Douglas I. Rouse

Effects of Soil Water Content and Soil Temperature on Efficacy of Metham-Sodium Against Verticillium dahliae

Soil column studies were conducted to investigate the influence of soil water content and temperature on the efficacy of metham-sodium and its degradation product methyl isothiocyanate against Verticillium dahliae. The viability of the microsclerotia (MS) of the fungus in the top 30 cm of fumigated and control columns was measured. Temperatures for studies were 2 or 22°C, and the soil water content, expressed as soil matric potential, varied from -23 (wet), -113 (moist), to -2485 J/kg (dry). There was a significant interaction of soil water content and temperature on the efficacy of metham-sodium against V. dahliae MS. For the low soil temperature (2°C) the fumigant was more effective against MS of the fungus in wet than in moist or dry soil Soil water content did not affect fungicidal activity of metham-sodium when the soil columns were maintained at 22°C. These results suggest that the fumigant has a greater efficacy against V. dahliae in wet/cold soil conditions compared to the other conditions tested. Consequently, it was recommended that metham-sodium be applied to fields by chemigation in late fall or early spring to obtain efficacious results.

Methyl isothiocyanate volatilization from fields treated with metam-sodium

Emission of methyl isothiocyanate (MITC) from fields treated with metam-sodium (sodium N-methyldithiocarbamate) is a potential environmental and human safety hazard. Concentrations of MITC at three heights above four arable fields were measured following the application of metam- sodium at a rate of 480 litre ha ˇ1 (166 kg AI ha ˇ1 ). Two of these fields were treated by injection into a center-pivot irrigation system (chemigation), while in the other two fields the fumigant was applied through injection directly into the soil. Generally, higher MITC air concentrations were observed above chemigated than above injected fields. Maximum MITC air concentrations were 11.2 and 7.4 mg m ˇ3 recorded 10cm above ground 6-8 h following application and the minimum concentrations were 0.7 and 0.2 m gm ˇ 3 observed at 200 cm 30-35h after application above chemigated and injected fields, respectively. The estimated MITC respiratory exposure a worker might encounter during the re-entry period ranged between 1.37 and 0.03 mg day ˇ1 in chemigated fields and between 0.35 and 0.02mg day ˇ1 in the injected fields. These results suggest that application of the fumigant through injection reduced MITC volatilization losses in comparison with the chemigation method, thus posing a relatively lower risk of exposure to MITC emissions. # 2000 Society of Chemical Industry

Synergism of Pratylenchus penetrans and Verticillium dahliae Manifested by Reduced Gas Exchange in Potato

ABSTRACT The effects of solitary and concurrent infection by Pratylenchus pene-trans and Verticillium dahliae on gas exchange of Russet Burbank potato (Solanum tuberosum) were studied in growth chamber experiments. Treatments were P. penetrans at low, medium, and high density; V. dahliae alone at one initial density; the combination of the nematode at these three densities and V. dahliae; and a noninfested control. Gas exchange parameters of leaf cohorts of different ages in the different treatments were repeatedly measured with a Li-Cor LI-6200 portable photosynthesis system. At 45 days after planting, joint infection significantly reduced net photosynthesis, stomatal conductance, and transpiration of 1- to 25-day-old leaf cohorts. Intercellular CO(2) levels were significantly increased by co-infection, especially in older leaves. The synergistic effect of co-infection on gas exchange parameters was greater in the oldest cohort than in the youngest cohort. No consistent effects on leaf gas exchange parameters were observed in plants infected by the nematode or the fungus alone. The relationship between the assimilation rate and stomatal conductance remained linear regardless of solitary or concomitant infection, indicating that stomatal factors are primarily responsible for regulating photosynthesis. The significant reduction of gas exchange in leaves of co-infected plants without reduction in intercellular CO(2) concentrations suggests that nonstomatal factors also play a role when both organisms are present.

Disease Progress Based on Effects of Verticillium dahliae and Pratylenchus penetrans on Gas Exchange in Russet Burbank Potato.

