Uninfested Plots Research Articles

Structural characteristics of black spruce (Picea mariana) infested with eastern spruce dwarf mistletoe (Arceuthobium pusillum), Minnesota, USA

Disturbances are important processes within forested ecosystems, influencing stand dynamics, species composition, and ecosystem services, with effects across multiple spatial scales. Interacting effects across scales can influence the suitability and results of management actions, necessitating a thorough understanding of disturbances at multiple scales. Eastern spruce dwarf mistletoe (Arceuthobium pusillum Peck, hereafter ESDM) is a native disturbance agent of lowland black spruce (Picea marina (Mill.) B.S.P.) in Minnesota, USA. ESDM results in mortality at the individual tree scale but is commonly managed through stand-level silvicultural treatments; ESDM can also influence landscape-scale forest composition. Despite its role as a mortality agent of black spruce, little is known about the relationship between structure and infestation at scales finer than the stand. Here, our objective was to quantify the relationship between ESDM infestation and structure at the stand, plot, and neighborhood scales. Twenty-five black spruce stands, 12 infested and 13 uninfested, were sampled using fixed radius plots; variables collected relate to ESDM infestation severity, forest structure, and forest composition. The relationship between forest structure and ESDM infestation presence was analyzed at the stand, plot, individual tree, and neighborhood (a hybrid scale focused on individual trees but including variables measured in the plot in which the tree is located) scales. Infested plots and neighborhoods had significantly lower trees per hectare and higher species richness and Shannon’s Diversity than uninfested plots and neighborhoods; these differences were not significant the stand scale. At the stand scale, stand structure and composition variables, including trees per hectare and species diversity metrics, shift with ESDM severity. However, at the tree level, the presence or absence of ESDM is more strongly related to structure than is severity.. When incorporating ESDM infestations in management planning and decision making, considering the appropriate scale is important. By documenting the effects of ESDM at multiple scales, we can deepen our understanding of stand dynamics and succession in lowland black spruce and use this information to develop new, ecologically based management approaches in these forests that more closely reflect the local disturbance regimes.

Impact of Western Bean Cutworm (Lepidoptera: Noctuidae) Infestation and Insecticide Treatments on Damage and Marketable Yield of Michigan Dry Beans.

The western bean cutworm, Striacosta albicosta (Smith) (Lepidoptera: Noctuidae), recently expanded its range from the western United States into the Great Lakes region of the United States and Canada, threatening eastern dry bean production. Our objectives were to better understand the relationship between cutworm infestation and damage in dry beans, and to determine the best method and timing of insecticide application to reduce pick. Infesting with at least one egg mass per 1.5 m, or as few as two larvae per 0.3 m, of row resulted in significantly more pod damage and pick than in uninfested plots. By 14 d after hatch, larvae were no longer on plants during the daytime; direct observations revealed that fifth instars climbed plants to feed between 2100 and 0600 hours, illustrating the impractically of using larval counts to make management decisions. There was a strong linear relationship between pod damage and percent pick, making scouting for pod damage a viable alternative to egg or larval scouting. Aldicarb soil insecticide or thiamethoxam-treated seed did not reduce cutworm damage. Instead, plots treated with these insecticides had significantly more pick than control plots, perhaps related to increased canopy growth or fewer natural enemies. The pyrethroid λ-cyhalothrin provided excellent control of cutworm when sprayed up to 18 d after infestation. Pick was similar among plots sprayed once up to 18 d after infestation or sprayed four separate times. In a field study, λ-cyhalothrin residue on field-treated foliage was 100% effective at controlling caterpillars up to 14 d after application.

Bark beetles and dwarf mistletoe interact to alter downed woody material, canopy structure, and stand characteristics in northern Colorado ponderosa pine

