than greenhouse assays for overall ratings of partial resistance, but could be useful for measuring specific components of resistance. Screening evaluations for late blight resistance should include standard cultivars with known reaction to Phytophthora infestans to reference the disease potential within the screening evaluation.

Control of Potato Tuber Rots Caused by Oomycetes with Foliar Applications of Phosphorous Acid

Phosphorous acid for control of tuber rots caused by Phytophthora infestans, P. erythroseptica, and Pythium ultimum was applied to foliage of potato cultivars at various application timings and rates under growing conditions in the Pacific Northwest at Othello and Mount Vernon, WA, and Bonners Ferry and Aberdeen, ID in 2001 to 2003. Efficacy was assessed by artificially inoculating harvested tubers. Mean incidence and severity of late blight tuber rot in tubers inoculated with US-8 and US-11 isolates of Phytophthora infestans usually were significantly less when the foliage from which the tubers were obtained was treated with phosphorous acid than when it was not treated at all locations. With two applications of phosphorous acid, late blight tuber rot in the tuber-resistant cv. Umatilla Russet was significantly less than for Ranger Russet. For phosphorous acid at a rate of 9.37 kg a.i./ha, late blight tuber rot control achieved with two applications at 2-week intervals was not consistently improved across locations by making an additional application 2 weeks later. In 2003, incidence and severity of late blight tuber rot did not differ significantly between the rates of 7.49 and 9.37 kg a.i./ha at both Othello and Mount Vernon. Late blight tuber rot incidence and severity were significantly less at a rate of 7.49 kg a.i./ha when the application schedule began at initial tuber bulking rather than when the first application was made 4 weeks after initial tuber bulking at Othello, but not Mount Vernon. Incidence of pink rot was significantly less in inoculated tubers from plots treated with three applications of phosphorous acid than in tubers from nontreated control plots at Mount Vernon in 2002 and 2003, Bonners Ferry in 2002, and Aberdeen in 2003. Pink rot severity was reduced significantly by both two and three phosphorous acid applications at Mount Vernon in 2002. Pink rot incidence, but not severity, was reduced significantly at all timings when either 7.49 or 9.37 kg a.i./ha was applied at Mount Vernon in 2003. Control of Pythium spp. by phosphorous acid was not evident in this study. Total tuber yield at harvest did not differ significantly among the phosphorous acid treatments and the nontreated control at Othello and Mount Vernon in 2001 and 2002.

Assessment of Laboratory Methods for Evaluating Potato Tubers for Resistance to Late Blight

Two laboratory methods (whole tuber and tuber slice) were evaluated and compared with field assessment of potato tubers for resistance to late blight caused by Phytophthora infestans. Altogether, the resistance responses of 20 cultivars and advanced selections were compared by the three methods. All three assays separated materials into resistant and susceptible groups. The majority of cultivars and advanced selections did not react in the same way within the field and tuber-slice assays but did react similarly within the whole-tuber assay. Some cultivars were susceptible in the field at harvest but were resistant in the whole-tuber assay. The differences may be due to the extent of lenticel and eye development during tuber formation. In contrast, other cultivars resistant in the field at harvest were susceptible in the whole-tuber assay. In this case, placement in the hill may affect whether or not the tuber is exposed to inoculum or whether aging of tubers during storage affects susceptibility to tuber blight. Our study shows that high levels of tuber resistance are available in certain commercial cultivars and advanced potato selections. For laboratory assessments to be reliable, however, both pre- and poststorage evaluations may be necessary.

Biodegradable plastic agricultural mulches and key features of microbial degradation

The development of biodegradable plastic mulch films for use in agriculture has been ongoing for decades. These films consist of mixtures of polymers with various additives. As a result, their physical and chemical properties differ from those of the pure polymers often used for in vitro enzymatic and microbial degradation studies, raising questions about the biodegradation capability of mulch films. Currently, standards exist for the biodegradation of plastics in composting conditions but not in soil. Biodegradation in soil or compost depends on a complex synergy of biological and abiotic degradative processes. This review discusses the physicochemical and structural properties of biodegradable plastic mulches, examines their potential for on-site decomposition in light of site-to-site variance due to environmental and biological conditions, and considers the potential for long-term effects on agroecosystem sustainability and functionality.

