Potato Plots Research Articles

Effect of Adult Colorado Potato Beetle Density on Dispersal Under Field Conditions

Adult Colorado potato beetles, Leptinotarsa decemlineata (Say) (Coleoptera: Chrysomelidae), released uniformly at densities of 2, 8, 16, and 32 beetles per plant, had an emigration rate independent of initial density per plant. The overwintered (postdiapause) and summer (prediapause) adult beetles emigrated at overall mean rates of 0.403 and 0.398 beetles over the first 4 h after release. The reduction in the mean percentage of released overwintered and summer beetles per plant over a period of 124 h after the initiation of the release followed an exponential decay model. Over that period, the emigration rate of overwintered populations changed faster than that of summer populations. The corresponding immigration rates into beetle-free areas (342.8 m2), which surrounded the RAs (37.4 m2) of the potato plots, were low. The overwintered and summer adult beetles immigrated at overall mean rates of 0.09 and 0.07 beetles over the first 4 h after release. A capture-mark-release-recapture test carried out using the eight beetles per plant density in 1998 showed that cross-contamination of the release plots was not a factor in the study.

Natural Occurrence of Phytophthora infestans on Black Nightshade (Solanum nigrum) in Wales.

There is only one published record of natural infection of black nightshade (Solanum nigrum L.) by Phytophthora infestans (Mont.) de Bary in England (3) and none from Wales. In August 2001, brown, necrotic leaf lesions with pale green margins were found on black nightshade weeds in a potato trial naturally infected with P. infestans at Henfaes Research Centre, University of Wales, Bangor. Although the plants were low growing with large, succulent leaves 4 to 5 cm long instead of having a more erect habit and smaller leaves, their identity was confirmed as S. nigrum; their atypical appearance may relate to the known phenotypic plasticity of this species (4). Infected leaflets incubated in moist chambers produced sporangia typical of P. infestans, and zoospores were released after chilling in water. Five isolates obtained from leaf fragments had growth on rye agar that was indistinguishable from that of P. infestans from potato. Detached leaflets of S. nigrum and S. tuberosum cv. Green Mountain inoculated with the S. nigrum isolates developed sporulating lesions under high humidity in 7 to 10 days; uninoculated controls remained symptomless. Inoculation of attached leaves of 10 potted S. nigrum plants resulted in seven plants developing necrotic lesions with a few sporangia 10 to 14 days later; sporulation developed mainly on lower leaves of plants that were older or had senesced. The remaining plants developed necrotic lesions with no sporulation, and P. infestans was reisolated from sporulating and nonsporulating lesions. All isolates were A1 mating type, metalaxyl-sensitive, and mitochondrial haplotype IIa, which are characteristics found commonly in isolates of P. infestans from potato in Wales (1). Single-sporangial isolates from each isolate were homozygous for glucose-6-phosphate isomerase and peptidase (Gpi 100/100, Pep 100/100). RG57 fingerprint analysis further established that all five black nightshade isolates were identical to each other and to some local P. infestans isolates from potato. P. infestans in Wales belongs to the new population (1), which may infect a wider host-range than the old US-1 clonal lineage. However, infected black nightshade was only found after late blight was widespread in potato fields. In subsequent years at the same site, weeds of S. nigrum have remained noninfected despite high levels of late blight pressure on adjacent potato plots. There is no evidence to suggest that this species acts as an overwintering host in Wales since it is an annual and lacks frost resistance. Field infection of S. nigrum by P. infestans has recently been reported in the Netherlands (2). Our observations confirm the potential of P. infestans to infect another solanaceous plant species. Alternative hosts may interfere with current disease control strategies because infected weeds would escape fungicide application and could serve as reservoirs of inoculum throughout the growing season.