Due to the recent outbreaks of bark beetles in western U.S.A., research has focused on the effects of tree mortality on forest conditions, such as fuel complexes and stand structure. However, most studies have addressed outbreak populations of bark beetles only and there is a lack of information on the effect of multiple endemic, low level populations of biotic disturbance agents that could influence forest dynamics, fuel heterogeneity, and species composition. Downed woody material, fuel parameters and stand characteristics were assessed in areas of ponderosa pine (Pinus ponderosa) infested with southwestern dwarf mistletoe (Arceuthobium vaginatum subsp. cryptopodum), and in areas with endemic populations of mountain pine beetle (Dendroctonus ponderosae) and Ips spp. (Coleoptera: Curculionidae, Scolytinae) in northern Colorado. Both endemic bark beetles and dwarf mistletoe were associated with more dense stands than uninfested plots and resulted in reducing basal area of live trees. The amount of downed woody material was positively related to time since tree mortality and basal area of bark beetle-attacked trees. There was an increase of up to 10% for fine and coarse downed woody material for every increase of 1m2ha−1 basal area of mortality from mountain pine beetle that was 4–15years after death. The average intensity of dwarf mistletoe infections on live trees was positively related to amount of 10-h downed woody material and negatively associated with percent live ponderosa pine crown. Fuel bed and canopy characteristics were dependent on the presence of both dwarf mistletoe and mountain pine beetle-caused mortality. Together these endemic biotic disturbances have an effect on downed woody material biomass accumulation, fuel arrangements, stand densities and species composition.

Seasonal dispersal of the Western corn rootworm (Diabrotica virgifera virgifera) adults in Bt and non-Bt maize fields

32 experimental plots were sown at Borovce (district Trnava) in Slovakia during 2008 with 8 maize hybrids, including coleopteran-protected MON 88017, the lepidopteran-protected MON 89034, the stacked product, MON 89034 × MON 88017, that same hybrids isoline that did not have the Bt trait or traits, and 4 commercial hybrids. Part of the fourth repetition was unexpectedly infested and damaged with the Western corn rootworm (WCR). High numbers of the WCR adults were found in the damaged plots on July 8. Adult WCR abundance remained concentrated in this area until August 12–19. The beetles apparently began to disperse during August 19–September 2, and after September 2. The trap captures increased in the uninfested areas up to September 16, no significant difference having been found in the adult WCR captures between the infested and uninfested plots, including the plots with MON 88017. Bt-maize hybrids containing MON 88017 strongly influenced the level of the WCR damage as assessed by lodging of the maize plants, and they also influenced the numbers of the WCR adults before maize flowering.  We found that intensive movement of the WCR adults from the place of their origin started later in the vegetation season.

Soybean Yield Response to Plant Distribution in Fusarium virguliforme Infested Soils

Sudden death syndrome (SDS), caused by the fungal pathogen Fusarium virguliforme, causes significant yield reductions in soybean [Glycine max (L.) Merr.] in the United States. Appropriate recommendations to manage SDS for growers in Iowa and the Upper Midwest are limited. The research objective was to determine the response of SDS foliar disease incidence, severity, and yield to row spacing and seeding rate. In 2008 and 2009, at two Iowa locations, in fields with histories of SDS, SDS‐susceptible and SDS‐resistant cultivars were planted at 38‐ and 76‐cm row spacing at seeding rates of 185,000, 309,000, and 432,000 seeds ha−1 in plots infested with and without the pathogen. Sudden death syndrome incidence and severity were very low; however, infested plots had greater SDS disease incidence and severity than uninfested plots. A row spacing × infestation interaction indicated 7% greater yield in narrow rows (38 cm) than wide rows (76 cm) in uninfested plots, with no yield advantage to narrow rows in infested plots. Soil infestation reduced soybean seed mass (7%) in narrow rows, explaining the yield reduction for narrow rows with greater SDS. The two highest seeding rates had increased SDS incidence but yielded 9% greater than the lowest seeding rate. The susceptible cultivar had greater SDS incidence and severity and yielded 7% less than the resistant cultivar. This study indicates that in infested plots with greater SDS symptom expression, the yield advantage of narrow rows may be negated; therefore, cultivar selection is crucial when planting in narrow rows to maximize yield.

Evaluating Potential Fire Behavior in Lodgepole Pine-Dominated Forests after a Mountain Pine Beetle Epidemic in North-Central Colorado

Abstract A mountain pine beetle outbreak in Colorado lodgepole pine forests has altered stand and fuel characteristics that affect potential fire behavior. Using the Fire and Fuels Extension to the Forest Vegetation Simulator, potential fire behavior was modeled for uninfested and mountain pine beetle-affected plots 7 years after outbreak initiation and 10 and 80% projected tree fall using measured and projected fuel and stand characteristics. Under 90th percentile weather conditions, uninfested plots exhibited proportionally more crown fire than infested plots. Plots predicted to have crown fire were composed mainly of nonhost conifer species and had a lower and more continuous canopy than infested plots. Where surface fire was predicted to occur, live lodgepole pine was the only conifer present, and plots had significantly lower tree mortality from fire than plots predicted to have crown fire. Mountain pine beetle-induced changes in stand and fuel characteristics resulted in increased intensity of surface fire behavior. Furthermore, with 80% infested tree fall, potential smoke production was predicted to be higher. Tree species composition of stands pre and postbark beetle outbreak is important when identifying mountain pine beetle-caused changes to potential fire behavior.

