Dwarf Essex Research Articles

Can relative sensitivity of cover crop species to chloroacetamide and pyrazole herbicides be exploited to design interseeding mixtures?

AbstractDrill-interseeding cover crops into standing corn (V3 to V5 growth stage) provides opportunities for increasing functional diversity in cropping systems by facilitating use of cover crop mixtures that include grass, legume, and brassica species. Designing herbicide-based weed control programs that negotiate tradeoffs between crop protection and environmental goals when interseeding mixtures remains a major challenge. The objective of this study was to use greenhouse-based dose-response assays to describe the relative sensitivity of 12 cover crop species that differ in traits, including taxonomic group and seed mass, to chloroacetamide (acetochlor, dimethenamid, S-metolachlor; Group 15 herbicides as categorized by the Weed Science Society of America) and pyrazole (pyroxasulfone; Group 15) herbicides. Nonlinear models were fit, and effective doses were estimated (ED50) to compare relative sensitivities of cover crop species to each herbicide. Brassica species (winter canola, Dwarf Essex rape, daikon radish) were less sensitive than small-seeded legumes (medium red clover, crimson clover) and grasses (annual ryegrass) in response to each chloroacetamide, but similar in response to pyroxasulfone. Austrian winter pea, a large-seeded legume, was less sensitive to each herbicide compared to other legumes (crimson clover, medium red clover, hairy vetch). Cereal rye and triticale were less sensitive to dimethenamid, S-metolachlor, and pyroxasulfone compared to annual ryegrass, but similar sensitivities were observed for acetochlor. These results suggest that relative differences in sensitivity to chloroacetamide and pyrazole herbicides could be exploited when designing cover crop mixtures for interseeding. It is imperative, however, to use these findings in conjunction with field-based observations of cover crop injury potential in interseeded systems.

Soil properties after one year of interseeded cover cropping in topographically diverse agricultural landscape

Planting cover crops within or following a cash crop may improve soil-based ecosystem services due to increased plant diversity and a longer duration of live vegetation coverage. We examined the effect of three different cover cropping systems on soil properties after one year of a three-year organic transition rotation at four agricultural field sites with contrasting topographical positions, namely depressions, slopes, and summits. The four studied systems were (1) cereal rye (Secale cereal L.) planted after corn (Zea mays L.) harvest (Rye); (2) a mixture of cold susceptible cover crop species, namely, oat (Avena sativa), winter pea (Pisum sativum), and radish (Raphanus sativus), interseeded into corn (WK); (3) a mixture of cold tolerant cover crop species, namely, annual ryegrass (Lolium multiflorum), Dwarf Essex rapeseed (Brassica napus), and crimson clover (Trifolium incarnatum), interseeded into corn (WH); and (4) a no-cover control (NC). While soil moisture was affected by topography, interseeding cover crops into corn did not influence soil moisture levels at the 0–10 cm depth for the studied year. Soil NO3- content was markedly higher in the WK system compared to cereal rye and WH cover crop treatments. The difference was especially pronounced in depressions and summits. Soil N mineralization rates followed the pattern WH>WK>Rye>NC and the effects were most pronounced in slopes. Soil microbial biomass C was the highest in depressions followed by summits and slopes, and in depressions the WH had higher microbial biomass than the other systems. There were no effects of cover crops and topography on soil C mineralization one year after the organic transition was initiated. The WH system increased the fraction of 0.053–2 mm aggregates and decreased > 2 mm aggregates in depressions. The results suggest that the effect of cover cropping can become evident already one year after the organic transition begins. The WH mixture interseeded into the cash crop was an optimal cover crop choice for improving soil characteristics as well as decreasing soil N leaching risks during organic transition in undulating agricultural terrain. However, the magnitude of the benefit provided by WH was mediated by topography.

Effects of cover crops on soil CO2 and N2O emissions across topographically diverse agricultural landscapes in corn-soybean-wheat organic transition