Fluxes of N2O from farmed peat soils in Finland

SummaryAgricultural peat soils are important sources of nitrous oxide (N2O). Emissions of N2O were measured from field plots of grass, barley, potatoes and fallow on a peat field in northern Finland during 2000–2002 and in southern Finland in 1999–2002. In the north the mean annual fluxes of N2O (with their standard errors) during 2 years were 4.0 (±1.2), 13 (±3.0) and 4.4 (±0.8) kg N ha−1 from the plots of grass, barley and fallow, respectively. In the north there were no significant thaw periods in the middle of winter. As a result, the thawing in the spring did not induce especially large N2O emissions. Emissions of N2O were larger in the south than in the north. In the southern peat field the mean annual fluxes during 3 years were 7.3 (±1.2), 15 (±2.6), 10 (±1.9) and 25 (±6.9) kg N2O‐N ha−1 for grass, barley, potato and fallow plots, respectively. Here, the largest single episodes of emission occurred during the spring thaw each year, following winter thaw events. An emission factor of 10.4 kg N2O‐N ha−1 year−1 for the N2O emission from the decomposition of the peat results from these data if the effect of fertilization according to the IPCC default emission factor is omitted. The direct effect of adding N as fertilizer on N2O emissions was of minor importance. On average, 52% of the annual N2O flux entered the atmosphere outside the cropping season (October–April) in the north and 55% in the south. The larger N2O fluxes from the peat soil in the south might be due to the more humified status of the peat, more rapid mineralization and weather with more cycles of freezing and thawing in the winter.

Ground Beetle (Coleoptera: Carabidae) Assemblages in Mulched and Non-Mulched Garden Plots

Abstract Ground beetles (Coleoptera: Carabidae) are important generalist predators in a variety of habitats, where they prey on many kinds of pest insects. We used pitfall traps for weekly collections by Master Gardener volunteer researchers for 13 weeks in 1998 and 12 weeks in 1999 to assess species composition and temporal patterns of ground beetles in plots of potatoes in home gardens in Illinois and Indiana. Potato plots were either mulched with straw or were left unmulched, having bare soil between plant rows. A total of 82 individuals of 9 species was found in 1998 in mulched plots, versus 79 individuals of 10 species found in unmulched plots. In 1999, 639 individuals of 23 species were collected in mulched plots, versus 380 individuals of 31 species in unmulched plots. Significantly more Cyclotrachelus sodalis (LeConte) were found in mulched plots than in unmulched plots in 1999, whereas there were no significant differences between mulched and unmulched plots in numbers of Cyclotrachelus convivus ...

Soil and total ecosystem respiration in agricultural fields: effect of soil and crop type

A study was made of the effect of soil and crop type on the soil and total ecosystem respiration rates in agricultural soils in southern Finland. The main interest was to compare the soil respiration rates in peat and two different mineral soils growing barley, grass and potato. Respiration measurements were conducted during the growing season with (1) a closed-dynamic ecosystem respiration chamber, in which combined plant and soil respiration was measured and (2) a closed-dynamic soil respiration chamber which measured only the soil and root-derived respiration. A semi-empirical model including separate functions for the soil and plant respiration components was used for the total ecosystem respiration (TER), and the resulting soil respiration parameters for different soil and crop types were compared. Both methods showed that the soil respiration in the peat soil was 2–3 times as high as that in the mineral soils, varying from 0.11 to 0.36 mg (CO2 )m −2 s −1 in the peat soil and from 0.02 to 0.17 mg (CO2 )m −2 s −1 in the mineral soils. The difference between the soil types was mainly attributed to the soil organic C content, which in the uppermost 20 cm of the peat soil was 24 kg m −2 , being about 4 times as high as that in the mineral soils. Depending on the measurement method, the soil respiration in the sandy soil was slightly higher than or similar to that in the clay soil. In each soil type, the soil respiration was highest on the grass plots. Higher soil respiration parameter values (Rs0, describing the soil respiration at a soil temperature of 10 ◦ C, and obtained by modelling) were found on the barley than on the potato plots. The difference was explained by the different cultivation history of the plots, as the potato plots had lain fallow during the preceding summer. The total ecosystem respiration followed the seasonal evolution in the leaf area and measured photosynthetic flux rates. The 2–3-fold peat soil respiration term as compared to mineral soil indicates that the cultivated peat soil ecosystem is a strong net CO2 source.