Interacting disturbances: wildfire severity affected by stage of forest disease invasion

Sudden oak death (SOD) is an emerging forest disease causing extensive tree mortality in coastal California forests. Recent California wildfires provided an opportunity to test a major assumption underlying discussions of SOD and land management: SOD mortality will increase fire severity. We examined prefire fuels from host species in a forest monitoring plot network in Big Sur, California (USA), to understand the interactions between disease-caused mortality and wildfire severity during the 2008 Basin Complex wildfire. Detailed measurements of standing dead woody stems and downed woody debris 1-2 years prior to the Basin fire provided a rare picture of the increased fuels attributable to SOD mortality. Despite great differences in host fuel abundance, we found no significant difference in burn severity between infested and uninfested plots. Instead, the relationship between SOD and fire reflected the changing nature of the disease impacts over time. Increased SOD mortality contributed to overstory burn severity only in areas where the pathogen had recently invaded. Where longer-term disease establishment allowed dead material to fall and accumulate, increasing log volumes led to increased substrate burn severity. These patterns help inform forest management decisions regarding fire, both in Big Sur and in other areas of California as the pathogen continues to expand throughout coastal forests.

Western Dwarf Mistletoe and Prescribed Fire Behavior—A Case Study from Crater Lake National Park

This study investigates the influence of parasitic western dwarf mistletoe on prescribed fire behavior and limb flammability of ponderosa pine. Fall season prescribed fires were applied to 14 plots (7 infested and 7 uninfested) in an 81.5 ha mixed-conifer stand at Crater Lake National Park. I measured stand structure variables and surface fuel composition before and after burning and monitored fire behavior during burns. To assess the influence of mistletoe infection on limb flammability in a more controlled setting, I burned infected and uninfected limbs (n = 18 each) in a burn chamber. Results show that plots at Crater Lake infested with mistletoe burned at higher temperatures for shorter periods than uninfested plots. Plots with greater duff biomass recorded higher temperatures, while those with greater accumulations of litter burned longer. All plots experienced a significant reduction in total surface fuel loads following fire, however there was no difference in fuel reduction between uninfested and infested plots, suggesting mistletoe has little influence on fire behavior in this forest type. Infected branches lost a greater proportion of their mass during burn chamber tests, primarily in the form of needles. These results indicate that dwarf mistletoe has little influence on the behavior of low-intensity surface fires, but further research is needed to assess the role of mistletoe in other fire conditions and stand types.

Stand characteristics and downed woody debris accumulations associated with a mountain pine beetle (Dendroctonus ponderosae Hopkins) outbreak in Colorado

Lodgepole pine (Pinus contorta Dougl. ex Loud.)-dominated ecosystems in north-central Colorado are undergoing rapid and drastic changes associated with overstory tree mortality from a current mountain pine beetle (Dendroctonus ponderosae Hopkins) outbreak. To characterize stand characteristics and downed woody debris loads during the first 7 years of the outbreak, 221 plots (0.02ha) were randomly established in infested and uninfested stands distributed across the Arapaho National Forest, Colorado. Mountain pine beetle initially attacked stands with higher lodgepole pine basal area, and lower density and basal area of Engelmann spruce (Picea engelmannii [Parry]), and subalpine fir (Abies lasiocarpa (Hook.) Nutt. var. lasiocarpa) compared to uninfested plots. Mountain pine beetle-affected stands had reduced total and lodgepole pine stocking and quadratic mean diameter. The density and basal area of live overstory lodgepole declined by 62% and 71% in infested plots, respectively. The mean diameter of live lodgepole pine was 53% lower than pre-outbreak in infested plots. Downed woody debris loads did not differ between uninfested plots and plots currently infested at the time of sampling to 3 or 4–7 years after initial infestation, but the projected downed coarse wood accumulations when 80% of the mountain pine beetle-killed trees fall indicated a fourfold increase. Depth of the litter layer and maximum height of grass and herbaceous vegetation were greater 4–7 years after initial infestation compared to uninfested plots, though understory plant percent cover was not different. Seedling and sapling density of all species combined was higher in uninfested plots but there was no difference between infested and uninfested plots for lodgepole pine alone. For trees ≥2.5cm in diameter at breast height, the density of live lodgepole pine trees in mountain pine beetle-affected stands was higher than Engelmann spruce, subalpine fir, and aspen, (Populus tremuloides Michx.), in diameter classes comprised of trees from 2.5cm to 30cm in diameter, suggesting that lodgepole pine will remain as a dominant overstory tree after the bark beetle outbreak.