Agriculture represents about 10 % of total greenhouse gases (GHG) emission in the US, in which carbon dioxide (CO2) and nitrous oxide (N2O) can greatly contribute to global warming and climate change. Organic farming has been increasingly popular as it provides long-term environmental benefits including GHG mitigation. Judicious use of cover crops in crop rotation systems has been considered an effective approach to sustain crop productivity during organic transition period. Here, we examined effects of cover crops on soil CO2 and N2O fluxes after the first year of organic transition in corn-soybean-wheat rotation across three contrasting topographies, particularly depression, slope, and summit at Kellogg Biological Station located in Southwest Michigan, USA. Three cover cropping systems used in this study were (1) cereal rye; (2) a mixture of cold susceptible species, namely, oat, winter pea, and radish, frost-killed in winter (WK); and (3) a winter hardy mixture of cold tolerant species, namely annual ryegrass, Dwarf Essex rapeseed, and crimson clover (WH). We also examined the effect of the size of incorporated plant residue fragments on soil CO2 and N2O emissions. Soil GHG emissions were the lowest on slopes as compared to depressions and summits. Cover crop mixtures including WH and WK increased soil CO2 fluxes emitted from the soil while cereal rye decreased it. Cutting plant residues prior to incorporation tended to decrease CO2 emission, suggesting that reducing fragment size of incorporated residue has a potential to mitigate GHG emissions from agricultural soils. In contrast, differences in N2O emissions for any of the studied effects were not statistically significant, likely due to very high variability of N2O fluxes. Soil temperature, moisture and total aboveground biomass were strongly associated with soil CO2 and N2O fluxes from soil.

Effect of Brassica crop-based biofumigation on soilborne disease suppression in woody ornamentals

Soilborne diseases are the most economically important problem for ornamental nursery producers in the southeastern United States. The use of cover crops selected based on their biofumigant activity to improve soilborne disease management in woody ornamental production was assessed. Replicated pot bioassays were established as greenhouse trials in sterilized clay loam soil which had pre-existing populations of Rhizoctonia solani or Phytophthora nicotianae. Selected Brassica crops were seeded directly into the soil and flowering cover crops were incorporated 15 cm deep into the same pots and covered with polyethylene for 2 or 4 weeks. Volatile compounds released during the biofumigation process were collected at different time intervals. Soil type and moisture affected ITC release. Hydrangea or viburnum rooted cuttings were grown in the biofumigated (2 or 4 weeks) and non-biofumigated control pots and root rot disease severity was evaluated at the end of each bioassay. Yellow mustard (Sinapis alba, ‘White gold’), turnip (Brassica rapa, ‘Purple top forage’), arugula (Eruca vesicaria ssp. sativa, ‘Astro’), Mighty mustard (B. juncea, ‘Pacific gold’), rape (B. napus, ‘Dwarf essex’), mustard green (B. carinata, ‘Amara’) and brown mustard (B. juncea, ‘Kodiak’) cover crops were effective in suppressing R. solani and P. nicotianae. Similar disease suppression was observed whether biofumigation was performed for 2 or 4 weeks. Phytotoxicity was not observed on viburnum and hydrangea woody ornamental plants after either the 2 or 4 weeks biofumigation period with any of the tested cover crops. Viburnum and hydrangea grown in mustard green-, arugula- and turnip-incorporated soil had significantly higher whole plant and root fresh weights compared with the inoculated, non-biofumigated control plants. Although mustard green and arugula are not used currently as commercial biofumigation cover crops, they also showed promise for controlling soilborne pathogens of woody ornamental plants under greenhouse conditions.

Pathogenicity of Rhizoctonia solani and Phytophthora nicotianae to Brassicaceae cover crops

Soil-borne diseases can reduce nursery crop performance and increase costs to nursery producers. In particular, soil-borne diseases caused by Phytophthora nicotianae and Rhizoctonia solani are the most economically important problems of Southeastern United States nursery producers. Methyl bromide was widely used as a standard treatment for management of soil-borne diseases until the implementation of the Montreal protocol. Since then, many chemical and non-chemical soil-borne disease management methods have been tested, but are not yet providing effective and consistent results like methyl bromide. Cover crops that belonged to the Brassicaceae family can be incorporated into the soil to control soil-borne diseases and this process is widely known as biofumigation. But, the use of Brassicaceae cover crops has not been widely explored as a method of controlling soil-borne diseases in woody ornamental nursery production. The objective of this study was to evaluate Brassicaceae cover crops for susceptibility to most destructive soil-borne pathogens of nursery production, P. nicotianae and R. solani, to identify effective cover crops that can be used in the biofumigation process in woody ornamental nursery production. Brassica species intended to be used in the fresh market or biofimigation were screened for their susceptibility to R. solani and P. nicotiane in an environmentally controlled greenhouse. At the end of experiments, plant growth data (plant height, width and fresh weight), total damping-off were recorded, and cover crop root systems were assessed for disease severity using a scale of 0–100% roots affected. Among the tested 15 cover crops in the Brassicaceae family, oilseed radish (Raphanus sativus L.), yellow mustard “White Gold” (Sinapis alba L.), turnip “Purple Top Forage” (Brassica rapa L.), arugula “Astro” (Eruca vesicaria (L.) Cav. ssp. sativa (Mill.) Thell.), mighty mustard® “Pacific Gold” (B. juncea (L.) Czern.), brown mustard “Kodiak” (B. juncea (L.) Czern.), rape “Dwarf Essex” (B. napus L.) and mustard green “Amara” (B. carinata A. Braun) showed numerically lower root rot disease severity and total damping-off in topsoil which had pre-existing populations of R. solani or P. nicotinanae compared to other cover crops. Since these above mentioned Brassicaceae crops shows the ability to withstand the higher disease pressure from R. solani and P. nicotinanae under the greenhouse conditions they can be used in the further experiments to evaluate their ability in biofumigation. Further research is necessary to evaluate the performance of these cover crops under the field conditions.