Natural occurrence of Phytophthora infestans on woody nightshade (Solanum dulcamara) in Ireland

Woody nightshade (Solanum dulcamara) is a common hedgerow plant in the UK and Ireland, a herbaceous perennial and one of only two native Solanum spp. Solanum dulcamara is a known host of Phytophthora infestans (Erwin & Ribeiro, 1996), but infection is rarely reported and there is only one British record, from Harpenden, England (Hirst & Steadman, 1960). There have been no published reports from Ireland. On 2 September 1998, leaf lesions similar to those of P. infestans on potato were found on a naturally occurring S. dulcamara at the Agriculture & Food Science Centre, Belfast, Northern Ireland. The plant was growing within 100 m of an experimental potato plot inoculated with P. infestans. When two infected leaves of S. dulcamara were incubated for 24 h under high humidity, pathogenic growth developed around the lesion margins. This had a hyaline mycelium bearing lemon-shaped sporangia which released motile zoospores after chilling in water, consistent with P. infestans. Three isolates were obtained from this plant during September 1998. Growth on rye agar was indistinguishable from that of local potato isolates of P. infestans. Detached leaflets of S. dulcamara and S. tuberosum were inoculated with the isolates from woody nightshade and kept in a humid chamber. Both hosts became infected, and profuse sporulation occurred within 7 days and was confirmed as P. infestans by morphological, biochemical and molecular characteristics. Inoculations of attached leaves of potted S. dulcamara plants resulted in necrotic lesions with a few sporangia; sporulation was mostly on fallen leaves. Phytophthora infestans was reisolated and its identity confirmed as before. In September 1999, numerous leaf lesions were again observed on the same plant. White sporulation occurred on the abaxial surface and comprised sporangia characteristic of P. infestans. The three 1998 isolates were A1 mating type, sensitive to metalaxyl, mitochondrial haplotype IIa, characteristics typical of P. infestans from potato in Northern Ireland (Carlisle et al., 2001). A 1999 isolate was A1, metalaxyl-sensitive, mtDNA haplotype Ia. All were monomorphic and homozygous for glucose-6-phosphate isomerase and peptidase (Gpi 100/100, Pep 100/100), as confirmed with single-spore isolates. RG-57 fingerprint analysis confirmed the close similarity of the woody nightshade and potato isolates. Phytophthora infestans in Ireland belongs to the new population sensuSpielman et al. (1991), which may infect a wider host range than the old US-1 clonal lineage. However, S. dulcamara infections have only been found when blight is already widespread in potato fields, and there is no evidence to suggest that woody nightshade acts as an overwintering host in Ireland.

Transgenic Bt potato and conventional insecticides for Colorado potato beetle management: comparative efficacy and non‐target impacts

AbstractField studies were conducted in 1992 and 1993 in Hermiston, Oregon, to evaluate the efficacy of transgenic Bt potato (Newleaf®, which expresses the insecticidal protein Cry3Aa) and conventional insecticide spray programs against the important potato pest, Leptinotarsa decemlineata (Say), Colorado potato beetle (CPB), and their relative impact on non‐target arthropods in potato ecosystems. Results from the two years of field trials demonstrated that Newleaf potato plants were highly effective in suppressing populations of CPB, and provided better CPB control than weekly sprays of a microbial Bt‐based formulation containing Cry3Aa, bi‐weekly applications of permethrin, or early‐ and mid‐season applications of systemic insecticides (phorate and disulfoton). When compared with conventional potato plants not treated with any insecticides, the effective control of CPB by Newleaf potato plants or weekly sprays of a Bt‐based formulation did not significantly impact the abundance of beneficial predators or secondary potato pests. In contrast to Newleaf potato plants or microbial Bt formulations, however, bi‐weekly applications of permethrin significantly reduced the abundance of several major generalist predators such as spiders (Araneae), big‐eyed bugs (Geocorus sp.), damsel bugs (Nabid sp.), and minute pirate bugs (Orius sp.), and resulted in significant increases in the abundance of green peach aphid (GPA), Myzus persicae (Sulzer) – vector of viral diseases, on the treated potato plots. While systemic insecticides appeared to have reduced the abundance of some plant sap‐feeding insects such as GPA, lygus bugs, and leafhoppers, early and mid‐season applications of these insecticides had no significant impact on populations of the major beneficial predators. Thus, transgenic Bt potato, Bt‐based microbial formulations and systemic insecticides appeared to be compatible with the development of integrated pest management (IPM) against other potato pests such as GPA because these CPB control measures have little impact on major natural enemies. In contrast, the broad‐spectrum pyrethroid insecticide (permethrin) is less compatible with IPM programs against GPA and the potato leafroll viral disease.