Defender: A High-Yielding, Processing Potato Cultivar with Foliar and Tuber Resistance to Late Blight

The potato cultivar Defender is high-yielding, white-skinned, and notable for having foliar and tuber resistance to late blight infection caused byPhytophthora infestans (Mont.) de Bary. It was released in 2004 by the USDA-ARS and the agricultural experiment stations of Idaho, Oregon, and Washington. Defender is suitable for processing into french fries and other frozen potato products directly from the field or from storage. Defender also may be used for fresh markets in regions such as California, where cultivars with long tubers and white skin are traditionally grown. Resistances to late blight and other potato diseases make Defender an ideal candidate for organic potato production. Defender consistently produced greater total and U.S. No. 1 yields than ‘Russet Burbank’ in Idaho trials. In early harvest trials conducted in the western U.S., average total yields of Defender were 17% and 23% greater than yields for ‘Ranger Russet’ and ‘Shepody’, respectively. In full-season trials conducted in the western U.S., Defender averaged 10% and 15% higher yields than Ranger Russet and Russet Burbank, respectively. Specific gravity of Defender is consistently high, with values comparable to those of Ranger Russet; tuber ascorbic acid (Vitamin C) levels are also high. In addition to late blight, Defender also is resistant to tuber early blight (Alternaria solani, (Ellis & G. Martin), L.R. Jones & Grout), potato virus X, and net necrosis; it has moderate levels of resistance to Verticillium wilt (Verticillium dahliae, Kleb), pink rot, foliar early blight, corky ringspot, and Erwinia soft rot. Defender is susceptible to scab (common and powdery) and potato leafroll virus; it has moderate susceptibility to dry rot (Fusarium spp.) and potato virus Y. Susceptibilities to internal necrosis, tuber greening, and blackspot bruise also have been noted, but may be minimized through cultural and harvest practices.ResumenDefender es un cultivar de papa de alto rendimiento, piel blanca y excelente por su resistencia al tizón tardío causado porPhytophthora infestans (Mont.) de Bary, tanto a la infección foliar como a la de los tubérculos. Ha sido liberada el 2004 por USDA-ARS y las Estaciones Experimentales Agrícolas de Idaho, Oregon y Washington. Defender es apropiado para el procesamiento de papa frita y otros productos congelados de papa, directamente del campo o del almacén. Defender puede también ser usado en regiones como California, donde se cultivan tradicionalmente variedades de tubérculos alargados y piel blanca. La resistencia al tizón tardío y otras enfermedades de la papa hace de Defender un candidato ideal para la producción orgánica. Defender tuvo consistentemente un mayor rendimiento de tubérculos de grado “US No. 1” que Russet Burbank en pruebas en Idaho. En ensayos de cosecha temprana realizados en el oeste de EUA, el promedio de rendimiento total de Defender fue de 17 y 23% mayor que el de las variedades Ranger Russet y Shepody respectivamente. En ensayos de ciclo completo realizadas en el oeste, Defender tuvo rendimientos 10 y 15% mayores que Ranger Russet y Russet Burbank respectivamente. El peso específico de Defender es consistentemente alto, con valores comparables a los de Ranger Russet y los niveles de ácido ascórbico (Vitamina C) son también altos. Además de ser resistente al tizón tardío, Defender es también resistente al tizón temprano (Alternaria solani (Ellis & G. Martín) L. R. Jones & Grout), Virus X de la papa y necrosis en red. Tiene niveles moderados de resistencia a marchitez por Verticillium (Verticillium dahliae Kleb.), pudrición rosada, tizón foliar temprano, mancha corchosa en anillo y pudrición blanda causada por Erwinia. Defender es susceptible a la sarna (común y polvorienta) y al virus del enrollamiento; tiene moderada susceptibilidad a la pudrición seca (Fusarium spp.) y al virus Y de la papa. También se ha notado susceptibilidad a necrosis interna, verdeamiento del tubérculo y mancha negra, pero estas pueden minimizarse a través de prácticas culturales.