Co-managing soil and plant pathogens: effects of organic amendments on soil fertility and fungal pathogen survival

This study examined presence and persistence of the gray mold pathogen Botrytis cinerea in organic field soils and the dual effect of different organic soil amendments on soil fertility and survival of Botrytis cinerea – one of the most important broad-host-range plant pathogens in the world. Field soil was collected and assayed for two consecutive years from an organic farm in Maryland to determine baseline levels of B. cinerea inoculum. Then a controlled environment study was conducted in triplicate, where laboratory-reared B. cinerea survival structures (sclerotia) were buried in potted mixtures of field soil containing compost and chitin amendments and assessed over time for viability, fragmentation, and colonization by microbes antagonistic to B. cinerea growth. A one-year in-situ field trial was also conducted to evaluate dual effects of different combinations of soil amendment and cover crop treatments on pathogen persistence and soil fertility as they would occur in an organic grower’s field, under winter and summer conditions. Surveys revealed that the selected organic farm sustained high levels of soil-borne B. cinerea inoculum, with up to 1200 B. cinerea propagules per gram of soil, and field studies demonstrated that the fungus could persist for both a five-month over-winter (2–24% of sclerotia viable) and a five-month over-summer period (0–1% of sclerotia viable) in-situ. In general, compost and cover crop treatments increased persistence of the fungus compared to fallow (control) treatments in both controlled environment and field trials. On the other hand, application of chitin (crushed crab shells) facilitated fragmentation and mortality of sclerotia. Compost and chitin applications resulted in a two- to three-fold increase in recovery of sclerotia-associated microbial antagonists, while the addition of dwarf essex rape (Brassica napus) significantly reduced recovery compared to the control. In both the winter and summer field studies, all compost treatments significantly increased soil N, P, and pH, but the combined treatment of compost, chitin, and cover crop had the greatest increase in soil fertility. Based on molecular analysis, putative bacterial antagonists recovered from non-viable sclerotia included species of Bacillus, Paenibacillus, Stenotrophomonas, and Lactobacillus – all of which have been previously described to inhibit B. cinerea growth. Overall, our studies indicate that organic strategies to co-manage B. cinerea and soil fertility can be optimized by adding chitin to composted amendments.

Effects of Cover Crops on Pratylenchus penetrans and the Nematode Community in Carrot Production.

Cover cropping is a common practice in U.S. Midwest carrot production for soil conservation, and may affect soil ecology and plant-parasitic nematodes-to which carrots are very susceptible. This study assessed the impact of cover crops-oats (Avena sativa), radish (Raphanus sativus) cv. Defender, rape (Brassica napus) cv. Dwarf Essex, and a mixture of oats and radish-on plant-parasitic nematodes and soil ecology based on the nematode community in Michigan carrot production systems. Research was conducted at two field sites where cover crops were grown in Fall 2014 preceding Summer 2015 carrot production. At Site 1, root-lesion (Pratylenchus penetrans) and stunt (Tylenchorhynchus sp.) nematodes were present at low population densities (less than 25 nematodes/100 cm3 soil), but were not significantly affected (P > 0.05) by cover crops. At Site 2, P. penetrans population densities were increased (P ≤ 0.05) by 'Defender' radish compared to other cover crops or fallow control during cover crop growth and midseason carrot production. At both sites, there were few short-term impacts of cover cropping on soil ecology based on the nematode community. At Site 1, only at carrot harvest, radish-oats mixture and 'Dwarf Essex' rape alone enriched the soil food web based on the enrichment index (P ≤ 0.05) while rape and radish increased structure index values. At Site 2, bacterivore abundance was increased by oats or radish cover crops compared to control, but only during carrot production. In general, cover crops did not affect the nematode community until nearly a year after cover crop growth suggesting that changes in the soil community following cover cropping may be gradual.

