Raymond J. Taylor

Differences in Etiology Affect Mefenoxam Efficacy and the Control of Pink Rot and Leak Tuber Diseases of Potato

Data supplementing a previously published survey of North American isolates of Phytophthora erythroseptica and Pythium ultimum demonstrated that the proportion of the populations sensitive to mefenoxam remains high, 79.6 and 96.9% with EC50 sensitivities ranging from <0.01 to 0.9 µg ml-1 and <0.01 to 0.8 µg ml-1, respectively. Mefenoxam should provide control of these pathogens in most potato production areas. Factors affecting the development of pink rot and leak in potato tubers and the efficacy of mefenoxam to control these diseases with different etiologies were examined. Results confirmed that P. erythroseptica is capable of directly infecting potato tubers causing pink rot, whereas Pythium ultimum requires a wound to infect and cause leak. Mefenoxam was applied to replicated field plots as a single in-furrow application at planting, as an in-furrow application at planting followed by an additional sidedress application 3 weeks after planting, as a single foliar application when tubers were 7 to 8 mm in diameter, and as two foliar applications when the tubers were 7 to 8 mm in diameter and 14 days later. The recommended label rate plus two additional lower application rates were used with each method. For tubers challenge-inoculated after harvest, mefenoxam was found to be more effective in controlling pink rot relative to leak over all application methods. The greatest level of pink rot control (89%) was attained with the in-furrow at planting and sidedress application. All rates tested provided similar levels of control with this application method, but this method provided only a modest level of leak control (35%), and leak was not controlled by foliar applications of mefenoxam at any rate tested. In contrast, the foliar applications of mefenoxam resulted in 10 to 50% control of pink rot. Since the isolates of both pathogens were highly sensitive to me-fenoxam, disease-specific control was attributed to differences in disease etiology. Therefore, the use of mefenoxam to control pink rot in the field and storage appears to be well founded.

Assessment of Resistance of Tubers of Potato Cultivars to Phytophthora erythroseptica and Pythium ultimum

Tubers of 34 potato cultivars were examined for their susceptibility to infection by zoospores of Phytophthora erythroseptica and mycelia of Pythium ultimum. Incidence of infected tubers (%) and penetration of rot (mm) were the parameters used to determine the susceptibility of each cultivar. Tubers of cv. Atlantic appeared to have some resistance to infection and colonization by Phytophthora erythroseptica. Cvs. Russet Norkotah and Snowden were the most susceptible to infection by P. erythroseptica. Snowden was found to be highly susceptible to P. erythroseptica, but it was the most resistant to Pythium ultimum. Cvs. FL-1625 and FL-1867 also were less susceptible to P. ultimum than the other cultivars. Cvs. Superior, Itasca, and Dark Red Norland were the most susceptible to P. ultimum. Cultivar susceptibility should be considered when making disease management decisions, particularly in fields where these soilborne diseases are a recurring problem.

Biological Significance of Mefenoxam Resistance in Phytophthora erythroseptica and Its Implications for the Management of Pink Rot of Potato

Tubers from plants treated with in-furrow and foliar applications of mefenoxam were inoculated with eight isolates of Phytophthora erythroseptica having varying levels of sensitivity to the fungicide. Two isolates with effective concentration causing 50% reduction of mycelial growth (EC50) values of 0.02 and 0.04 μg ml-1 were categorized as being mefenoxam sensitive. Isolates with EC50 values >1.0 μg ml-1 were designated as insensitive to mefenoxam and were grouped two each into low intermediate (EC50 = 1.1 and 5.3 μg ml-1), high intermediate (EC50 = 26 and 74 μg ml-1), and resistant (EC50 ≥ 100 μg ml-1). The biological significance of these isolates was examined by quantifying disease control. P. erythroseptica isolates classified in the resistant group infected a significantly greater proportion of untreated tubers than isolates in any other group. Mefenoxam reduced infection frequency of sensitive isolates by as much as 37%. Mefenoxam did not provide disease control of any isolate possessing insensitivity to the fungicide, with the greatest decrease in control observed with the low intermediate group. Aggressiveness indices, representing tuber infection frequency and depth of penetration, were calculated for untreated and mefenoxam-treated tubers. According to these indices, both isolates classified in the resistant group and high intermediate isolate 252-4 were more aggressive than sensitive isolates in the absence of mefenoxam pressure, and significantly so in the presence of mefenoxam. These results suggest that pink rot may become more severe in fields known to contain P. erythroseptica populations with mefenoxam EC50 values >1.0 μg ml-1 if the fungicide is applied. These factors should be considered when developing strategies to manage pink rot and mefenoxam-resistant populations of P. erythroseptica.