Pre-visible Detection of Grub Feeding in Turfgrass using Remote Sensing

Japanese beetle grubs (Popillia japonica Newman) are rootfeeding pests of turfgrass in the Midwest and eastern United States causing millions of dollars of damage annually. To reduce unnecessary pesticide output by applying only where needed, turfgrass managers need a practical, noninvasive method to locate patchy infestations before unsightly damage has occurred. Spectrometer data and multispectral aerial imagery were evaluated for detecting pre-visible symptoms of grub damage in turfgrass, to facilitate sitespecific grub management. Using spectrometer reflectance data, first derivatives of reflectance, narrow-band vegetation indices, and linear combinations of multiple bands/indices, infested turfgrass plots were distinguished from un-infested plots 10 to16 days before visual differences appeared in the year when visual ratings were conducted. Pre-visible symptoms of grub feeding were not detected using the position of the red edge or edge of the red edge. Results using multispectral imagery were mixed, with early symptoms detected in only one of two years.

Carbon and nitrogen cycling immediately following bark beetle outbreaks in southwestern ponderosa pine forests

Bark beetle infestation is a well-known cause of historical low-level disturbance in southwestern ponderosa pine forests, but recent fire exclusion and increased tree densities have enabled large-scale bark beetle outbreaks with unknown consequences for ecosystem function. Uninfested and beetle-infested plots ( n = 10 pairs of plots on two aspects) of ponderosa pine were compared over one growing season in the Sierra Ancha Experimental Forest, AZ to determine whether infestation was correlated with differences in carbon (C) and nitrogen (N) pools and fluxes in aboveground biomass and soils. Infested plots had at least 80% of the overstory ponderosa pine trees attacked by bark beetles within 2 years of our measurements. Both uninfested and infested plots stored ∼9 kg C m −2 in aboveground tree biomass, but infested plots held 60% of this aboveground tree biomass in dead trees, compared to 5% in uninfested plots. We hypothesized that decreased belowground C allocation following beetle-induced tree mortality would alter soil respiration rates, but this hypothesis was not supported; throughout the growing season, soil respiration in infested plots was similar to uninfested plots. In contrast, several results supported the hypothesis that premature needlefall from infested trees provided a pulse of low C:N needlefall that altered soil N cycling. The C:N mass ratio of pine needlefall in infested plots (∼45) was lower than uninfested plots (∼95) throughout the growing season. Mineral soils from infested plots had greater laboratory net nitrification rates and field resin bag ammonium accumulation than uninfested plots. As bark beetle outbreaks become increasingly prevalent in western landscapes, longer-term biogeochemical studies on interactions with other disturbances (e.g. fire, harvesting, etc.) will be required to predict changes in ecosystem structure and function.

Susceptibility of ponderosa pine, Pinus ponderosa (Dougl. ex Laws.), to mountain pine beetle, Dendroctonus ponderosae Hopkins, attack in uneven-aged stands in the Black Hills of South Dakota and Wyoming USA

Mountain pine beetle, Dendroctonus ponderosae Hopkins can cause extensive tree mortality in ponderosa pine, Pinus ponderosa Dougl. ex Laws., forests in the Black Hills of South Dakota and Wyoming. Most studies that have examined stand susceptibility to mountain pine beetle have been conducted in even-aged stands. Land managers increasingly practice uneven-aged management. We established 84 clusters of four plots, one where bark beetle-caused mortality was present and three uninfested plots. For all plot trees we recorded species, tree diameter, and crown position and for ponderosa pine whether they were killed or infested by mountain pine beetle. Elevation, slope, and aspect were also recorded. We used classification trees to model the likelihood of bark beetle attack based on plot and site variables. The probability of individual tree attack within the infested plots was estimated using logistic regression. Basal area of ponderosa pine in trees ≥25.4 cm in diameter at breast height (dbh) and ponderosa pine stand density index were correlated with mountain pine beetle attack. Regression trees and linear regression indicated that the amount of observed tree mortality was associated with initial ponderosa pine basal area and ponderosa pine stand density index. Infested stands had higher total and ponderosa pine basal area, total and ponderosa pine stand density index, and ponderosa pine basal area in trees ≥25.4 cm dbh. The probability of individual tree attack within infested plots was positively correlated with tree diameter with ponderosa pine stand density index modifying the relationship. A tree of a given size was more likely to be attacked in a denser stand. We conclude that stands with higher ponderosa pine basal area in trees >25.4 cm and ponderosa pine stand density index are correlated with an increased likelihood of mountain pine beetle bark beetle attack. Information form this study will help forest managers in the identification of uneven-aged stands with a higher likelihood of bark beetle attack and expected levels of tree mortality.