The impact of cerium oxide nanoparticles on the salt stress responses of Brassica napus L.

Dwindling high quality water resources and growing population are forcing growers to irrigate crops with water of high salinity. It is well recognized that salinity negatively affects plant physiology and biochemistry, and represents one of the most serious threats to crop production and food security. Meanwhile, engineered nanoparticles (ENPs) are increasingly detected in irrigation water and agricultural soils due to the rapid advancement of nanotechnology. Previous research has demonstrated that ENPs such as cerium oxide nanoparticles (CeO2-NPs) exert significant impact on plant growth and production. However, almost all previous studies were conducted in well controlled environment. Knowledge on how ENPs affect plant development in a stressed condition is almost empty. The goal of the present study was to understand the physiological and biochemical changes in Brassica napus L. (canola) cv. ‘Dwarf Essex’ under synergistic salt stress and CeO2-NPs effects. Two salinity levels: 0 (control) and 100 mM NaCl, and three CeO2-NPs concentrations: 0 (control), 200 and 1000 mg kg−1 dry sand and clay mixture, were employed. As expected, 100 mM of NaCl significantly hindered plant growth and negatively affected the physiological processes of canola. Plants treated with CeO2-NPs had higher plant biomass, exhibited higher efficiency of the photosynthetic apparatus and less stress in both fresh water and saline water irrigation conditions Overall, our results demonstrated that CeO2-NPs led to changes in canola growth and physiology which improved the plant salt stress response but did not completely alleviate the salt stress of canola.

Green manure crops for integrated pest management in upland taro fields in Hawaii

Taro (Colocasia esculenta) is a non-graminaceous monocot consumed primarily for its starchy corm. To determine the best green manure crops to rotate with upland-grown taro, 10 genotypes of seven green manure species were evaluated in a greenhouse study to determine their ability to suppress root-knot nematode Meloidogyne javanica. These species were: black hollyhock (Alcea rosea L.); canola (Brassica napus L.); cabbage (B. oleracea L.); French marigold (Tagetes patula L.), sorghum-sudangrass [Sorghum bicolor (L.) Moench nothosubsp. drummondii (Steud.) de Wet ex Davidse]; sunn hemp (Crotalaria juncea L.); and yellow mustard (Sinapis alba L.). Plants were inoculated with eggs of M. javanica and after 6 weeks, nematode eggs, and reproduction factor (Rf) of M. javanica were determined. Marigolds were non-hosts to M. javanica; other crop species that were poor hosts to M. javanica included canola 'Dwarf Essex', sorghum-sudangrass cvs. Piper and Sordan 79, black hollyhock 'Nigra', and sunn hemp. Based on results of this greenhouse study, eight green manure treatments (yellow mustard cultivar 'Ida Gold', French marigolds cultivars 'Nema-gone' and 'Golden Guardian', sorghum-sudangrass cultivar 'Sordan 79' and 'Tastemaker', sunn hemp, an unplanted plot, and a clean fallow control plot) were selected and grown for 3 months in a field trial in Pepeekeo, Hawaii. Each treatment was blocked four times in a randomized complete block design. Based on this field trial, marigold cultivars, sorghum-sudangrass hybrids, and sunn hemp appeared to be the best green manure crops for rotation in an upland taro cropping system.

Green Manure Crops for Management of Meloidogyne javanica and Pythium aphanidermatum