Management of soilborne diseases of potato

A large number of soilborne diseases that affect potato are important in the United States. In the Midwestern USA, early dying, involving Verticillium dahliae and Colletotrichum coccodes and the storage rots, pink rot and leak tuber rot, caused by Phytophthora erythroseptica and Pythium ultimum are among the most serious. Various aspects of the biology and aetiology of these pathogens have been investigated to improve our understanding of the factors involved in disease development. These studies have addressed knowledge gaps and have assisted in the development of management strategies and tactics for each disease. Disease management integrates cultural practices, use of agrochemicals and to some degree, host resistance. Unfortunately, cultural management practices alone are currently inadequate to control these diseases, causing the potato industry to become over-reliant on the use of agrochemicals for effective management. Current research efforts are directed at the identification and incorporation of genetic resistance into cultivars with acceptable horticultural characteristics to provide more effective disease management.

Resistance to phytophthora erythroseptica and pythium ultimum in a potato clone derived from s. berthaultii and s. etuberosum

Tubers of several potato clones and cultivars were screened for susceptibility to infection by zoospores of Phytophthora erythroseptica (causal agent of pink rot) and mycelia of Pythium ultimum (causal agent of leak) over a three-year period, from 2003-2005. Incidence of infected tubers (%) and penetration of rot (mm) were the parameters used to determine the susceptibility of each potato clone. Responses of each potato clone were compared to cultivars with known resistance or susceptibility to these pathogens. Tubers of cultivars Atlantic and Snowden have moderate resistance to infection and colonization by P. erythroseptica and P. ultimum, respectively, and were used as the resistant checks. Cultivars Russet Norkotah and Red Norland are susceptible to infection by both pathogens. A number of potato clones demonstrated resistance to pink rot equal to or greater than the control cultivar Atlantic, including Etb 6-5-2, ND5822C-7, ND6956b-13, ND7443Ab-44, ND7443Ab-181, ND7818-1Y and J101K6A22. In addition to demonstrating the highest resistance to pink rot, Etb 6-5-2 was the only clone that demonstrated resistance to leak greater than or equivalent to the resistant cultivar Snowden. Etb 6-5-2 is a backcross derivative from a somatic hybrid of Solanum etuberosum and Solanum berthaultii and will be investigated further as a potential source of resistance to these two storage rot diseases.ResumenLos tubérculos de varios clones y cultivares de papa fueron tamizados para susceptibilidad con zoosporas de Phytopthora erythroseptica (agente causal de pudricion rosada) y con micelio Pythium ultimum (agente causal de pudricion acuosa o gotera) en un periodo de tres aÑos, del 2003 al 2005. La incidencia de tubérculos infectados (%) y la profundidad de la pudriciön (mm) fueron los parámetros usados para determinar la susceptibilidad en cada clon de papa. Las respuestas de cada clon fueron comparadas con las de cultivares de conocida resistencia o susceptibilidad a estos patögenos. Tubérculos de los cultivares Atlantic y Snowden que tienen resistencia moderada a la infecciön y colonizacion por P. erythroseptica y P. ultimum respectivamente, fueron usados como testigo resistente. Los cultivares de Russet Norkotah y Red Norland son susceptibles a la infeccion de ambos patögenos. Varios clones de papa mostraron resistencia a la pudriciön rosada, igual o mayor que el testigo Atlantic, incluyendo, Etb 6-5-2, ND5822C-7, ND6956b-13, ND7443Ab-44, ND7443Ab-181, ND7818-1Y y J101K6A22. Además de demostrar muy alta resistencia, Etb 6-5-2 fue el único clon que demoströ resistencia a la gotera en mayor proporciön o equivalente al cultivar resistente Snowden. Etb 6-5-2 es producto de una retrocruza derivada de un híbrido somático de Solanum etuberosum y S. berthaultii y será investigada adicionalmente como fuente potencial de resistencia a estas dos pudriciones de tubérculos almacenados.