Recruitment by Flight and Walking in a One-Generation Colorado Potato Beetle (Coleoptera: Chrysomelidae) Environment

Adult Colorado potato beetle, Leptinotarsa decemlineata (Say), recruitment on potato plots by flight or by walking occurred throughout the 1991–1993 crop seasons. Most recruitment occurred by walking. Yearly recruitment by flight ranged from 9.5 to 49.4%. It is suggested that the proportion of recruitment occurring by flight is determined largely by environmental factors such as air temperature and solar radiation. The presence of well-developed flight muscles is necessary for flight but was not a limiting factor until late August and even then only for part of the population. The proportion of beetles with developed thoracic muscle masses, developed ovaries, high abdominal fat content levels, and sperm in the spermatheca did not significantly differ between beetle populations recruited by flight or by walking but did differ between the overwintered and summer groups. A significant proportion of beetles that colonized potato plots by flight emigrated from the plots by walking on the same day throughout each crop season. This suggests that most beetles use both flight and walking to disperse throughout the day depending on the “task” and environmental conditions. A large proportion of summer adults in prediapause condition retain the ability to disperse by flight when conditions are appropriate such as in 1991 in this study. Results confirm that potato beetle recruits counted on plants or collected in ground traps have an overall 0.41 sex ratio but those collected in flight traps have a 0.52 sex ratio. There was no indication that the sex ratio plays a significant role in determining the relative contribution of walking and flight to recruitment. A diapausing switch in early August responsible for prediapausing conditions in the summer population of beetles occurs in New Brunswick. The switch did not entirely prevent dispersal to the crop by flight as reported elsewhere for more southern locations.

Effect of crop-specific field management and N fertilization on N2O emissions from a fine-loamy soil

Agricultural soils are a major source of atmospheric N2O. This study was conducted to determine the effect of different crop-specific field management and N fertilization rates on N2O emissions from a fine-loamy Dystric Eutrochrept. Fluxes of N2O were measured for two years at least once a week on plots cropped with potatoes (Solanum tuberosum) fertilized with 50 or 150 kg N ha−1 a−1, winterwheat (Triticum aestivum) fertilized with 90 or 180 kg N ha−1 a−1, corn (Zea mays) fertilized with 65 or 130 kg N ha−1 a−1, and on an unfertilized, set-aside soil planted with grass (mainly Lolium perenne and Festuca rubra). The mean N2O emission rate from the differently managed plots was closely correlated to the mean soil nitrate content in the Ap horizon for the cropping period (April to October, r 2 = 0.74), the winter period (November to March, r 2 = 0.93, one outlier excluded), and the whole year (r 2 = 0.81). N2O emissions outside the cropping period accounted for up to 58% of the annual emissions and were strongly affected by frost-thaw cycles. There was only a slight relationship between the amount of fertilizer N applied and the annual N2O emission (r 2 = 0.20). The mean annual N2O-N emission from the unfertilized set-aside soil was 0.29 kg ha−1. The annual N2O-N emission from the fertilized crops for the low and the recommended rates of N fertilization were 1.34 and 2.41 kg ha−1 for corn, 2.70 and 3.64 kg ha−1 for wheat, and 5.74 and 6.93 kg ha−1 for potatoes. The high N2O emissions from potato plots were due to (i) high N2O losses from the interrow area during the cropping season and (ii) high soil nitrate contents after the potato harvest. The reduction of N fertilization (fertilizer was applied in spring and early summer) resulted in decreased N2O emissions during the cropping period. However, the emissions during the winter were not affected by the rate of N fertilization. The results show that the crop-specific field management had a great influence on the annual N2O emissions. It also affected the emissions per unit N fertilizer applied. The main reasons for this crop effect were crop-specific differences in soil nitrate and soil moisture content.