To determine the potential to suppress root-knot nematode Meloidogyne javanica, 10 genotypes of seven green manure species were evaluated in a greenhouse study. These species were: black hollyhock (Alcea rosea L.); canola (Brassica napus L.); cabbage (B. oleracea L.); French marigold (Tagetes patula L.), sorghum–sudangrass [Sorghum bicolor (L.) Moench nothosubsp. drummondii (Steud.) de Wet ex Davidse]; sunn hemp (Crotalaria juncea L.); and yellow mustard (Sinapis alba L.). Plants were inoculated with eggs of M. javanica and after 6 weeks, nematode eggs and reproduction factor (Rf = final egg population density/initial egg population density) were determined. Marigolds were non-hosts to M. javanica; other crop species that were poor hosts to M. javanica included canola cv. Dwarf Essex, sorghum–sudangrass cvs. Piper and Sordan 79, black hollyhock cv. Nigra, and sunn hemp. Based on low Rf, four groups of species were selected for further evaluation in the greenhouse to determine the response to both M. javanica and another crop pathogen, Pythium aphanidermatum. These four groups of green manure crops were: 1) seven marigold genotypes; 2) four Brassicaceae genotypes; 3) seven sorghum–sudangrass hybrids; and 4) four other species [black hollyhock, sunn hemp, elecampane (Inula helenium L.), and black-eyed Susan (Rudbeckia hirta L.)]. Plants were inoculated with a factorial combination of M. javanica and P. aphanidermatum (none, each alone, and in combination) and repeated four times in a split-plot experimental design (whole plots were factorial treatments and subplots were green manure crop genotypes). Six weeks after inoculation, plants were harvested and measured for fresh and dry weights of shoots and roots and Rf of M. javanica. Adverse effects of P. aphanidermatum were characterized by dead or dying roots and measured by reduced plant biomass. Negative synergistic effects were observed in several marigold and Brassicaceae genotypes, in which the combined effects of M. javanica and P. aphanidermatum reduced shoot and root growth more severely than either treatment alone. Marigold T. erecta cv. Orangeade, sorghum–sudangrass cvs. Graze-All, Piper, and Sordan 79, and sunn hemp appeared to be resistant to M. javanica and P. aphanidermatum, either alone or in combination. Based on results of greenhouse trials, eight green manure crops (yellow mustard cv. Ida Gold, French marigolds cvs. Nema-gone and Golden Guardian, sorghum–sudangrass cvs. Sordan 79 and Tastemaker, sunn hemp, unplanted plot, and a control plot with weed mat) were selected and grown for 3 months in a field trial in Pepeekeo, HI. Each treatment was replicated four times in a randomized complete block design. Shoot biomass was sampled at 1, 2, and 3 months after planting. Plant–parasitic nematodes were counted before planting and at 4 months after planting. Dry weight biomass averaged across three sampling dates was greatest for the two sorghum–sudangrass hybrids followed by those of two marigold cultivars that did not differ from them. No significant differences in populations of root-knot nematodes were found. Based on this field trial as well as greenhouse trials, marigold cultivars, sorghum–sudangrass hybrids, and sunn hemp appeared to be non-hosts or poor hosts to reniform (Rotylenchulus reniformis) as well as root-knot nematodes and well adapted to the environmental conditions found along the Hamakua Coast of the Hawaii Island.

Effect of biofumigation and chemical fumigation on soil microbial community structure and control of pepper Phytophthora blight

Soil biofumigation with brassica plant residues has been shown to significantly suppress soilborne pathogen. However, little published data reported the impact of biofumigation on microbial community structure in pepper (Capsicum annuum L.) production systems under field conditions. Biofumigation with rapeseed (Brassica napus 'Dwarf Essex') meal and chemical fumigation with dazomet were tested to control the pepper disease caused by Phytophthora capsici. BF treatment showed the lowest disease incidence among these treatments. Effects on soil bacterial and fungal communities were assessed by denaturating gradient gel electrophoresis and the results showed that the biofumigation increased bacterial diversity and decreased fungal diversity. There was a negative correlation between soil bacterial diversity and disease incidence and a positive correlation between soil fungal diversity and disease incidence. Cloning of the microbial community showed that the microbial community structures were altered by biofumigation. Soil was also evaluated for their chemical properties. Biofumigation increased soil content of total N, NO3(-)-N, available P and available K. A significant correlation between soil microbial community structures and soil chemical properties was found. Overall, these results indicated that biofumigation reduced disease incidence of pepper through altering soil microbial community structures.

Increased pepper yields following incorporation of biofumigation cover crops and the effects on soilborne pathogen populations and pepper diseases

The use of brassica cover crops and their associated degradation compounds as biofumigants to manage soilborne pathogens could offer vegetable growers an alternative to the restricted broad-spectrum fumigant methyl bromide. Biofumigation was tested in two experiments to manage Rhizoctonia solani, Sclerotium rolfsii, and Pythium spp., and the diseases they cause on pepper. Field plots were seeded in fall 2010 and spring 2011 to oilseed radish (Raphanus sativus L.), ‘Pacific Gold’ mustard (Brassica juncea (L.) Czern), or ‘Dwarf Essex’ winter rapeseed (Brassica napus L.). Cover crops were disked into soil in spring 2011 and immediately covered with virtually impermeable film (VIF) to reduce the escape of volatile pesticidal compounds. Controls included fallow plots with (CVIF) and without (fallow) VIF. Green bell pepper was transplanted into all plots. Concentrations of isothiocyanates (ITCs), the brassica degradation compounds primarily responsible for pesticidal activity, were highest following incorporation of mustard. Rapeseed yielded the second highest ITC concentrations. Radish yielded very low ITC concentrations in experiment 1, and none during experiment 2. ITCs also were detected in low concentrations in CVIF treatments. All treatments that received VIF reduced populations of R. solani compared to fallow, with no differences between biofumigation treatments and CVIF. Biofumigation treatments did not reduce populations of Pythium spp. or S. rolfsii compared to CVIF. Pepper stunting was significantly lower in treatments that received VIF compared to fallow, with no consistently significant differences between biofumigation treatments and CVIF. Pythium isolated from roots of stunted peppers was identified as Pythium aphanidermatum. Biofumigation treatments did not reduce plant mortality. Pepper yields were highest in biofumigation treatments compared to CVIF, and CVIF yields were higher than fallow yields.