Influence of Tillage and Method of Metam Sodium Application on Distribution and Survival of Verticillium dahliae in the Soil and the Development of Verticillium Wilt of Potato

The effect of plowing and deep-rip tillage, in combination with chemigation or shank injection of metam sodium, onVerticillium dahliae populations and disease development was assessed in two fields with differing soil types and potato rotations. Soil samples were collected on a geo-referenced basis at depths of 0 to 10 cm and 10 to 20 cm before tillage, after tillage, after chemical application, and before planting and assayed for the presence of the pathogen. Propagules ofV. dahliae were detected at 140 of 141 sites sampled prior to tillage. Most (74.4% in heavy, sandy loam; 63.1% in light, loamy sand) were concentrated in upper 10 cm of the soil profile. Plowing redistributed inoculum vertically while deep-rip tillage did not. In the non-chemical treated areas of both fields, theVerticillium population reached a maximum between 25 July and 8 August before declining to near pre-tillage levels. Overall, the population generally was lower in the field with heavy soil, higher organic matter content, and a 3-year crop rotation. Metam sodium appeared to be most effective when shank injected, as the levels of inoculum in both fields declined by 60% to 80% following this application method. Chemigation was ineffective in the lighter soil, but the inoculum density in the deep-rip tillage area of the field with the heavier soil declined by nearly 20% in the upper and 60% in the lower strata following this treatment. The number ofV. dahliae propagules at the 10- to 20-cm depth in the plowed area of the same field was reduced by 25% following chemigation, but remained unchanged in the upper strata. Wilt was reduced in both fields by as much as 50% with shank injection of metam sodium with concomitant increases in total yield, marketable yield and gross income. Increases in total yield were significant (P<0.05) for the main effect of chemical, in the plowed area of the field with the heavier soil type following shank injection. These data suggest that growers might benefit from altering their tillage and chemical application practices as part of an integrated approach to managing Verticillium wilt.ResumenSe probó en dos campos con diferentes tipos de suelo y rotaciones de papa, el efecto de labranza y de aradura profunda, en combinación con la aplicación de productos químicos por el agua de riego e inyección (shank injection) de metasodio en el suelo, sobre poblaciones deVerticillium dahliae y desarrollo de la enfermedad. Las muestras de suelo se colectaron por zonas geográficas, a profundidades de 0 a 10cm y 10 a 20 cm antes del barbecho, después del barbecho, después de la aplicación de productos químicos y antes de la siembra y se hizo la prueba de presencia del patógeno. Los propágulos deV. dahliae se detectaron en 140 de los 141 lugares de muestreo antes del barbecho. La mayoría (74.4% en suelos areno-arcillosos pesados; 63.1% en suelos areno-arcillosos ligeros) estaban concentrados en los 10cm superiores del suelo. La labranza redistribuyó el inóculo verticalmente, mientras que la aradura profunda no lo hizo. En las áreas de ambos campos que no fueron tratadas químicamente, la población deVerticillium alcanzó el máximo entre el 25 de julio y el 8 de agosto antes de disminuir a niveles de pre-barbecho. Sobre todo, la población fue generalmente más baja en campos con suelo pesado, alto contenido de materia orgánica y una rotación de tres años. El metasodio se mostró más efectivo cuando se inyectó, ya que los niveles de inóculo en ambos campos disminuyeron en un 60–80% con este método de aplicación. La aplicación de productos químicos por medio del riego no fue efectiva en suelo más ligero pero la densidad de inóculo en áreas de suelo con aradura profunda y suelo más pesado disminuyó cerca de 20% en la parte superior y 60% en el estrato inferior después de este tratamiento. El número de propágulos deV. dahliae a 10 a 20 cm de profundidad en el área arada del mismo campo se redujo después de la aplicación de productos químicos por riego en un 25% pero permaneció igual en el estrato superior. La marchitez se redujo en ambos campos en un 50% con el metasodio inyectado al suelo con el concomitante incremento del rendimiento total, rendimiento comerciable e ingresos totales. Los incrementos en el rendimiento total fueron significativos (P<0.05), principalmente por el efecto del producto químico, en áreas del campo con suelo pesado después de inyección de producto químico. Estos datos sugieren que los productores pueden obtener beneficios alterando la labranza y las prácticas de aplicación de productos químicos como parte de un manejo integrado para marchitez por Verticillium.