Effects of Western Balsam Bark Beetle on Spruce-Fir Forests of North-Central Wyoming

Abstract Western balsam bark beetle, Dryocoetes confusus (Coleoptera: Scolytidae), has caused widespread mortality of subalpine fir (Abies lasiocarpa) in western North America throughout the past decade. The objectives of this study were to document the effects of this mortality, relate mortality to pre-existing stand conditions, and investigate the role of storm-damaged fir in beetle population dynamics in north-central Wyoming. Transect cruise lines and pairs of infested and uninfested plots were installed to detect changes in the forest overstory and understory and to determine associations between stand conditions and beetle-caused fir mortality. On average, beetles killed more than 70 trees/ac over the last several years. This mortality resulted in significant decreases in: subalpine fir basal area, trees per acre, stand density index, and the percentage of subalpine fir stems in the overstory. Small, but significant increases were detected in the understory; herbaceous plant abundance increased in the infested plots compared with the noninfested plots. Moreover, significant positive linear relationships were found between the amount of fir mortality and the percentage of subalpine fir trees in a stand, subalpine fir basal area, and subalpine fir stand density index. In addition, a significant positive linear relationship was found between the percentage of wind-caused downed fir logs in an area and the percentage of logs utilized by western balsam bark beetle. The blowdown events that occurred in the mid-1990s in combination with a high percentage of fir component has provided ideal conditions for continued beetle expansion. West. J. Appl. For. 18(4):259–266.

Effects of irrigation method on chile pepper yield and Phytophthora root rot incidence

A field experiment was conducted in 1995 and 1996 to examine the effects of different irrigation methods on yields and Phytophthora root rot disease of chile plants ( Capsicum annum New Mexico `6–4'). Three irrigation methods, daily drip, 3-day drip, and alternate row furrow irrigation, were applied to plots infested with P. capsici and uninfested plots. For both years, the drip irrigation (either daily or 3-day) created higher marketable green chile yields than the alternate row furrow irrigation ( p < 0.05), and the yields between the daily and 3-day drip irrigation were statistically similar. The effect of irrigation on marketable combined yields was similar to that on green chile yields. In 1995, root rot disease incidence in the infested plots was significantly higher under alternate row furrow irrigation than for daily and 3-day drip irrigation. There was no disease development in the uninfested plots regardless of the irrigation method. The disease decreased green chile yield by 55% ( p < 0.1), and combined yield (green + red chile) by 36% ( p < 0.1) in 1995 compared to that in uninfested plots in alternate row furrow irrigation. In 1996, however, no disease occurred in any treatment. The results suggested that drip irrigation increases chile yield through providing either favorable soil moisture conditions or unfavorable conditions for Phytophthora propagation.

Impact of an introduced ant on native rain forest invertebrates: Pheidole megacephala in monsoonal Australia.