ABSTRACT The interactive effects of concomitant infection by the nematode Pratylenchus penetrans and the fungus Verticillium dahliae on symptom expression in Russet Burbank potato was studied in growth chamber experiments. Treatments were P. penetrans at three initial densities, V. dahliae at one inoculum density, the combination of the nematode at these three densities and the fungus, and a noninfested control. Gas exchange was measured nondestructively in leaf cohorts of different ages, one to three times weekly, with a LI-COR portable photosynthesis system. The single-pathogen treatments had no effect on assimilation or transpiration rates, but joint infection had a significant impact. In concomitant infection, photosynthesis was impaired more than transpiration, so estimates of leaf health were based on carbon assimilation rates only. Reductions in assimilation rate were apparent before the onset of visual symptoms. Assimilation rates decreased as much as 44% in the top, and newest, leaves of concomitantly infected plants, compared to rates in control plants. Even so, the health of newly produced leaves did not become progressively worse through time. With light use efficiency less than 0.20 mol of CO(2) fixed per mol of photosynthetically active radiation used as the criterion for disease incidence, disease progressed acropetally from the oldest to the youngest leaves. In plants infected with P. penetrans (0.8 nematodes per cm(3) of soil) in combination with V. dahliae, all leaves in cohorts 1 and 2 were symptomatic by 45 days after planting, and leaves in cohorts 3 to 6 became symptomatic at weekly intervals thereafter. For the control and single-pathogen treatments, the first time that light use efficiency fell below 0.20 in all leaves in cohort 1 was 71 days after planting. Concomitant infection reduced leaf life span by about 3 weeks. Both visual and physiological symptom expression were invariant to differences in initial nematode inoculum densities ranging from 0.8 to 2.5 nematodes per cm(3) of soil in one experiment and from 1.3 to 4.1 nematodes per cm(3) of soil in a second experiment.

Leaching of Methyl Isothiocyanate in Plainfield Sand Chemigated with Metam-Sodium

Soil column studies were undertaken to investigate the influence of soil water content and irrigation on leaching, distribution and persistence of methyl isothiocyanate (MITC) in a sandy soil chemigated with the soil fumigant metam-sodium. No leaching was obtained from columns at low water content (0.042 or 0.074 cm 3 cm -3 ). However, 8.4 (±2.8), 34.2 (±7.4) and 119.4 (±8.3) μg of MITC leached from columns at 0.105, 0.137 and 0.168 cm 3 cm -3 water content, respectively. Increased leaching resulted from sprinkler application of 25-mm of water to columns at 0.137 cm 3 cm -3 water content. Leaching of MITC constituted only a small fraction of the amount applied even in the worst case. Methyl isothiocyanate persisted in soil for 15 days at 2°C in varying amounts under the different water regimes. Relatively high amounts of MITC residues (8-12 mg kg -1 soil) were detected in the top 25-cm layer of all the soil columns. Degradation was the major pathway of dissipation for the chemical despite the soil water regime.