Tuber yield, soft rot resistance, bruising resistance and processing quality in a population of potato (cv. crystal) somaclones

Several important postharvest characteristics were evaluated in protoplast-derived somaclones after two consecutive growing seasons (1987, 1988). None of the somaclones had tuber yields significantly greater than the protoplast donor Crystal mother clone (MC), however, 8 of 37 clones had significantly lower yields. Of the 25 somaclones selected forErwinia soft rot resistance testing, 1 clone demonstrated significantly elevated resistance to soft rot while 12 clones were found to be significantly more susceptible relative to MC. Over half (20) of the 33 somaclones tested showed significant improvement in resistance to tuber bruising while only a single clone was more susceptible than MC. Based upon Agtron color analysis, 5 somaclones showed significantly enhanced chip color when processed directly out of cold storage (6 C). Six clones had significantly improved chip color after a reconditioning period (20 C). Four of the clones with significantly better cold processing characteristics were also among the 6 with significantly enhanced processing quality after reconditioning. Some clones showed improvement in two or more characteristics. Clone 72–93 had significantly elevated resistance to both bacterial soft rot and tuber bruising while the bruise resistant clones 74-14, 71-41, 71-18 and 71-2 demonstrated significant improvement in both cold and reconditioned processing characteristics.

Effect of Application Method and Rate on Residual Efficacy of Mefenoxam and Phosphorous Acid Fungicides in the Control of Pink Rot of Potato

Experiments were conducted to examine the effectiveness of rate and method of phosphorous acid application for controlling pink rot of potato (Solanum tuberosum) caused by Phytophthora erythroseptica. Replicated small-plot and replicated split commercial field trials were established in commercial production fields in Minnesota from 2006 to 2009. Fungicides were applied in-furrow at planting, or as one, two, or three foliar applications via ground sprayer, irrigation system (chemigation), or fixed-wing aircraft. Phosphorous acid efficacy was compared to mefenoxam, the fungicide commonly utilized to manage pink rot, either by determining natural infections in the field or by inoculating eyes of harvested tubers using a mefenoxam-sensitive and -resistant isolate of P. erythroseptica via postharvest challenge inoculation. In replicated small plot trials, both in-furrow and two foliar applications of mefenoxam controlled tuber rot in the field, and significantly controlled tuber rot in storage. Phosphorous acid also reduced tuber rot in the field when applied two or three times to the foliage. Although phosphorous acid was ineffective when applied in-furrow, one, two, and three foliar applications and a postharvest application of phosphorous acid controlled mefenoxam-sensitive and -resistant isolates of P. erythroseptica during storage for 187 days, while mefenoxam failed to control the resistant isolate. In replicated split commercial field trials, two aerial applications of phosphorous acid were as effective as three applications in reducing pink rot incidence in tubers inoculated postharvest. Three aerial applications were as effective as three chemigation applications in replicated split commercial field trials in 2008, but provided significantly greater protection than chemigation in 2009.

A Foliar Blight and Tuber Rot of Potato Caused by Phytophthora nicotianae: New Occurrences and Characterization of Isolates

Phytophthora spp. are pathogenic to many plant species worldwide, and late blight, caused by Phytophthora infestans, and pink rot, caused by P. erythroseptica, are two important diseases of potato. Another Phytophthora sp., P. nicotianae, was recovered from pink-rot-symptomatic tubers collected from commercial fields in Nebraska, Florida, and Missouri in 2005, 2006, and 2007, respectively. P. nicotianae also was recovered from foliage obtained from commercial potato fields in Nebraska and Texas exhibiting symptoms very similar to those of late blight. Isolates of P. cactorum also were recovered from foliar infections in a commercial potato field in Minnesota in 2005. Natural infection of potato foliage by P. cactorum and infection of wounded potato tuber tissue via inoculation with zoospores of P. capsici are reported here for the first time. Isolates of P. nicotianae, regardless of origin, were primarily of the A1 mating type. All isolates of P. nicotianae and P. cactorum were sensitive to the fungicide mefenoxam. Optimum growth of P. nicotianae, P. erythroseptica, and P. cactorum in vitro occurred at 25°C; however, only P. nicotianae sustained growth at 35°C. Regardless of the tissue of origin, all isolates of P. nicotianae and P. cactorum were capable of infecting potato tubers and leaves. However, isolates of P. nicotianae were less aggressive than P. erythroseptica isolates only when tubers were not wounded prior to inoculation. Pink rot incidence varied significantly among potato cultivars following inoculation of nonwounded tubers with zoospores of P. nicotianae, ranging from 51% in Red Norland to 19% in Atlantic. Phytophthora spp. also differed significantly in their ability to infect potato leaves. Highest infection frequencies were obtained with P. infestans and levels of infection varied significantly among P. nicotianae isolates. The rate of foliar lesion expansion was similar among isolates of P. nicotianae and P. infestans. Whereas P. infestans infections yielded profuse sporulation, no sporulation was observed with foliar infections of P. nicotianae.