Pheidole megacephala is an exotic ant species that has severely affected native invertebrate biodiversity throughout the tropics. Its impacts have been documented extensively in relatively depauperate invertebrate communities, but not in species-rich habitats such as tropical rain forests. Here we describe the local distribution of P. megacephala and its impacts on native invertebrate assemblages in and around a rain forest patch at Howard Springs, in Australia's monsoonal tropics. P. megacephala was found to be confined to a single area of approximately 25 ha, with its distribution centered on drainage lines and the rain forest. Significant but weak correlations were found between its abundance and vegetative canopy cover (positive) and distance from the rain forest (negative). In the most heavily infested area within the rain forest, the abundance of P. megacephala was 37-110 times that of total native ant abundance found within uninfested plots, as measured by pitfall traps. The abundance and richness of native ants and other invertebrates were significantly reduced in litter samples, pitfall catches and foliage beats where P. megacephala was present, inversely relative to the abundance of P. megacephala. Only two individuals of a single native ant species were found within the most infested plot, with native ant richness being reduced to about half in the least infested plot. The most persistent functional groups of native ants in infested plots were Cryptic species, which forage primarily within soil and leaf litter, and Opportunists, which exhibit highly generalised foraging behaviour. The highest abundance of P. megacephala corresponded with a 42-85% decrease in the abundance of other native invertebrates. Insect larvae were totally absent from foliage beats collected at the most heavily infested plot. P. megacephala was found overall to be expanding its range, averaging 12 m range expansion in the dry season and contracting 7 m in the wet season. It is able to spread into surrounding savanna habitats by occupying relatively sheltered microsites, such as beneath logs and at the bases of trees. However, it is unlikely to attain high population densities in open savanna habitats because of its relative intolerance of desiccation, and the prevalence of behaviourally dominant native ant species. Howard Springs is currently the only rain forest patch in monsoonal Australia known to be infested by P. megacephala, but clearly this ant is a serious potential threat to the region's rain forest invertebrate fauna.

The Influence of Compost Amendment or Straw Mulch on the Reduction of Gas Exchange in Potato by Verticillium dahliae and Pratylenchus penetrans.

Single potato plants (Solanum tuberosum cv. Superior) were grown in microplots in soil that was fumigated and then infested with Verticillium dahliae, Pratylenchus penetrans, or both to evaluate the effects of these pathogens and of cultural treatments with spent mushroom compost or straw mulch on gas exchange of potato leaves. Photosynthesis and transpiration of terminal leaflets of a cohort of similar-aged leaves were measured once a week from the time of expansion until they senesced. Over all measurements, gas exchange per unit leaf area was less for plants in microplots infested with V. dahliae or P. penetrans than for those in uninfested plots. For leaves that expanded in early June, gas exchange was similar immediately after leaf expansion but declined more quickly when microplots were infested with one or both pathogens compared to no infestation. Overall, leaf gas exchange was increased by compost amendment but not affected by straw mulch. Compost amendment prevented some of the decline in gas exchange due to infestation by one or both pathogens. For leaves that expanded in July, compost increased the gas exchange immediately after expansion in both infested and non-infested plots.

Striga hermonthica Seed Inoculum Rate Effects on Maize Hybrid Tolerance and Susceptibility Expression

A parasitic plant, Striga hermonthica (Del.) Benth., infests millions of hectares of arable land in sub‐Saharan Africa, and it threatens production of major cereal crops including maize (Zea mays L.). Erratic natural infestation is considered a major constraint to assess host plant resistance to the parasite. The objective of this study was to evaluate the effects of different S. hermonthica seed inoculum rates on a tolerant and a susceptible maize genotype. Individual maize plants of the tolerant (8322‐13) and the susceptible maize hybrid (8338‐1) were inoculated with six rates of germinable Striga seeds: 750, 1500, 2250, 3000, 3750, and 4500. Uninfested plots served as the control. The experiment was conducted at two locations, Mokwa (two seasons) and Abuja (one season), in Nigeria. Increasing Striga inoculum level increased Striga emergence and the amount of damage to the host plant, plant height, and grain yield of the host crop. The tolerant hybrid showed less host plant damage (P &lt; 0.01) and that fewer S. hermonthica plants emerged (P &lt; 0.05) than the susceptible hybrid. Among the four traits studied, host plant damages core was most correlated with grain yield (r = −0.65**). Average grain yield of the tolerant hybrid, 8322‐13, was 97% higher than that of the susceptible hybrid, 8338‐1, across the six inocolum rates. The tolerant hybrid produced similar grain yields under all levels of the inoculum; the yield reductions ranged from 27% (2250 seeds) to 35% (4500 seeds), while the yield reduction of the susceptible hybrid ranged from 43% (750 seeds) to 74% (3750 seeds). Based on the results this study, we conclude that host plant tolerance is the key element for stabilizing crop yield under S. hermonthica infestation.