Some approaches to prediction of potato early dying disease severity

There are two control procedures available to growers who have fields that may develop severe potato early dying. The first, use of long crop rotation sequence, may result in only marginal control. The second, soil fumigation, is costly; and while providing effective control in many cases, has failed in others. Because of the cost of fumigation and its limited effectiveness in some situations, it may be helpful to predict the potential severity of the disease and subsequent yield loss. One approach to disease and yield loss prediction could be to rely on historical knowledge of early dying severity and yield in a field. Another approach is to use soil inoculum levels as an indicator of potential disease severity and yield loss. Because severity of potato early dying may vary from year to year in a given field as a result of differing environmental conditions, neither historical knowledge nor estimated soil inoculum levels may be sufficient for predicting disease severity. These methods may, however, provide an estimate of the potential for severe early dying to occur. Modelling the impact of early dying on plant growth and yield as a function of environmental parameters and presence of inoculum may provide a means of setting confidence limits of an estimated disease severity and yield loss based on historical knowledge or knowledge of inoculum load.ResumenHay dos procedimientos de control disponibles para los agricultores que tienen campos donde podría desarrollarse la muerte prematura de la papa en grado severo. El primero, aplicación de secuencia larga en la rotación de cultivos, podria resultar en un control marginal solamente. El segundo, fumigación del suelo, es costoso y, mientras en muchos casos produce un control efectivo, en otros ha fracasado. Debido al costo de la fumigación y su limitada eficacia en algunas situaciones, puede ser útil poder predecir la severidad potencial de la enfermedad y la pérdida subsecuente en rendimientos. Un método de predicción de la enfermedad y de la pérdida en rendimiento podría ser el de confiar en el conocimiento histórico de la severidad de la muerte prematura y del rendimiento en un campo. Otro método es el de utilizar los niveles de inóculo en el suelo como un indicador de la severidad potencial de la enfermedad y de la pérdida en rendimientos. Debido a que la muerte prematura de la papa puede variar de un año a otren un campo dado como resultado de variaciones en las condiciones ambientales, ni el conocimiento histórico, ni un estimado de los niveles de inóculo en el suelo puede ser suficiente para poder predecir la severidad de la enfermedad. Estos métodos pueden, sin embargo, proporcionar un estimado del potencial de que la muerte prematura ocurra en grado severo. El concebir el impacto de la muerte prematura en el crecimiento delas plantas y el rendimiento como una función de los parámetros ambientales y de la presencia del inóculo puede proporcionar un medio para establecer limites de confianza respecto a un estimado de la severidad de la enfermedad y de la pérdida de rendimiento, basándose en los conocimientos históricos o en los conocimientos de los niveles de inóculo.

Root deterioration in the potato early dying syndrome: Causes and effects of root biomass reductions associated with colonization byVerticillium dahliae

Russet Burbank potatoes were grown in the greenhouse in a split-root culture system to assess the impact of pathogens associated with the Potato Early Dying Syndrome (PED) on root deterioration. Plants were grown with all, half, or none of the root system in steamed soil infested withVerticillium dahliae, or in field soil infested withV. dahliae and other PED pathogens. The direct impact of pathogens on root deterioration versus indirect effects resulting from premature senescence of the whole plant was evaluated. Root biomass did not differ significantly between root halves from the same plant in infested and uninfested soil. Root biomass from plants with both halves of the root system in uninfested soil was usually greater than from plants with one or both halves growing in infested soil. Root biomass decreased rapidly after plant stems were inoculated with conidial suspensions ofV. dahliae. Root deterioration in PED was associated with premature senescence of foliar tissue and occurred independently of root colonization by root-colonizing pathogens.ResumenPapas del cultivar Russet Burbank fueron sembradas en el invernadero en un sistema de cultivo de raíces divididas con al propósito de determinar el impacto de patógenos asociados al síndrome de muerte temprana (PED) en el deterioro de raíces. Las plantas crecieron con todo, la mitad y nada de su sistema radicular en un suelo esterilizado e inoculado conVerticillium dahliae ó en suelo de campo infestado conV. dahliae y otros patógenos que causan muerte temprana en papa (PED) el impacto directo de los patógenos en el deterioro de las raíces versus los efectos indirectos resultantes de una senescencia prematura en toda la planta fueron evaluados. La biomasa de raíces no definió significativamente entre mitades de raíces de una misma planta en suelo infestado y no-infestado. La biomasa de raíces de plantas con ambas mitades de su sistema radicular en suelo no-infestado fue generalmente mayor que aquella de plantas con una ó ambas mitades creciendo en suelo infestado. La biomasa de raices decreció rápidamente luego que los tallos de las plantas fueron inoculados con una suspensión de conidias deV. dahliae. El deterioro de raíces en PED fue asociado con senescencia prematura del follaje y ocurrió independientemente de la colonización de las raíces por patógenos colonizadores de raices.