Partial resistance toPhytophthora infestans in fourSolanum crosses

Thirty progeny from each of fourSolanum crosses were evaluated in the field at Mount Vernon, WA, in 1996 and 1997 for partial resistance toPhytophthora infestans. Of the four parents, three have high levels of partial resistance toP. infestans; one derived from somatic hybridization ofS. bulbocastanum, the other two from traditional breeding efforts for multiple disease resistance. Data were collected from each cross to estimate area under the disease progress curve (AUDPC), days to 5% disease severity threshold (DT5), and sporangia production (SP). All of these variables differed significantly among the progeny within each cross in each year. Correlation analysis indicated that DT5 was highly correlated with AUDPC for all four populations for both years. Log-transformed SP was significantly (P>0.001) correlated to AUDPC values for one population in both years, but the significance of the correlation was variable between years for the remaining three crosses. The variable DT5, which is composed of three components (infection efficiency, latent period, and lesion growth rate), was the most important in identifying progeny with partial resistance to late blight in all four crosses in this study.ResumenTreinta progenies de cada uno de los cuatro cruzamientos deSolanum fueron evaluadas en los campos de Mount Vernon, WA en 1996 y 1997, para comprobar su resistencia parcial aPhytophthora infestans. De los cuatro progenitores, tres muestran altos niveles de resistencia parcial aP. infestans; uno, derivado de la hibridación somática deS. bulbocastanu; los otros dos, producto de esfuerzos tradicionales de mejoramiento para conferir resistencia a múltiples enfermedades. Los datos fueron recolectados de cada cruce, con el fin de estimar la curva de progreso de la enfermedad (AUDPC por sus siglas en inglés), los días al 5% de inicio severo de la enfermedad (DT5) y la producción de esporangio (PE). Todas estas variables fueron significativamente diferentes entre la progenie de cada cruce, cada año. Los análisis correlativos indicaron que DT5 fue la correlación más alta con AUDPC para las cuatro poblaciones en ambos años. El registre transformado de PE fue significativamente (P⩽0.001) correlativo con los valores de la AUDPC para una población en los dos años, pero el significado de la correlación fue variable entre años para los tres cruces restantes. La variable DT5 -que consta de tres componentes (eficacia de la infección, período de latencia y tasa de crecimiento de la lesión)- fue la más importante para identificar las progenies con resistencia parcial al tizón tardío en los cuatro cruces de este estudio.

Foliar fungicides as protective seed piece treatments for management of late blight of potatoes.

Fungicides receiving Section 18 emergency exemptions for management of the foliar phase of late blight of potato were evaluated as protective treatments for control of tuberborne inoculum of Phytophthora infestans in greenhouse and field studies. Three Section 18 products, Acrobat MZ (dimethomorph + mancozeb), Curzate M-8 (cymoxanil + mancozeb), and Tattoo C (propamocarb hydrochloride + chlorothalonil), when applied to the seed piece prior to inoculation with P. infestans, significantly (P ≤ 0.05) increased sprout emergence, compared with the inoculated water control. Of the three Section 18 products, Curzate M-8 was frequently the most effective. The increase in plant emergence with the protective seed piece treatment was seen with cultivars that differ in susceptibility to late blight, two genotypes (US-8 and US-11) of P. infestans, zoospore and sporangial inocula, and at different inoculation sites on the seed piece. In greenhouse studies, percent emergence with the protective seed piece treatments averaged 74% for cv. White Rose and 44% for cv. Shepody, compared with 9 and 8% for the inoculated water control of the respective cultivars. In field studies, when inoculum was placed either adjacent to or 4 cm away from the sprout, plant emergence of the inoculated water control was 12 and 36%, respectively. Application of the fungicides to the seed piece prior to inoculation increased emergence by an average of 700 and 212% for the respective inoculation sites. There was no evidence for translocation of the fungicides to the emerging foliage in concentrations high enough to prevent foliar infection from airborne inoculum of P. infestans. Control of tuberborne inoculum of P. infestans with appropriate seed piece treatments will result in an increase in plant emergence and improved crop uniformity.

Effect of Registered Potato Seed Piece Fungicides on Tuber-borne Phytophthora infestans

Curative applications of thiophanate-methyl + mancozeb to blighted seed pieces of three potato (Solanum tuberosum) cultivars significantly reduced the amount of surface area colonized by Phytophthora infestans compared with treatment with thiophanate-methyl or no fungicide under laboratory conditions. Percent blighted tuber surface area over six tests averaged 3.5, 11.4, and 21.2% for the three treatments, respectively. Seed pieces inoculated with P. infestans US-8 or US-11 incubated, and then treated with the same fungicides and planted at Oregon and Washington field sites, respectively, had higher emergence across locations for thiophanate-methyl + mancozeb than for thiophanate-methyl (30 versus 12.5%). Emergence of healthy thiophanate-methyl + mancozeb-treated seed pieces averaged 93%. When protective applications of these fungicides, mancozeb, or fludioxinil were made to healthy potato seed pieces prior to inoculation with P. infestans, plant emergence in the greenhouse was significantly increased with thiophanate-methyl + mancozeb compared with fludioxinil (92 versus 36%) in Oregon and with thiophanate-methyl or fludioxinil (90 versus 20 and 24%) in Washington. Inoculation of healthy seed pieces reduced plant stand in all greenhouse tests; however, treatment with thiophanate-methyl + mancozeb or mancozeb alone prior to inoculation resulted in sprout emergence similar to that of the noninoculated control. In a late blight management program, treatment of seed pieces with a registered fungicide that has activity against P. infestans contributes to an increase in plant emergence and improved crop uniformity. However, for maximum benefit, the fungicide must be in place before coming in contact with the pathogen.