Oospore Survival and Pathogenicity of Single Oospore Recombinant Progeny from a Cross Involving US-17 and US-8 Genotypes of Phytophthora infestans

Oospores of Phytophthora infestans produced in vitro and in planta, from a cross between US-17 and US-8 genotypes, were exposed to a variety of environments and their survival was assessed. Additionally, the pathogenic characteristics of some resultant progeny isolates were assessed. Viability of oospores as measured by plasmolysis declined slightly over a period of 18 months whether they were stored in water at 4°C, in soil at 18°C, or in soil under natural field conditions. In comparison, viability as measured by germination was lower overall but appeared to increase after storage in soil. Oospores produced in planta were buried in the field in the fall of 1998, and were capable of infecting both tomato and potato leaflets when recovered in May 1999. Single oospore progeny (n = 53) from the in vitro cross were analyzed individually for genetic and pathogenicity characteristics. All 53 progeny tested for restriction fragment length polymorphisms with probe RG57 were hybrids. All but one progeny produced sporulating lesions on detached potato or tomato leaflets in growth chamber tests, but most lesions were smaller and developed more slowly than those produced by either parental isolate. In a further test of pathogenicity, under field conditions, none of a subset of 10 A2 progeny was capable of initiating a detectable epidemic in small plots of either potatoes or tomatoes.

Evaluation of compost and straw mulching on soil-loss characteristics in erosion plots of potatoes in Prince Edward Island, Canada

The intensive use of Prince Edward Island (PEI) farmland for potato ( Solanum tuberosum L.) production draws concern because of soil physical degradation under the intense tillage and other heavy traffic associated with the mechanisation of this crop, and the resulting vulnerability of sloping fields to hydraulic erosion. Growing public consciousness and pressure in favour of sustainable agriculture have engendered farmer interest in a possible place for commercial farm by-products in commercial cropping. Using existing natural erosion plots in PEI, the effects of compost amendment and straw mulching were assessed in terms of soil loss and soil physical condition on a fine sandy loam (Orthic Podzol). The treatment materials are local by-products of potato farming systems, and were used for an insight into the likelihood of sustainable commercial use. Compost had no effect on soil loss. Mulching reduced soil loss by almost 50%. Both treatments increased soil water content by 6–7%. Among the treatment effects on soil physical characteristics, soil penetration resistance below the root zone was reduced almost 20%; and, in the third year of the study, soil aggregate stability increased 7% with compost. Shear strength showed a strong ( p≤0.05) regression relationship ( R 2=0.835) with bulk density.

Mulching potatoes: Aspects of mulch management systems and soil erosion

Intensive potato production occupies much of the sloping arable land of Prince Edward Island, Canada, and is at the center of soil erosion concerns in this province. Corrective considerations have turned recent attention to mulching, but there is limited knowledge of its workability or effectiveness in potato systems. This study looks at the effect of mulching on soil loss from potatoes grown on standard erosion plots, and examines a relatively simple approach to assessing soil-surface splash detachment (splash erosion) under mulch-management systems with potatoes. Three sizes of splash cup (25, 50, and 100 mm in diameter) were used under simulated rainfall at 150 mm·h−1 for 10 minutes to measure splash erosion on potato plots under mulch-management systems which, respectively, left surface coverage of ∼ 5%, ∼ 15%, and ∼20%. The lowest straw coverage gave up to 56% more erosion than either of the two higher coverages. The 25-mm splash cup yielded 14% more sediment splash than the 100-mm splash cup on the basis of unit surface area of soil in the splash cup (unit area). Regression modeling of unit-area splash against straw cover showed an exponential decay in splash detachment with increasing straw cover. On the erosion plots, soil loss with mulching was half of what it was without mulching; and soil water retention was 5% greater with mulching.

First Report of Verticillium dahliae Causing Wilt on Solanum cardiophyllum and Solanum ehrenbergii.