Validation of a simple potato growth model in the North Central United States

A simple potato growth model was validated in 1985 and 1986 at two dryland sites, Grand Forks, ND and Rosemount, MN, and one irrigated site, Hancock, WI. At each site, Russet Burbank potatoes were grown to maximize yield. Leaf area index and leaf, stem, root, and tuber biomass data were collected from replicated experiments every two weeks. Soil matric potentials, minimum and maximum temperatures, and solar radiation also were measured. Among sites, crop development rates and solar radiation receipts were similar, but final tuber yields differed by as much as 35 Mg/ha. Model predictions of leaf, stem, and tuber dry weights were similar to observed data both in terms of maximum values attained and seasonal development except for the experiment in Grand Forks in 1986 where the model over-estimated final tuber yield by 23 Mg/ha (115%). Soil aeration or salinity, factors not currently modeled, may account for this discrepancy. Modeled yield responses to defoliation were compared with field data from one of the experimental locations. At levels of defoliation below fifty percent, proportional yield reductions were similar between observed and modeled crops.CompendioEn 1985 y 1986, se validó un modelo simple de crecimiento de la papa en dos localidades de tierras sin riego, Grand Forks, ND, y Rosemount, MN, y una localidad irrigada, Hancock, WI. En cada localidad se sembró papa del cultivar Russet Burbank, para maximizar el rendimiento. Cada dos semanas, se recolectó de experimentos repetidos, información sobre el indice de área de hoja, y sobre la biomasa de hoja, talle y raíz. Se midieron también los potenciales considerados de suelo, las temperaturas mínima y máxima, y la radiación solar. Las tasas de desarrollo del cultivo entre las localidades y las radiaciones solares interceptadas fueron similares, pero los rendimientos finales en tubérculos difirieron hasta en 35 Mg/ha. Con la excepción de un experimento, los pronósticos del modelo para los pesos secos de hojas, tallos, y tubérculos fueron similares a los datos observados, tanto en términos de valores máximos logrados como en lo concerniente al desarrollo estacional. Sin embargo, para Grand Forks, en 1986, el modelo sobre estimó el rendimiento final en tubérculos en aproximadamente 23 Mg/ha (115%). La aereación del suelo o su salinidad —factores no considerados en el modelo—pueden ser la causa de esta diferencia. Las respuestas de los rendimientos a la defoliación fueron comparadas con los datos de campo de una de las localidades experimentales. Una concordancia razonable fue observada entre los rendimientos de campo y los pronosticados bajo niveles similares de defoliación.

Factors contributing to seasonal fluctuations in rust severity on Ribes missouriense caused by Cronartium ribicola.

Cronartium ribicola, causal agent of white pine blister rust, is a macrocyclic heteroecious rust that cycles between white pines and members of the genus Ribes, which are typically wild plants in North America. To improve predictability of inoculum available for infection of ecologically and commercially important white pines, this research was conducted to identify the factors that influence the development and persistence of uredinia and telia on Ribes in their natural habitats. Numbers of infectious C. ribicola rust lesions (with potentially sporulating rust sori) on tagged Ribes missouriense plants in the woods fluctuated during the season. Changes in numbers of infectious rust lesions were related to rain that occurred 13 days earlier. In field experiments, supplemental leaf wetness provided for 2 days on Ribes shoots resulted in the development of rust lesions more frequently than on control shoots. Viable inoculum and susceptible hosts were present, and the environment was the limiting factor for disease development. Lesion necrosis and leaf abscission contributed to decreases in numbers of infectious rust lesions. Higher lesion density was significantly related to earlier leaf abscission. Telial fruiting bodies occurred in low numbers from early June throughout the remainder of the season.

A field study on the influence of Verticillium dahliae and Pratylenchus penetrans on gas exchange of potato

Field experiments were conducted for three consecutive years to study the effects of low populations of Verticillium dahliae and Pratylenchus penetrans on leaf gas exchange of Russet Burbank potato. Treatments were P. penetrans, V. dahliae, the combination of the nematode with the fungus, and a no-pathogen control. Gas exchange was measured nondestructively on young, fully expanded, asymptomatic leaves one to three times per week starting the ninth week after planting. Infection with either pathogen alone had little or no effect on leaf gas exchange parameters. However, co-infection by both pathogens resulted in reduced leaf light use efficiency (mole of CO2 fixed per mole of photon), lower leaf stomatal conductance, lower leaf water use efficiency (mole of CO2 fixed per mole of water used), and increased intercellular CO2 compared with the no-pathogen control. These effects, additive relative to the impact of each pathogen alone, were first observed 9 weeks after inoculation in the first 2 years of the study and 15 weeks after inoculation in the third year.