Influence of Summer Cover Crops and Mycorrhizal Fungi on Strawberry Production in the Southeastern United States

The effects of eight summer cover crop treatments combined with two arbuscular mycorrhizal (AM) fungal inoculants on strawberry growth and yields were examined in a 2-year field experiment. Cover crop treatments included 1) sudangrass [Sorghum bicolor (L.) Moench cv. Piper]; 2) pearl millet [Pennisetum glaucum (L.) R.Br. cv. 102 M Hybrid]; 3) soybean [Glycine max (L.) Merrill cv. Laredo]; 4) velvetbean [Mucuna deeringiana (Bort) Merr. cv. Georgia Bush]; 5) sudangrass/velvetbean combination; 6) pearl millet/soybean combination; 7) a non-mycorrhizal host consisting of rape (Brassica napus L. var. napus cv. Dwarf Essex) and buckwheat (Fagopyrum esculentum Moench) in Year 1 and Year 2, respectively; and 8) no cover crop control. Strawberry tips were inoculated with either a native mixture of several AM fungal species or a single species sold commercially, Glomus intraradices. Cover crop treatments were assessed for their aboveground biomass and nutrient uptake as well as their impacts on weed abundance and diversity, soil nutrients, and parasitic nematode populations. Cover crop and AM treatments were assessed for their impact on strawberry growth, yields, AM root colonization, and nutrient uptake. Grass-based cover crop treatments, particularly pearl millet, produced the most aboveground biomass. In both years, all cover crop treatments reduced summer weed biomass compared with the control. Neither cover crop nor AM treatments had an effect on overall strawberry plant growth or yields in either year, although some differences existed at specific growth periods. The results suggest that cover crops are a viable strategy for reducing summertime weeds and that background, native populations of AM fungi in the soil may be just as effective as a commercially available species. It is likely that no overall yield benefit was found among treatments for two reasons: 1) nutrients, especially nitrogen, were not limiting; and 2) the cover crop growth window may have been too short for a significant impact on strawberries over two seasons.

Ecological Relationships of Verticillium Wilt Suppression of Potato by Green Manures

Studies show that once a suppressive effect has been established, a green manure treatment for a single season is sufficient to either maintain or to re-establish the control of Verticillium wilt of potato. Verticillium wilt, caused by Verticillium dahliae Kleb., was controlled on the first years of Russet Burbank potato cropping following 2–3 successive years of green manure treatments. Following this control, yields were increased with the first year of potato cropping, but were reduced during the second consecutive year of cropping with potato. In the second year of continuous cropping with Russet Burbank, total yields were reduced by 10% and U.S. #1 tubers by 40%, and these yield reductions were accompanied by increased soil populations of V. dahliae by >5- fold. Yet, with a single season of an earlier–used green manure treatment (e.g. Austrian winter pea, sudangrass, Dwarf Essex rape, Bridger rape, oats, rye, or sweet corn), Verticillium wilt was again controlled and yields of Russet Burbank were increased to higher levels or to levels that were equivalent to those initially observed following either 2 or 3 successive seasons of green manure treatment. Green manures also demonstrated significant effects on microbial activities, which were inversely related to the incidence of Verticillium wilt. More specific effects on microbial activity and soilborne ecology were shown with increased populations of Fusarium spp. Verticillium wilt was negatively correlated with infections of potato feeder roots and stem-ends of potato tubers by F. equiseti. Exemplifying the effect of a green manure on fungal ecology, sudangrass treatments were shown to have lasting effects on Fusarium avenaceum populations with quantitative changes that extended 5 years beyond the time of soil incorporation with sudangrass.