Solanum cardiophyllum Lindl and Solanum ehrenbergii (Bitt) Rydb are wild edible potato plants found throughout central Mexico (2). These plants are not cultivated, but farmers collect tubers for their own consumption and to sell at local markets (2). Wilted plants were observed in experimental plots of these wild potatoes established near Chapingo, Mexico, during spring 1983. Initial symptoms included wilting and dark yellowing of lower leaves. As the disease advanced, all of the foliage became chlorotic and the plants wilted and eventually died. Disease incidence was 13.4% for S. ehrenbergii and 0.2% for S. cardiophyllum. Verticillium dahliae Kleb. was consistently isolated from the roots and lower stems of diseased plants of both Solanum species. The isolating procedure consisted of thoroughly rinsing roots and lower stems with tap water and cutting roots and stems into 3- to 6-cm sections that were placed in 10% bleach for 3 to 5 min. Bleach excess was removed with sterile paper, and the tissue sections were cut into smaller pieces (0.5 cm) and placed on potato dextrose agar (PDA) plates. Cultures of Verticillium produced numerous dark microsclerotia of various shapes and sizes (0.05 to 0.1 mm); erect, slender, hyaline, and branched conidiophores; and elliptical and hyaline, single-celled conidia characteristic of V. dahliae (1). Pathogenicity studies were conducted in a greenhouse on 2-month-old S. cardiophyllum and S. ehrenbergii plants grown from tubers. Inoculum was obtained from colonies growing on PDA for 10 days producing abundant conidia. Conidial suspensions were obtained by flooding the plate cultures with sterile distilled water, filtering the suspension with two layers of cheesecloth, and adjusting the inoculum to 1.0 × 106 conidia/ml (3). Ten ml of the conidial suspension were applied to each of four holes 5 cm deep and 3 to 5 cm next to the crown of each plant. Symptoms similar to those observed on field-grown plants were observed 15 days after inoculation, and V. dahliae was re-isolated from lower stems and roots. All inoculated plants were dead 4 weeks after inoculation. Water-inoculated plants remained healthy throughout the experiments. This is the first report of V. dahliae on S. cardiophyllum and S. ehrenbergii.

Effect of local inoculum on the spread of sweet potato virus disease: limited infection of susceptible cultivars following widespread cultivation of a resistant sweet potato cultivar

A study compared the spread of sweet potato virus disease (SPVD) into crops of two moderately resistant and initially SPVD‐free sweet potato cultivars in northern and southern Mpigi, Uganda. Whiteflies, the vector of sweet potato chlorotic stunt crini virus (SPCSV), a component cause of SPVD, were similarly abundant in farmers' sweet potato fields around Namulonge in northern Mpigi, and Kanoni in southern Mpigi. However, mean incidence of SPVD in farmers' crops neighbouring the trials was higher at Kanoni (13.3%) than at Namulonge (2.8%). Furthermore, spread of SPVD into initially SPVD‐free sweet potato plots of two only moderately resistant cultivars was greater in plots at Kanoni than in plots at Namulonge. The SPVD‐resistant New Kawogo was the most common cultivar grown in farmers' fields at Namulonge and had few diseased plants, whereas susceptible cultivars with relatively high incidences of disease predominated at Kanoni. Final SPVD incidence in each trial was positively correlated with a measure combining the proximity and level of inoculum in surrounding fields. The study demonstrates the importance of local SPVD inoculum in determining the rate of spread of the disease into fields and implies that the widespread cultivation of a resistant variety limits infection of susceptible cultivars grown nearby.

Definition and distribution of aggressive pathotypes of Synchytrium endobioticum in Ukraine1

Aggressive pathotypes of Synchytrium endobioticum have been detected in Ukraine since 1963, mainly in the mountainous western part of the country. Potato crops are regularly surveyed to detect possible outbreaks. It has been observed that the aggressive forms attack the above‐ground parts of the potato plant, as well as the underground parts, and this can assist detection. Possible foci are investigated by planting a standard replicated test with a susceptible cultivar and a cultivar resistant to the common pathotype. The distribution of S. endobioticum in Ukraine is summarized. Outbreaks are particularly found in potato plots adjoining farm buildings.