Nitrogen Effects on Striga hermonthica Infestation, Grain Yield, and Agronomic Traits of Tolerant and Susceptible Maize Hybrids

A phytoparasite, Striga hermonthica (Del.) Benth., infests millions of hectares of cultivated fields of cereal crops in sub‐Saharan Africa. Yield losses are often 70 but can be as high as 100% and farmers often abandon infested fields and move to new areas. The objectives of this study were to investigate the effects of nitrogen on S. hermonthica infestation and the subsequent grain yield and agronomic traits of tolerant and susceptible maize hybrids (Zea mays L.). Two tolerant and two susceptible hybrids were grown under six levels of N (0–150 kg ha−1), with and without Striga seed infestation. The trials were conducted for 3 yr at Mokwa, Nigeria. Each plant was infested with approximately 3000 germinable Striga seeds, and the density of Striga that emerged, Striga damage on the host plant, plant height, stalk lodging, and grain yield were measured. Striga infestation (emergence and host damage), maize grain yield, and plant height were significantly affected by N rates. Two tolerant hybrids showed significantly lower Striga emergence and host plant damage symptoms than the susceptible hybrids (P&lt; 0.001). Interactions between infested and uninfested plots and hybrid (tolerant vs. susceptible) for grain yields and stalk lodging were significant (P &lt; 0.001). Striga infestation reduced grain yields of two susceptible hybrids by 49%, and of two tolerant hybrids by 24%. The two tolerant hybrids produced on average 87% greater grain yields than the two susceptible hybrids under low N rates (0– 60 kg ha−1) and 51% greater yields under high N (90–150 kg ha−1). Among the plant traits measured, Striga damage score had the highest correlation with grain yield (r = − 0.60**).

Regeneration of Susceptible Native Species Following a Decline of Phytophthora cinnamomi over a Period of 20 Years on Defined Plots in the Grampians, Western Victoria

Regeneration of susceptible species following a major decline in the population and distribution of the pathogen Phytophthora cinnamomi Rands is reported from a reassessment of infested plots in open forest, woodland and heathland of the Grampians. The distribution of P. cinnamomi and changes in the vegetation were measured on quadrats set up in 1976 and 1977 and checked biennially until 1984. In 1995, P. cinnamomi was detected in only 15.6% of the 345 soil and root samples tested and was present in only 28.6% of the infested quadrats compared with 100% of the samples and infested quadrats in all previous assessments. Both the restricted distribution and the isolation of P. cinnamomi from susceptible species without symptoms were evidence of changes in the disease which may have been associated with factors such as a reduced supply of susceptible host roots or an alteration in pathogenicity or in the environment. Changes were recorded in structure and species composition of infested plant communities compared with those that remained pathogen-free, but few recent deaths or dieback symptoms were observed in 1995. On the infested plots, several mature stringybark eucalypts such as Eucalyptus baxteri and E. obliqua had died, and the survivors had reduced crown density and dieback of the major branches. However, regeneration in the form of 8-year-old saplings and young seedlings showed few or no symptoms despite the isolation of P. cinnamomi from the roots. The understorey of infested plots comprised a dense growth of field-resistant plants such as Leptospermum spp. and sedges entwined with dodder laurel, whereas the uninfested plots contained a species-rich heath flora of which 54% were known to be susceptible to P. cinnamomi. Regeneration and continuing survival of 24 susceptible species from 11 different families were recorded as small plants on various infested quadrats. However, 12 susceptible species from 7 different families had failed to regenerate. Pathogen populations may increase with the renewed supply of susceptible roots, but independent of pathogen potential, the emerging susceptible species may not be able to compete effectively against the dense field-resistant understorey in infested plots in order to re-establish the original diverse heathland understorey.

A Method for Infesting Small Field Plots with Soybean Cyst Nematode

AbstractField experimentation with soybean cyst nematode (Heterodera glycines Ichinohe) is difficult, because of problems in obtaining plots with desired population densities. Therefore, a technique was developed for infesting small field plots with soybean cyst nematode. In 1990 and 1991, cysts, eggs, and infective juveniles were mixed with soil and applied with a commercial fertilizer spreader. Infected soybean [Glycine max (L.) Merr.] root segments were distributed by hand. Infested soil and roots were incorporated with a tractor‐drawn herbicide incorporator, and plots were planted to susceptible soybean. At‐planting egg densities in 1990 and 1991 averaged 184 and 100 eggs 100 cm−3 soil, respectively, representing 78 and 90% of expected egg densities. Resultant low densities of encysted eggs, which are highly aggregated, probably prevented detection of eggs in some plots. No soybean cyst nematode was detected in uninfested plots at planting, although some contamination was noted at harvest. This method may be appropriate for establishing soybean cyst nematode population densities for some small‐plot research.