Brassica Cover Crops for Nitrogen Retention in the Mid‐Atlantic Coastal Plain

Brassica cover crops are new to the mid-Atlantic region, and limited information is available on their N uptake capabilities for effective N conservation. Forage radish (Raphanus sativus L. cv. Daikon), oilseed radish (Raphanus sativus L. cv. Adagio), and rape (Brassica napus L. cv. Dwarf Essex) were compared with rye (Secale cereale L. cv. Wheeler), a popular cover crop in the region, with regard to N uptake ability and potential to decrease N leaching at two sites in Maryland. Plants were harvested in fall and spring for dry matter and N analysis. Soil samples from 0 cm to 105 to 180 cm depth were obtained in fall and spring for NH(4)-N and NO(3)-N analyses. Ceramic cup tension lysimeters were installed at depths of 75 to 120 cm to monitor NO(3)-N in soil pore water. Averaged across 3 site-years, forage radish and rape shoots had greater dry matter production and captured more N in fall than rye shoots. Compared with a weedy fallow control, rape and rye caused similar decreases in soil NO(3)-N in fall and spring throughout the sampled profile. Cover crops had no effect on soil NH(4)-N. During the spring on coarse textured soil, pore water NO(3)-N concentrations in freeze-killed Brassica (radish) plots were greater than in control and overwintering Brassica (rape) and rye plots. On fine textured soil, all cover crops provided a similar decrease in pore water NO(3)-N concentration compared with control. On coarse textured soils, freeze-killed Brassica cover crops should be followed by an early-planted spring main crop.

Effect of Incorporation of Brassica spp. Residues on Population Densities of Soilborne Microorganisms and on Damping-off and Fusarium Wilt of Watermelon.

Incorporating Brassica spp. residue to reduce populations of soilborne fungi and manage damping-off and Fusarium wilt of watermelon (Citrullus lanatus var. lanatus) was studied in two field experiments. Treatments included incorporating flowering Brassica napus cv. Dwarf Essex canola or B. juncea cv. Cutlass mustard and laying black polyethylene mulch at incorporation or 1 month after incorporation, methyl bromide, and a nontreated control. In both years, glucosinolates were identified and quantified in the shoots and roots of the flowering plants. In both years, the total concentration of glucosinolates incorporated per square meter was significantly higher for B. juncea than for B. napus. Isothiocyanates were inconsistently detected in the amended soils and none were detected more than 12 days postincorporation. After incorporation in 2004 and 2005, amended plots had higher populations of Fusarium oxysporum and Pythium spp. than the methyl bromide treatment, and in some treatments, populations were higher than in the control. Fluorescent Pseudomonas spp. were not suppressed in amended soils, and their populations were significantly higher in some amended treatments than those in methyl bromide-treated soils or nontreated control soils. Incidence of damping-off and severity of Fusarium wilt on seedless watermelon cv. Tri-X 313, which is susceptible to Fusarium wilt, were not consistently lower in brassica-amended soils or methyl bromide-treated plots than in nontreated control plots. Therefore, under spring conditions and methods used in this study, neither biofumigation nor methyl bromide fumigation in coastal South Carolina was an effective disease management tool for two soilborne pathogens of watermelon.

Cultural Management of Microbial Community Structure to Enhance Growth of Apple in Replant Soils

ABSTRACT Apple replant disease typically is managed through pre-plant application of broad-spectrum soil fumigants including methyl bromide. The impending loss or restricted use of soil fumigants and the needs of an expanding organic tree fruit industry necessitate the development of alternative control measures. The microbial community resident in a wheat field soil was shown to suppress components of the microbial complex that incites apple replant disease. Pseudomonas putida was the primary fluorescent pseudomonad recovered from suppressive soil, whereas Pseudomonas fluorescens bv. III was dominant in a conducive soil; the latter developed within 3 years of orchard establishment at the same site. In greenhouse studies, cultivation of wheat in replant orchard soils prior to planting apple suppressed disease development. Disease suppression was induced in a wheat cultivar-specific manner. Wheat cultivars that enhanced apple seedling growth altered the dominant fluorescent pseudo-monad from Pseudomonas fluorescens bv. III to Pseudomonas putida. The microbial community resident in replant orchard soils after growing wheat also was suppressive to an introduced isolate of Rhizoctonia solani anastomosis group 5, which causes root rot of apple. Incorporation of high glucosinolate containing rapeseed ('Dwarf Essex') meal also enhanced growth of apple in replant soils through suppression of Rhizoc-tonia spp., Cylindrocarpon spp., and Pratylenchus penetrans. Integration of these methods will require knowledge of the impact of the biofumigant component on the wheat-induced disease-suppressive microbial community. Implementation of these control strategies for management of apple replant disease awaits confirmation from ongoing field validation trials.