Yield, vitamin and mineral contents of organically and conventionally grown potatoes and sweet corn

An experiment was conducted for three years in a Pugwash sandy loam near Truro, NS. Five replicates of two treatments (organic and conventional) were established annually in different plot areas for potatoes ( Solanum tuberosum L. Superior) and sweet corn ( Zea mays L. var. saccharata `Sunnyvee' or `Pride and Joy'. The addition of pesticides, lime and NPK fertilizer to the conventional plots followed a soil test and provincial recommendations; lime, composted manure and insect control applications to the organic plots were according to the guidelines of the OCIA [Organic Crop Improvement Association, 1990. OCIA Certification Standards. OCIA, Bellefontaine, OH, 11 pp.] The compost was analysed for total N and applied to provide 260 kg N/ha for potatoes and 200 kg N/ha for sweet corn, which assumed 50% availability of the total N. Marketable yields were determined, and potato leaves and tubers, as well as sweet corn kernels and ear leaves were digested and analysed for 12 macro- and micronutrients. In addition, the vitamin C content of the tubers and the vitamin C and E contents of the kernels were analysed. Soil samples were also taken at harvest and analysed for essential nutrients and available N. Analysis of the three years of data showed that the yield and vitamin C content of the potatoes was not affected by treatments. However, the conventionally grown treatment outproduced the organically grown treatment for Pride and Joy (cv.) corn, but there was no difference between treatments in the yield of Sunnyvee (cv.) corn or the vitamin C or E contents of the kernels in any year. At p<0.11, four elements in potato tubers (P, Mg, Na, Mn) and four elements in potato leaves (N, Mg, Fe, B) were influenced by treatments, but only leaf Cu was affected in the sweet corn. Correspondingly, extractable P, Ca, Mg and Cu were higher in organically fertilized potato plots; only extractable Mg was affected by treatments in the sweet corn, with the Mg content higher in the organic plots. Only leaf P and K were significantly positively correlated with extractable P ( r=0.70) and K ( r=0.73) in the potato plots, while leaf Cu and kernel S were positively correlated with extractable Cu ( r=0.56) and S ( r=0.62) in the sweet corn plots.

Impact of Bacillus thuringiensis - insecticides on population dynamics and egg predation of the Colorado potato beetle in North Carolina potato plantings.

Field studies to assess the impact of Bacillus thuringiensis var. tenebrionis (Btt)-insecticides on Colorado potato beetle populations, egg survivorship and levels of predation on egg masses were conducted in replicated field research plots during two years. Stage-specific abundance of the Colorado potato beetle and predation on egg masses were monitored in Btt-treated and untreated potato plots in both years. The Btt-treatments significantly reduced densities of large (third and fourth instar) Colorado potato beetle larvae. The densities of large larvae remained below 0.5 and 3 per plant in the Btt-treatment while peak densities of 4.5 and 21 large larvae per plant occurred in the untreated control in 1992 and 1993, respectively. Regular sampling of egg masses indicated that predation rates in Btt-treated and untreated plots did not differ significantly although, in 1993, predation rates of up to 100% were recorded, only in Btt-treated plots. In a predator exclusion study carried out in 1992, survivorship of protected eggs was consistently higher than of eggs exposed to predation. Seasonal survivorship of exposed eggs was significantly lower in the Btt-treated than in untreated plots. Btt insecticides for control of Colorado potato beetles provided direct protection of the crop and were compatible with naturally-occurring biological control of Colorado potato beetle eggs due to predation.

A theoretical approach to soil porosity and sealing development using simulated rainstorms

Macroporosity, pore shape and size distribution were analysed in samples collected during a field rain simulation campaign. Soils were a Typic Eutrochrept and a Typic Udorthent, eroded phase, under different crops and treatments (1 lucerne, 1 potato and 2 bare plots). In each plot, 2 rainfall simulations were made and soil samples for thin section analysis were collected. Soil macroporosity decreased most strongly in the bare soil, then in the potato plot and least in the lucerne plot. The porosity reduction was mainly due to a decrease in size of elongated pores. The decrease of porosity was associated with an increase in runoff rate. The data on elongated pore dynamics were compared with equations based on a physical interpretation of the processes leading to sealing (mainly, porosity reduction due to sedimentation and impacting drops). The equations were compared successfully with the experimental trends. They can be considered as a first mathematical layout, which may be developed into a physically based algorithm.B.V.