Inheritance of mefenoxam resistance in selfed populations of the homothallic oomycetePhytophthora erythroseptica (Pethybr.), cause of pink rot of potato

The inheritance of mefenoxam resistance in the oomycetePhytophthora erythroseptica, causal organism of pink rot of potato, was investigated in two successive selfed generations of single oospore isolates. Fourteen parental isolates from three mefenoxam-sensitivity phenotypes (sensitive, intermediately resistant, and resistant) were used to produce S1 and S2 progeny isolates by selfing each isolate and germinating single oospore cultures from each isolate. The two generations of isolates were tested for mefenoxam sensitivity using the radial growth inhibition assay and calculating EC50 values. Twenty-four-hundred isolates from S1 and S2 generations were tested for mefenoxam sensitivity. Results demonstrated the lack of segregation for mefenoxam sensitivity among S1 and S2 progeny isolates from the resistant and sensitive parents. The majority of the S1 progeny isolates from the parents with intermediate resistance also possessed intermediate resistance, but demonstrated substantial quantitative shifts in response to mefenoxam more often toward increased insensitivity. A few isolates from the S1 progeny were sensitive to mefenoxam, and one isolate was mefenoxam-resistant. Similarly, the majority of the S2 progeny isolates from intermediately resistant parents also possessed intermediate resistance with some quantitative shifts in sensitivity to mefenoxam similar to those observed in S1 progeny, as well as a few isolates that were sensitive to mefenoxam. These results do not support the hypothesis that resistance to mefenoxam inP. erythroseptica is controlled by a single gene exhibiting incomplete dominance. Alternative hypotheses are proposed such as the genetics of mefenoxam resistance inP. erythroseptica is probably under the control of more than one major gene and perhaps some minor genes of additive effect.ResumenLa herencia de la resistencia del oomycetePhytophthora erythroseptica causante de la pudrición rosada de la papa al mefenoxam fue investigada en dos generaciones autofecundadas sucesivas de aislamientos provenientes de una sola oospora. Catorce aislamientos de tres fenotipos sensibles al mefenoxam (sensible, intermedio-resistente y resistente) fueron utilizados para producir los aislamientos progenle S1 y S2, autofecundando cada aislamiento y produciendo aislamientos provenientes de una sola oospora. Se probaron los aislamientos de las dos generaciones para sensibilidad al mefenoxam, utilizando la prueba de inhibición de crecimiento radial y calculando los valores EC50. Se probaron 2,400 aislamientos de las generaciones S1 y S2 para determinar su sensibilidad al mefenoxam. Los resultados demostraron la falta de segregación para sensibilidad a la acción del mefenoxam entre los aislamientos de las progenies S1 y S2 provenientes de progenitores resistentes y sensibles. La mayoría de los aislamientos de la progenie S1 provenientes de progenitores con resistencia intermedia también tuvieron resistencia intermedia, pero demostraron un cambio cuantitativo sustancial en respuesta al mefenoxam, con más frecuencia hacia una mayor insensibilidad. Unos pocos aislamientos de la progenie S1 mostraron sensibilidad al mefenoxam y un aislamiento mostró resistencia. De la misma manera, la mayoria de los aislamientos de la progenie S2 con progenitores de resistencia intermedia, tuvieron también resistencia intermedia con algunos cambios cuantitativos en sensibilidad, similares a aquellos observados en la progenie S1 e igualmente linos pocos aislamientos fueron sensibles. Estos resultados no apoyan la hipótesis de que la resistencia delP. erythroseptica al mefenoxam es controlada por un solo gen de dominancia incompleta. Se propone la hipótesis alternativa de que la resistencia genética delP. erythroseptica al mefenoxam está probablemente bajo el control de más de un gen mayor y tal vez de algunos genes menores de efecto aditivo.