Suppression of Specific Apple Root Pathogens by Brassica napus Seed Meal Amendment Regardless of Glucosinolate Content

ABSTRACT The impact of Brassica napus seed meal on the microbial complex that incites apple replant disease was evaluated in greenhouse trials. Regardless of glucosinolate content, seed meal amendment at a rate of 0.1% (vol/vol) significantly enhanced growth of apple and suppressed apple root infection by Rhizoctonia spp. and Pratylenchus penetrans. High glucosinolate B. napus cv. Dwarf Essex seed meal amendments did not consistently suppress soil populations of Pythium spp. or apple root infection by this pathogen. Application of a low glucosinolate containing B. napus seed meal at a rate of 1.0% (vol/vol) resulted in a significant increase in recovery of Pythium spp. from apple roots, and a corresponding reduction in apple seedling root biomass. When applied at lower rates, B. napus seed meal amendments enhanced populations of fluorescent Pseudomonas spp., but these bacteria were not recovered from soils amended with seed meal at a rate of 2% (vol/vol). Seed meal amendments resulted in increased soil populations of total bacteria and actinomycetes. B. napus cv. Dwarf Essex seed meal amendments were phytotoxic to apple when applied at a rate of 2% (vol/vol), and phytotoxicity was not diminished when planting was delayed for as long as 12 weeks after application. These findings suggest that B. napus seed meal amendments can be a useful tool in the management of apple replant disease and, in the case of Rhizoctonia spp., that disease control operates through mechanisms other than production of glucosinolate hydrolysis products.

Brassica vegetables as edible greens in Newfoundland

Turnip tops or greens, the early leaves of rutabaga (Brassica napus var. rapifera L.), are a traditional Newfoundland vegetable. Commercial farmers currently grow and market forage rape (B. napus L.) as greens. Our objectives were to determine why forage rape is now grown in preference to other Brassica crops and to examine potential greens alternatives. Seed from two cultivars each of three Brassicas [rutabaga, forage rape and forage kale (Brassica oleracea var. medullosa L.)] was used in: 1) a germination study at 5, 10, 15 and 20°C; 2) a growth study at constant temperature regimes of 12 and 18°C; 3) a 2 yr agronomic study; and 4) a sensory evaluation for appearance and taste as a boiled vegetable. Hobson rape, Dwarf Essex rape and the locally bred Brookfield rutabaga germinated, emerged and grew faster than both kale cultivars and Laurentian rutabaga at all controlled-temperature regimes. The two kale cultivars and Laurentian rutabaga did not exhibit adequate agronomic potential. Although the rape cultivars were among the top-yielding entries at most harvests, Brookfield rutabaga yielded greater leaf weight in both years of the agronomic study. Judges preferred the visual appearance of greens with dark green leaves, a characteristic of the forage rape cultivars studied, but favored the taste of boiled kale. Key words: Forage rape, kale, rutabaga, SPAD chlorophyll meter

Combined effects of Brassica napus seed meal and Trichoderma harzianum on two soilborne plant pathogens

The effects of soil amendment with rapeseed meal from Brassica napus cv. 'Dwarf Essex' (high glucosinolate concentrations) and 'Stonewall' (low glucosinolate concentrations) on the biological control activity of Trichoderma harzianum towards Sclerotinia sclerotiorum and Aphanomyces euteiches were evaluated. Trichoderma harzianum added to soil reduced myceliogenic germination of S. sclerotiorum by 94%, but did not affect carpogenic germination. In contrast, 100% reduction in carpogenic germination was observed in soil amended with Dwarf Essex meal, along with a 33% reduction in myceliogenic germination. With Stonewall meal as soil amendment, carpogenic germination was reduced by 44% and myceliogenic germination was not affected. Both Dwarf Essex and Stonewall meals inhibited colonization of sclerotia in soil by T. harzianum, from 100% to 0% and 8%, respectively, so that biocontrol activity of T. harzianum was reduced in the presence of either meal. Aphanomyces euteiches root rot of pea was significantly reduced by T. harzianum alone (100%), by amendment with Dwarf Essex meal alone (77%), and by T. harzianum in combination with Dwarf Essex meal (100%). Amendment with Stonewall meal alone did not control root rot, and combination of Stonewall meal with T. harzianum reduced the biocontrol efficacy of T. harzianum.