Role of Green Peach Aphid Flights in the Epidemiology of Potato Leaf Roll Disease in the Columbia Basin.

Three distinct and highly predictable green peach aphid (GPA) (Myzus persicae) flights that occur seasonally in the spring, summer, and fall were detected at a southern, central, and northern location in the Columbia Basin of the Northwestern United States. Intensity and timing of the flights was approximately the same at the three locations. Timing and number of alatae captured in the spring and summer flights was associated with heat unit accumulation. The spring flight, which originates on the overwintering peach tree host, colonized but did not introduce potato leafroll virus (PLRV) into virus-free potato plots. The summer flight, which originates from volunteer potatoes and spring herbs originally colonized by the spring flight, did introduce PLRV into virus-free potatoes. The fall flight was too late to affect potato production. When plots contained a point source of PLRV, the virus spread rapidly in a plant-to-plant mode to all plants in plots after aphids arrived in the spring. Rate of spread from point sources of infection was not affected by timing or intensity of the spring flight, but timing of virus spread in the plots depended on time of arrival of the aphids. Once PLRV was introduced to virus-free plots by the summer flight, virus spread to other plants within the plots. GPA overwintered on peach trees. Although GPA apterae and alatae were present on winter annual weed and crop hosts in the fall, none survived winters on these species. In addition to the GPA, one other vector of PLRV, Macrosiphum euphorbiae, was rarely collected in aphid traps. These results suggest that chemical control of aphids could be delayed until mid-July if PLRV-free potato seed were available.

Early-Season Applications of the FungusBeauveria bassianaand Introduction of the Hemipteran PredatorPerillus bioculatusfor Control of Colorado Potato Beetle

Abstract Unformulated conidia of the fungus Beauveria bassiana were applied to potato plots, with and without the hemipteran predator Perillus bioculatus, in an attempt to reduce field populations of Colorado potato beetle, Leptinotarsa decemlineata, and to compare their efficacy to currently utilized methods of chemical insecticide control. Dry conidia were mixed in a water/0.01% Silwet suspension and applied with a backpack sprayer, at a rate of 5 × 10 13 viable conidia/ha. Small nymphs (first and second instars) of the predator were released at the rate of one nymph/plant. Four rapid (at 3- to 4-day intervals) early-season (at the green row and touch in row growth stages of potato) foliar applications of the fungus and four releases (at weekly intervals) of the predator were made. The observed rate of mycosis in the treated larval populations was >90% 2 days after the last fungal application. The seasonal treatment effects on small (first and second instars) and large (third and fourth instars) larvae were examined. There were no differences in densities of small larvae in any of the treatments. However, the seasonal densities of large larvae were 41.4, 9.7, 20.7, and 9.1/plant in the control, Beauveria, chemical insecticides (esfenvalerate plus piperonylbutoxide and oxamyl plus carbofuran), and Beauveria–Perillus treatments, respectively. The respective seasonal percent efficacies (control corrected) of the treatments in reducing populations of large larvae were 76.6, 50.2, and 78.0% with the Beauveria, insecticide, and Beauveria–Perillus treatments. Large larvae cause most of the defoliation of potato plants; B. bassiana applied at 3- to 4-day intervals to early instars eliminated most of this pressure. Applications of the fungus provided substantial foliar protection and proved to be significantly more effective than parallel insecticide treatments and much more effective than no treatment. The role of the predator in limiting defoliation through predation on egg masses and small larvae was unclear. Although yields of marketable tubers showed no treatment differences, the significant reductions in larval densities, the prevalence of infection in field collections, and the substantial foliage protection confirmed the potential of B. bassiana conidia to infect and kill Colorado potato beetle for crop protection.